CONTENTS

A PERIODICAL OF PARTICLE PHYSICS

FALL 2001 VOL. 31, NUMBER 3

Guest Editor MICHAEL RIORDAN

Editors RENE DONALDSON, BILL KIRK

Contributing Editors GORDON FRASER JUDY JACKSON, AKIHIRO MAKI MICHAEL RIORDAN, PEDRO WALOSCHEK

Editorial Advisory Board PATRICIA BURCHAT, DAVID BURKE LANCE DIXON, EDWARD HARTOUNI ABRAHAM SEIDEN, GEORGE SMOOT HERMAN WINICK

Illustrations TERRY ANDERSON

Distribution C RYSTAL TILGHMAN

The Beam Line is published quarterly by the Stanford Linear Accelerator Center, Box 4349, Stanford, CA 94309. Telephone: (650) 926-2585. EMAIL: [email protected] FAX: (650) 926-4500

Issues of the Beam Line are accessible electroni- cally on the World Wide Web at http://www.slac. stanford.edu/pubs/beamline. SLAC is operated by Stanford University under contract with the U.S. Department of Energy. The opinions of the authors do not necessarily reflect the policies of the Stanford Linear Accelerator Center.

Cover: The Sudbury Neutrino Observatory detects neutrinos from the sun. This interior view from beneath the detector shows the acrylic vessel containing 1000 tons of heavy water, surrounded by photomultiplier tubes. (Courtesy SNO Collaboration) Printed on recycled paper 2 FOREWORD 32 THE ENIGMATIC WORLD David O. Caldwell OF NEUTRINOS Trying to discern the patterns of neutrino masses and mixing. FEATURES Boris Kayser

42 THE K2K NEUTRINO 4 PAULI’S GHOST EXPERIMENT A seventy-year saga of the conception The world’s first long-baseline and discovery of neutrinos. neutrino experiment is beginning Michael Riordan to produce results. Koichiro Nishikawa & Jeffrey Wilkes 15 MINING SUNSHINE The first results from the Sudbury 50 WHATEVER HAPPENED Neutrino Observatory reveal TO HOT DARK MATTER? the “missing” solar neutrinos. Neutrinos may have had a Joshua Klein significant impact on the structure of the Universe. 24 NEUTRINO EXPERIMENTS Joel R. Primack AT FERMILAB Neutrino physics is thriving in the Nation’s heartland. 58 CONTRIBUTORS Paul Nienaber DATES TO REMEMBER FOREWORD DAVIDO.CALDWELL

NEUTRINO TIMELINE

1930 Wolfgang Pauli predicts that HE PAST FEW YEARS have been an neutrinos exist. exceptionally exciting time for physicists 1956 Frederick Reines and involved in research on neutrinos. We now have Clyde Cowan discover two different confirmed (and one unconfirmed) the electron neutrino. pieces of evidence that neutrinos oscillate from 1961 Muon neutrinos are oneT type into another, which implies that they possess mass. discovered at the These experiments have provided the first convincing experimen- Brookhaven National tal evidence for new physics beyond the Standard Model, today’s Laboratory. dominant theory of elementary particles and the interactions 1968 Ray Davis and among them. For two decades, ever since this theory took firm colleagues begin the hold within the community during the late 1970s, particle physi- first solar neutrino cists have been looking in every possible corner and under every experiment. accessible rock for this new physics. Now that we have such 1989 Experiments at SLAC evidence, neutrino studies will lead the research into what lies and CERN prove that beyond the Standard Model. there are only three As a probe for new physics, neutrinos are unique among kinds of light neutrinos. particles. As far as we know today, they are elementary particles, 1998 Super-Kamiokande but unlike their fellow leptons and quarks, neutrinos are experiment reports unencumbered by electrical charge and do not take part in the conclusive evidence for complicated strong interactions. They seem about as elementary neutrino oscillations. as particles can be—possessing just the tiniest bits of mass and 2000 DONUT experiment engaging only in weak interactions with other leptons and quarks. reports direct observa- Neutrinos’ exceptionally small mass may in fact provide a valu- tion of tau neutrinos. able window into the very high-energy scales that are otherwise 2001 SNO experiment finds completely inaccessible at today’s particle accelerators. And evidence that solar despite the smallness of their mass, neutrinos may have played an neutrinos indeed important role in establishing the structure of the Universe during oscillate. the years immediately after the Big Bang.

2 FALL 2001 This special issue of the Beam Line appears at a pivotal moment in the evolution of the neutrino physics, when we take stock of the new advances and try to glimpse what discoveries may lie just over the horizon. A historical article by Michael Riordan helps familiarize readers with Wolfgang Pauli’s 1930 con- ception of the neutrino and its evolution over seven decades of research. Joshua Klein, Paul Nienaber, Koichiro Nishikawa, and Jeffrey Wilkes bring us up to date on recent neutrino experiments as well as others planned for the near future. Theorists Boris

Kayser and Joel Primack attempt to interpret the pattern of David O. Caldwell has been doing neutrino masses that seems to be emerging and assess what research on neutrinos since 1980. impact these ghostly particles might have had on the structure of He earned his Ph.D. in nuclear the Universe. physics at UCLA in 1953 and since Despite all the recent advances, there appears to be no end of 1965 has been a member of the questions about these elusive, fascinating particles and the effects physics department faculty at the that they may produce. This special Beam Line issue on neutrino University of California, Santa physics, edited by Michael Riordan, captures the excitement of Bartbara. A Guggenheim Fellow and a Fellow of the A merican this field. Physical Society, he is editor of the recently published book, Current UPDATE Aspects of Neutrino Physics.

AS THIS ISSUE was going to press, we learned about the extensive damage to most of the phototubes in the Super- Kamiokande experiment. This accident is not only devastating for that research group but also for all of physics. Super-Kamiokande has yielded so many insights about neutrinos and into physics beyond the Standard Model that the inevitable delays are a severe blow. We wish them as rapid a recovery as possible.

BEAM LINE 3 A Seventy-Year Saga of the Concep

by MICHAEL RIORDAN

HEN WOLFGANG PAULI conceived his The idea idea of the neutrino in 1930, it was substan- tially different from the ghostly particles rec- ognized today. That December he proposed of the this light, neutral, spin-1/2 particle as a “des- perate remedy” for the energy crisis of that Wtime: that electrons emitted in nuclear beta decay had a con- neutrino tinuous, rather than discrete, energy spectrum. The crisis had grown so severe by the late 1920s that Niels Bohr began to contemplate abandoning the sacrosanct law of energy con- has evolved servation in nuclear processes. Pauli could not countenance such radical unorthodoxy. Instead, he suggested that very substantially light poltergeists might inhabit the nucleus along with pro- tons and electrons. They should have a mass “of the same order of magnitude as the electron mass.” Normally bound over the within nuclei, they would have “about 10 times the pene- trating capacity of a gamma ray” after their emission. He could account for continuous beta-decay spectra by assuming seven decades that this particle “is emitted together with the electron, in such a way that the sum of the energies . . . is constant.” of its Pauli clearly thought of his ghosts as actual constituents of atomic nuclei, with a small mass and substantial interac- tion strength. He was trying not only to preserve energy con- existence. servation in nuclear processes but also to dodge the severe problems with spin and statistics that cropped up in nuclei— then imagined to consist only of protons and electrons. The Adapted from “Pauli’s Ghost: The Conception and Discovery of nitrogen nucleus, for example, was widely pictured as having Neutrinos,” by Michael Riordan, in 14 protons and 7 electrons, but it did not seem to obey Fermi Current Aspects of Neutrino Physics, D. O. Caldwell, Editor (Springer-Verlag statistics, as expected of any object containing an odd num- Berlin, 2001). ber of fermions. By adding seven more fermions to the heap,

4 FALL 2001 tion and Discovery of Neutrinos

Wolfgang Pauli, left, Niels Bohr, center, Erwin Schrödinger, and Lise Meitner at the 1933 Solvay Congress (Courtesy Niels Bohr Archives).

BEAM LINE 5 Pauli could explain why it behaved Thus was the idea of the neutrino like a boson. But nobody could fig- born, but it remained mostly an ure out how to cloister such light, intriguing possibility for years. Even speedy particles within the narrow after reading Fermi’s paper, Bohr was confines of a nucleus. still not convinced of its reality. “In James Chadwick’s 1932 discovery an ordinary way I might say that I do of a heavy fermion he dubbed the not believe in neutrinos,” Sir Arthur neutron resolved most of these prob- Eddington remarked, “Dare I say that lems. Composed of seven protons experimental physicists will not have and seven almost equally massive sufficient ingenuity to make neu- Graph from Fermi’s paper on the theory neutrons, the nitrogen nucleus now trinos?” of beta decay, showing how the shape of the emitted electron’s energy spec- had an even number of fermions in- trum varies with the possible mass of the side and could easily behave like a HEN George Gamow neutrino. boson. Enrico Fermi’s famous theory wrote “The Reality of of beta decay put the capstone on the W Neutrinos” in 1948, how- growing edifice. Instead of inhabit- ever, he could discourse about them ing the nucleus as constituents, the with confidence that they indeed electron and “neutrino” (a name existed. Although nobody had yet coined by Fermi in 1931 to mean detected one directly, there were sev- “little neutral object”) were to be cre- eral indirect experimental proofs of ated the moment a neutron trans- their reality.Sensitive measurements formed into a proton. Fermi even of the energy and momentum of went so far as to indicate how the beta-decay electrons and their energy spectrum of beta-decay recoiling nuclei in Wilson cloud electrons depends critically on the chambers indicated that substantial neutrino’s mass. By comparing his quantities of energy and momentum theoretical curves with the measured were missing. “This means some spectra near their high-energy end other particle must have been eject- point, he concluded that “the rest ed at the same time as the electron,” mass of the neutrino is either zero, he wrote. “These single-process or, in any case, very small in com- experiments leave little doubt that a parison to the mass of the electron.” third particle must be involved.” Shortly thereafter, Hans Bethe and Gamow even speculated that neu- Rudolf Peierls used Fermi’s theory to trinos might be involved in the show that the interaction of neutri- lethargic disintegration of the re- nos with matter had to be essentially cently discovered pi-mesons and negligible. In the energy range char- their lighter counterparts, then acteristic of beta-decay neutrinos, known as mu-mesons. After all, they would have a mean free path in some kind of invisible entity was water of more than 1000 light years! spiriting energy away from these Bethe and Peierls concluded “there two-body and three-body decay is no practically possible way of processes. Why not the same elusive observing the neutrino.” Pauli himself particle involved in beta decays? was dismayed. “I have done a terri- But little could be said conclu- ble thing,” he said. “I have postulated sively about a particle that had thus a particle that cannot be detected.” far evaded direct detection. Whether

6 FALL 2001 its antiparticle was a completely dis- tinct entity or just a different spin state of the same poltergeist could not then be determined. And the best attempts at measuring its mass could only establish an upper limit of about one-twentieth the electron mass. As the 1950s began, however, this situ- ation was about to change dramati- cally. In 1951, following atomic-bomb testing at Eniwetok atoll, Los Alamos physicist Frederick Reines began con- templating experiments in funda- mental physics he might attempt. The Manhattan Project had provided intense new sources of neutrinos that could be used to ascertain more about them. Reines and Clyde Cowan recognized that the recently developed organic scintillating liq- uids would allow them to build the massive detector required to observe such ghostly particles. Together with the intense neutrino fluxes gener- ated by atomic blasts or close to a fis- sion reactor, such a large detector might finally overcome the daunt- ingly miniscule probability of a neu- trino interacting. Reines and Cowan elected to search for evidence of the interaction

ν + p → n + e+, the idea of placing a detector with- Frederick Reines at work on an under- which should yield a prompt light in 100 meters of an atomic-bomb ex- ground experiment in a South African flash in the organic scintillator due plosion, they decided instead to put mine in 1966. (Courtesy University of to the positron’s annihilation with it close to one of the nuclear reactors California, Irvine) an atomic electron, followed a few then in operation. Their first exper- microseconds later by another flash iment, placed near one of the Han- due to neutron capture. (Savvy read- ford Engineering Works reactors used ers will protest that antineutrinos, to breed plutonium for the Manhat- not neutrinos, participate in such an tan Project, involved a 300-liter tank “inverse beta-decay” interaction, but of liquid scintillator. But the mar- this distinction was not clear at the ginal increase in signal they observed time.) After considering and rejecting with the reactor in operation was

BEAM LINE 7 gamma rays (see illustration at left). A delayed coincidence between the first and second gamma-ray bursts was taken as the signature of an an- tineutrino event; Reines and Cowan observed three events per hour with the reactor operating—much greater than backgrounds due to cosmic rays or accidental coincidences. Elated by their discovery, Reines and Cowan sent Pauli a telegram on June 14, 1956: “We are happy to inform you that we have definitely detected neutrinos from fission frag- ments by observing inverse beta- Diagram of the antineutrino-detection nearly swamped by cosmic-ray back- decay of protons.” According to scheme used in the Savannah River grounds. Reines, Pauli drank a whole case of experiment. Reines and Cowan then did a sec- champagne with his friends to cel- ond experiment at the Savannah ebrate the discovery and drunken- River reactor, which could generate ly penned a reply. “Thanks for the a flux of 10 trillion antineutrinos per message,” he wrote; “Everything square centimeter per second at a comes to him who knows how to position 11 meters away. (By then it wait.” was becoming recognized that the neutrino and antineutrino are dis- EVIEWING the status of tinct particles, with the latter be- neutrino physics a year later, ing produced in tandem with an elec- RReines and Cowan could cite tron in beta-decay.) The detector, a variety of major improvements in positioned 12 meters underground to the understanding of this previously reject cosmic-ray backgrounds, con- invisible poltergeist. Delicate mea- sisted of three tanks of organic scin- surements of the electron spectrum tillator, each viewed by 110 photo- in tritium beta decays had estab- tubes; between them sat two tanks lished that the neutrino’s mass was of water with dissolved cadmium less than 1/2000th that of the elec- chloride to promote neutron capture tron. The lack of evidence for double (see illustration on next page). beta-decay (in which two electrons An antineutrino from the reac- are emitted) indicated that it was tor occasionally interacted with a most probably a Dirac particle like proton in the water, producing a the electron, with neutrino and an- positron and a neutron. The positron tineutrino distinctly different enti- annihilated almost immediately with ties. And the failure of Brookhaven an atomic electron, yielding two scientist Ray Davis to find any Cl37 gamma rays that were detected in the → Ar37 conversions in a tank con- scintillator; about 10 microseconds taining a thousand gallons of carbon later, neutron capture by a cadmium tetrachloride placed near the Savan- nucleus resulted in another burst of nah River reactor could also be

8 FALL 2001 explained in the same way: antineu- trinos could not induce such tran- sitions, while neutrinos should have. The most striking advance in understanding neutrinos had come the previous year in the wake of the earthshaking discovery of parity violation. Tsung-Dao Lee and Chen- Ning Yang (among others) proposed to rescue the deteriorating situation by invoking a peculiarity of neutri- nos. If they were indeed Dirac par- ticles with absolutely no mass, neu- trinos themselves would violate parity because their spin vectors would always be aligned along their direction of motion, while the spins of antineutrinos would only point the opposite way. We say neutrinos are “left-handed” and antineutrinos “right-handed.” “Since this new Artist’s conception of the detector used model for the neutrino does not obey MAJOR MYSTERY in by Reines and Cowan in their Savannah the simple parity principle, no re- the late 1950s was whether River experiment. Tanks I, II, and III con- action involving such a neutrino can Athe neutral particles emitted tained liquid scintillator viewed on each be expected to conserve parity,” in pion and muon decays were the end by 55 phototubes. Tanks A and B, wrote Reines and Cowan. same neutrino and antineutrino as containing 200 liters of water with dis- A further consequence of this observed in nuclear decays—or some- solved cadmium chloride for neutron hypothesis was that the probability thing else. Because the strengths of capture, served as the target volume. of reactor-produced antineutrinos these interactions were similar, and interacting with protons had to be because they also violated parity, it twice as large—an effect that Reines was widely assumed that the very and Cowan had already begun to same particles were involved. But observe. But the most convincing if this were the case, then muons proof came from a sensitive experi- should occasionally have been seen ment at Brookhaven led by Maurice decaying into an electron and a Goldhaber. He determined the spin photon (µ → e + γ). If so, the neutrino direction of the recoiling nucleus that and antineutrino generated in three- emerged after a europium-152 body decays of muons could oc- nucleus had captured one of its casionally annihilate each other, atomic electrons and emitted a neu- yielding a photon in addition to the trino. From this he concluded that departing electron. Theorists calcu- the neutrino is always left-handed, lated that such processes should just as Lee and Yang had suggested. occur about once in every 10,000 Thus the neutrino and antineutrino muon decays, but accurate mea- appeared to be distinctly different, surements indicated that nothing massless entities. like this occurred in many millions

BEAM LINE 9 Plan view of the two-neutrino experiment of events. One way to accommodate chamber from aluminum plates. If at Brookhaven National Laboratory. this apparent discrepancy was to say neutrinos produced in pion or kaon Pions and kaons produced by protons that two different kinds of neutrinos decays (e.g., π → µ + ν) were distinct hitting a beryllium target at far left were involved in muon decay. from those in beta-decays, they decayed, yielding neutrinos (and anti- Intrigued by these questions and expected to see only the long, pene- neutrinos) that traveled from left to right, penetrating the massive steel shielding the possibility of resolving them by trating tracks of muons generated by and striking the detector at the far right. making neutrino beams, Melvin neutrinos that interacted in the alu- Schwartz, Leon Lederman, Jack minum. However,Schwartz recalled, Steinberger and their colleagues be- “If there had been only one kind of gan planning an experiment at neutrino, there should have been as Brookhaven. Spurred by his discus- many electron-type as muon-type sions with T. D. Lee, Schwartz rec- events.” ognized that intense, high energy In the initial run of this experi- beams of protons soon to be available ment, which began in late 1961, they from its Alternating Gradient Syn- recorded 34 events in which there chrotron would allow them to gen- appeared a single muon track origi- erate neutrino beams with sufficient nating in the aluminum plates. There intensity (see illustration above). were another 22 events having a With energies ranging up to sev- muon and other particles, plus six eral billion electron volts, these neu- ambiguous events that might have trinos had interaction probabilities been interpreted as electrons. But more than a hundred times greater comparisons with actual electron than reactor-born neutrinos, but a events from a separate run showed large, massive detector was still little similarity. required to observe a sufficient num- Thus the neutrinos produced in ber of them. Schwartz and his col- tandem with muons in pion and leagues elected to build a 10 ton spark kaon decay are distinct from those

10 FALL 2001 produced together with electrons in nuclear beta decay. After Schwartz’s pivotal experiment, particle physi- cists began calling the former “muon neutrinos” (or νµ) to distinguish these particles from the latter, known as “electron neutrinos” (or νe). When- ever a positive muon decays, for example, it yields a positron, an elec- tron neutrino and a muon anti- + + – neutrino (µ → e + νe + νµ). By 1962 physicists recognized four distinct “leptons,” or light particles: the elec- tron, the muon, and their two respec- tive neutrinos (plus antiparticles).

HEORETICAL and exper- interactions, thereby yielding their One of the first neutral current events imental advances over the characteristic parity-violating prop- observed in the Gargamelle bubble ensuing decade resulted in a erty. chamber at CERN. A muon neutrino T traveling invisibly from left to right strikes revolutionary new picture of the sub- An inescapable requirement of an atomic electron, which makes the atomic realm that came to be known this unification of the electromag- gently arcing track in this photograph. as the Standard Model of particle netic and weak interactions is the (Courtesy CERN) physics. In this theory, leptons and existence of “neutral currents” that other particles called “quarks” are occur due to exchange of another regarded as elementary, point-like massive, but neutral, boson Z. entities. The electromagnetic and Instead of converting into a muon weak interactions, previously thought when it interacts with a nucleus via of as distinct forces with vastly dif- the exchange of a W boson, for fering strengths, are now considered example, a muon neutrino can in- to be just two different aspects of one stead glance away unaltered, re- and the same “electroweak” inter- maining a muon neutrino. action. The extreme feebleness of the After years of searching, neutral weak interaction arises because it oc- currents were discovered in 1973 by curs via the exchange of ponderous an international collaboration of spin-1 particles known as “gauge physicists working at the European bosons,” which are difficult to con- Center for Nuclear Research (CERN) jure up out of sheer nothingness. In on the Gargamelle bubble chamber. beta decay, for example, a neutron Filled with liquid freon, it was coughs up a massive, negatively exposed to beams of muon neutrinos charged W boson and transforms into and antineutrinos from CERN’s pro- a proton (n → p + W−); the W− imme- ton synchrotron. The initial evidence diately converts into an electron plus came from a few rare events in which − − – its antineutrino (W →e + νe). Only these spookinos rebounded elasti- the left-handed electrons and neu- cally from atomic electrons (for – – trinos (and their right-handed anti- example, νµ + e → νµ + e ), and particles) participate in these weak imparted energy to them. These

BEAM LINE 11 families were recognized by 1976: the first includes the up and down quarks plus the electron and electron neu- trino, while the second contains the charm and strange quarks plus the muon and muon neutrino. Particle physicists could now discern a highly satisfying symmetry among the ele- mentary entities in their new ontology.

HREE YEARS earlier, two Japanese theorists had sug- T gested that a third family of quarks and leptons might exist. Makoto Kobayashi and Toshihide Maskawa were seeking a way to in- corporate the mysterious phenom- enon of CP violation within the emerging structure of what was soon One of the first anomalous eµ events recognized as the Standard Model. observed after a “muon tower” had been electrons left wispy tracks in the Discovered in 1964 by James Cronin, added in 1975 to the SLAC-LBL detector chamber, whereas the neutrinos or Valentine Fitch, and their colleagues, at the SPEAR collider. The muon travels antineutrinos crept away undetected. this phenomenon indicated that—at upwards through the tower in this com- Subsequent confirmation came from least in certain decays of kaons— puter reconstruction of the detector’s Gargamelle and two experiments at Nature is asymmetric under the cross section, while the electron moves downward. Fermilab in which muon neutrinos combined operations of charge con- scattered inelastically from nuclei. jugation (C) and parity inversion (P). By the mid-1970s, it was clear that They observed that kaons behave neutrinos (and, in fact, all the other differently, that is, if one replaces par- leptons and quarks) were capable of ticles by antiparticles and views their a new kind of weak interaction in interactions in a mirror. Kobayashi which they maintained their iden- and Maskawa could not obtain this tity instead of transforming into a CP violation using only the two partner lepton (or quark). The dis- known families of quarks and lep- covery of these neutral currents, tons. But if they added a third family together with conclusive evidence to the mix, including two more for a fourth, or charm, quark c to quarks plus another charged lepton accompany the initial trio—up u, and its neutrino, they discovered that down d, and strange s—provided it arose naturally. strong support for the Standard Mod- At about the same time, Martin el. Quarks and leptons come in pairs: Perl and his colleagues were begin- u and d, e and νe; c and s, µ and νµ. ning their search for another heavy, What’s more, quark and lepton pairs charged lepton using the SLAC-LBL can be grouped into families of four, detector at the new electron-positron often called “generations.” Two such collider SPEAR. If such a heavy lepton

12 FALL 2001 Particle physicists

could now discern

a highly satisfying

λ existed, he reasoned, pairs of them symmetry among The issue was settled in 1989 should be produced in high-energy by terrestrial experiments at new electron-positron collisions: electron-positron colliders—the SLC the elementary at SLAC and LEP at CERN—that were e+ + e− → λ+ + λ−. capable of producing hordes of Z bosons. Within the structure of the Depending on their mass, such lep- entities in their Standard Model, additional kinds of tons could have a variety of decays, neutrinos give this particle additional two of which would be similar (by pathways to decay, thereby shorten- analogy) to the muon’s. By searching new ontology. ing its lifetime. By making precision for events in which one of these measurements of the Z resonance hypothetical motes decayed into an peak, physicists at the two facilities electron and the other into an concluded that there are three, and oppositely-charged muon (plus un- only three, kinds of “conventional” seen neutrinos and antineutrinos), light, weakly interacting neutrinos. they hoped to find evidence for the difficulties of making a sufficiently Confirming the cosmological pre- existence of a new heavy lepton— intense beam of tau neutrinos and dictions, these experiments put a and (again by analogy) a third detecting them unambiguously. firm lid on the complexity of the neutrino. Meanwhile, the two quarks expected Universe. Its table of fundamental By 1974 Perl’s group began to find in the third family—the bottom b entities could have only three such “anomalous eµ events” in the and top t quarks—were isolated at families of quarks and leptons (see data samples being collected on the Cornell and Fermilab, leaving only illustration below). SLAC-LBL detector, and by 1975 they the elusive tau neutrino remaining had dozens (see diagram on the left). to be discovered. So dominant had But convincing their colleagues that the Standard Model become that few these events were conclusive evi- particle physicists seriously ques- dence for a heavy lepton took longer. tioned its existence. In 1977, confirmation of the SLAC Cosmological arguments about results began to come in from the nucleosynthesis of light elements DORIS electron-positron collider in during the first few minutes of Hamburg. Further experiments on the Big Bang required that a third SPEAR and DORIS showed the mass neutrino—and perhaps even a of this tau lepton τ to be around 1.78 fourth—ought to exist. The abun- GeV and identified its decay into a dance of primordial helium-4 syn- − − pion and tau neutrino (τ → π + ντ). thesized during this process is By the following su m mer, there was determined by the expansion rate little doubt among particle physicists of the Universe at that time, which regarding the existence of the tau is sensitively related to the number lepton. of different kinds of light neutrinos. As measurments became more TILL, IT TOOK physicists accurate during the 1980s, primordial over two more decades to helium-4 was observed to contribute The quarks and leptons in the Standard Sachieve the direct detection of about a quarter of the visible mass in Model. There are three, and only three, a free tau neutrino, well separated the Universe, suggesting a third (and families of quarks and leptons. The last from its point of production. The a remotely possible fourth) kind of to be isolated, by the DONUT experiment problem occurred due to the neutrino. at Fermilab, was the tau neutrino,ντ.

BEAM LINE 13 Direct experimental evidence for another fundamental entity into the the tau neutrino came finally in 2000, minimalist ontology of his day. No seventy years after Pauli conceived doubt the enduring success of his his novel idea. By examining millions bold scheme has encouraged theo- of particle tracks left on special three- rists of later decades to repeat this dimensional photographic film, exercise whenever a cherished sym- physicists working on the DONUT metry or conservation law appears to experiment at Fermilab found four be violated. In this subtle way, the events that could only be interpreted ghost of Wolfgang Pauli still haunts as a tau neutrino colliding with a the way particle physics is practiced nucleus and producing a tau lepton. today. (For further details, see the article by Paul Nienaber, this issue.) The third and final neutrino had finally left subtle, indirect footprints that con- firmed its long-anticipated existence.

HE IDEA of the neutrino has therefore evolved sub- T stantially since its concep- tion in the early 1930s. Pauli’s ten- uous hypothesis was just the starting point for a lengthy process of theo- retical and experimental elaboration that continues today. Where he at first regarded neutrinos as nuclear consituents, Fermi showed how they can instead be created in nuclear For Further Information transformations. Where Pauli sought a minimal way to preserve the con- Good summaries of the history of servation of energy and angular neutrinos can be found at: momentum in individual beta decays, physicists have recently http://wwwlapp.in2p3.fr/ neutrinos/aneut.html established that at least two—and most likely all three—of the neutri- http://www.ps.uci.edu/~superk/ nos have a tiny bit of mass. And neutrino.html where he speculated that this mass might well be greater than an elec- On Wolfgang Pauli, see: tron’s, physicists now think it must http://www.physicstoday.com/pt/ be at least a million times smaller. vol-54/iss-2/p43.html But despite the great transforma- tions that have occurred in the idea http://www-groups.dcs. of the neutrino since 1930, Pauli st-andrews.ac.uk/~history/ deserves due credit for being the one Mathematicians/Pauli.html daring enough to take the great conceptual leap of introducing

14 FALL 2001 MINING SUNSHINE

by JOSHUA KLEIN

HE COMMUTE is only four kilometers, but it can take an hour each way. The problem is neither traffic The first Tnor mass-transit delays, but the fact that the trip takes you 6800 feet below the surface of the Earth. Much of the time is taken up by the required safety rituals: donning the reflective cov- results eralls and hardhat, lacing up the steel-toed work boots, buckling the headlamp battery into the safety belt and placing your name from the tag on the peg board. The schedule of a busy mine is driven by the need to move Sudbury equipment—not people—from level to level. You wait by the mine shaft until the “on-shift boss” decides it is convenient to bring Neutrino you down and calls for your levels: “Sixty-two to seven- thousand!—going down.” Both miners and physicists then climb into an open steel box (the “cage”), which is suspended by a cable Observatory as thick as a man’s leg. The cage comfortably fits twenty people; it usually holds closer to sixty. On a good day, you can place both reveal the feet on the floor. While the drop downward starts slowly, it eventually reaches a “missing” top speed of about 2200 feet/minute. Headlamps provide the only light, and you can just make out the rock wall of the shaft as it solar neutrinos. rushes upward past your face. Occasionally the darkness is broken by a brief glimpse of one of the shallower mining levels blinking by so fast that you barely register an image of a lighted tunnel receding away. By the 4000-foot level, your ears start to feel the additional pressure, and even the most seasoned miners snort and work their jaws trying to adjust.

BEAM LINE 15 depth are so great that the rock would quickly crumble. A stiff breeze blows at your back, ventilation that keeps the air throughout most of the mine near 70°F all year round—much cooler than the natural 100°F of the rock. In some pockets where the ventilation doesn’t reach, the air is as hot and humid as a summer day in Phila- delphia. The (quite literal) light at t he end of this tunnel shines on two plain- looking blue doorways—one for peo- ple and one for railcars. The accu- mulated dirt and mud on your boots m ust be washed off before you enter, and once inside, the contrast with the conditions in the drift could not be more striking: bright lights, dust- free air, spotless floors, and white painted walls suddenly make it hard to remember that you’re at the bot- tom of one of the world’s deepest mines. To go further inside, you need The SNO detector just before completion If they don’t stop to let off miners to clean off even more: the boots, of the bottom of the acrylic vessel. The on one of the shallower levels, the hardhat, and coveralls co me off, and acrylic used for this vessel is over 2.5 trip down to the 6800-foot level takes everyone must shower and change inches thick and built to hold 1000 tons less than four minutes. As you step into clean clothes and a hairnet. of heavy water. off the cage, a sign long-ago covered Computers, fax machines, coffee in grime declares that this is “The makers, telephones—they make it Home of the Sudbury Neutrino almost impossible to convince your- Observatory.” To the miners, it is self that there is a mile and a half just another level rich with nickel. of rock above your head! And there The commute is not yet over, is more: a water purification plant, a however, as there is still over a mile’s control room where shift operators worth of hiking left at this level. The monitor and alter the way data are dark and the dust and the mud are taken, and a darkened, domed area typical of any of the mine’s other tun- with rack upon rack of flickering nels (called “drifts”), as is the fact data-acquisition electronics. Only that the rock above your head—near- the fact that there are no windows— ly a mile and a half of it—is kept from no way to see the Sun from a mile collapsing only by a brute-force com- and a half underground—reminds bination of bolts 30-feet long, steel you of where you are. Which is per- screening, and concrete. Without haps ironic, given that the princi- that support, the pressures at this pal purpose of the Sudbury Neutrino

16 FALL 2001 Observatory (SNO) is to study sun- probe the details of the solar core. shine—not the sunshine visible to Yet, while Davis did prove that the the human eye, but sunshine made Sun emits neutrinos, his measure- of neutrinos, which travel to the bot- ments also produced a surprise: com- tom of the mine more easily than pared to the detailed calculations sunlight through a pane of glass. made by John Bahcall and others, he detected far fewer neutrinos than THE SOLAR NEUTRINO expected. The five experiments that PROBLEM came after his also saw similar deficits. The mystery of these “miss- The sunshine that flows through ing” neutrinos is known as the solar windows—and down to the bottom neutrino problem, the oldest puz- of mines—is born in a tremendous zle in experimental particle physics. nuclear reactor that rests at the cen- Essentially there are three possi- ter of the Sun. Sir Arthur Eddington ble ways in which neutrinos from the first suggested that the energy stored Sun might seem to be missing. First, Ray Davis, circa 1966. The photograph in atomic nuclei provides the Sun’s the measurements may simply be was taken in the Homestake gold mine power, pointing out in 1920 that this incorrect: the neutrinos are present, in South Dakota shortly before the first source was “well-nigh inex- but we have undercounted them. But solar neutrino experiment began opera- haustible”and “sufficient . . . to the different detectors and ap- tion. (Courtesy John Bahcall) maintain [an] output of heat for 15 proaches used by the various solar billion years.” (See John Bahcall’s neutrino experiments make a com- article in the Winter 2001 Beam Line, mon error between them very Vol. 31, No. 1.) But proving that the unlikely. Second, the theory of the Sun is tapping nuclei for energy is not Sun may be wrong in some way; easy—it requires the detection of perhaps the Sun’s interior properties some specific signature other than are different from what is expected, the light and heat that we see and feel and fewer neutrinos are being cre- (most of which is coming directly ated. The theory of the Sun correctly from the boiling surface of the Sun, predicts other solar characteristics, not its center). Fortunately for us, the such as the way in which the Sun’s neutrino is a common by-product surface vibrates in response to seis- of nuclear reactions, and therefore mic activity, and it is therefore observation of solar neutrinos on unlikely that we would grossly mis- Earth can prove Eddington’s hypothe- calculate the number of neutrinos. sis. We are left with a third possibility: It was not until 1967, however, that the problem is not due to our that Brookhaven National Labora- measurement of neutrinos or our tory scientist Ray Davis and his co- understanding of the Sun, but to workers constructed the first detec- something about the neutrino itself. tor to look for neutrinos coming from Davis’s experiment therefore pro- the Sun. Davis’s experiment was not vided much more than a new way to only expected to be a triumphant study the Sun: it raised questions confirmation of the theory but also about a part of fundamental particle to usher in a new era of solar physics physics we thought we already in which neutrinos would be used to understood. Our best theory of

BEAM LINE 17 elementary particles, the Standard Model, holds that there are three dif- ferent types (or flavors) of neutrinos—

electron neutrinos (written νe), muon neutrinos (νµ), and tau neutrinos (ντ) each closely associated with the cor- responding charged particle. The Sun can produce only electron neutrinos, and therefore the six solar-neutrino experiments prior to SNO have searched primarily for this one fla- vor. But if the Standard Model is wrong, and the three flavors of neu- trinos are not completely distinct but can change from one type into an- other, we may have an explanation for the solar neutrino problem. If the electron neutrinos born in the cen- ter of the Sun change into one of the other types on their way to Earth, then the earlier experiments will have recorded fewer neutrinos than expected; the changelings would sail right through the detectors without being noticed. The changing of neutrinos back and forth from one flavor to anoth- er is usually referred to as neutrino oscillations (see Maury Goodman’s article in the Spring 1998 Beam Line, Vol. 28, No. 1). Strong evidence that neutrinos can oscillate from one fla- vor to another was reported in 1998 by the Super-Kamiokande experi- ment in Japan, which observed muon neutrinos produced in the Earth’s up- per atmosphere by cosmic rays. What Super-Kamiokande observed was that the number of muon neutrinos measured depends on where the muon neutrinos were produced— above the detector, where they needed to travel only 10–100 km be- Cut-away view of the SNO detector, showing the acrylic vessel containing 1000 tons fore observation, or below it all the of heavy water at its core. way on the other side of the Earth, where they would have to travel

18 FALL 2001 thousands of kilometers to reach the tor and count the number of argon the neutrinos many opportunities for detector. If muon neutrinos can os- atoms that had accumulated, deduc- an accidental collision. Solar neu- cillate into another type of neutrino, ing from this evidence how many trino detectors therefore tend to be they would produce exactly the type solar neutrinos must have traversed very big: the Super-Kamiokande of up-down, distance-dependent dif- the detector. Other experiments that experiment, for example, holds over ference in the number of neutrinos used gallium rather than chlorine 50,000 tons of water. witnessed by Super-Kamiokande (see operated in a similar way: essentially There is one further difficulty in John Learned’s article in the Winter visiting the beach now and then just trying to detect neutrinos from the 1999 Beam Line, Vol. 29, No. 3). to see if any additional footprints had Sun. To return one last time to our For solar neutrinos, however, the been made. invisible man, imagine that the beach distance from the production point A different approach was taken by is crowded with swimmers and sun- (the Sun) to the detection point (the the Kamiokande experiment—and bathers each of whom also leaves Earth) does not vary enough to allow subsequently by Super-Kamiokande footprints. Trying to figure out which the kind of measurements made with —which was able to view the neu- footprints are meaningful (especially Super-Kamiokande’s atmospheric trino “footprints” as they were being given that the number made by the muon neutrinos. Rather than look made. In these experiments, the other beachgoers is far larger than for distance-dependent effects, the detectors were made primarily of those made by our invisible stroller) way to prove that the electron neu- water, but rather than changing the is an extremely difficult task. Cosmic trinos produced in the Sun are atoms of water into something else, rays are one example of this problem, oscillating into muon neutrinos or neutrinos scattered their electrons in since they can create Cerenkov light tau neutrinos is to search directly for the same way that one billiard ball in the same way neutrino interactions evidence of these neutrinos. SNO is collides with another. Each time this do. And we are being bombarded by designed to do just that: determine happened, the scattered electron pro- cosmic rays all the time: if each cos- whether or not solar neutrinos other duced a cone-shaped flash of blue mic ray were a raindrop hitting the than electron neutrinos are arriving light called Cerenkov light, which surface of the Earth, we would be from the Sun. If this were true, it was quickly converted into electri- soaked by a steady downpour would not only solve the long-stand- cal signals by photon detectors placed amounting to about one inch every ing solar neutrino problem, but also outside the water volume. few hours. The bottom of a mine is provide the most direct evidence yet The footprint metaphor however therefore an ideal place to study neu- that neutrinos do indeed oscillate. fails to highlight the most critical trinos because most cosmic rays are problem of neutrino detection. Neu- stopped by the rock before they reach THE SUDBURY NEUTRINO trino interactions with matter are so the detector, while neutrinos are un- OBSERVATORY weak that the chances of a footprint affected. To solar neutrinos, a kilo- occurring are incredibly small; a typ- meter of rock hardly matters at all, Like an invisible man on the ical neutrino can travel easily nor do the thousands of kilometers beach who can be “seen” only by through a light year’s worth of lead. below the detector through which tracking his footprints, neutrinos can Even then, every so often—essentially they travel upward during the night. be observed only by the traces they just by accident—one does collide The Sudbury Neutrino Observa- leave behind as they pass through a with something, such as a chlorine tory uses the same basic technique detector. We cannot see them atom in Davis’s experiment or an as the Kamiokande experiments— directly. In Davis’s experiment, these electron in the Super-Kamiokande looking for Cerenkov light produced traces came in the form of atoms of experiment. To increase the chances by neutrinos interacting in water. A the element argon, which were cre- of observing neutrinos, a detector total of 7000 tons of water sits in a ated when neutrinos collided with must therefore contain lots of cavern 22 m wide by 34 m high, atoms of chlorine. Every so often the material—lots of chlorine or lots of carved out of rock 2000 m beneath physicists would flush out the detec- electrons, for example—thus giving the surface of INCO Ltd.’s Creighton

BEAM LINE 19 Mine in Sudbury, Ontario (see illus- tration on page 18). At SNO’s depth— deeper than any other solar neutrino experiment—only three cosmic rays pass through the detector each hour. The Cerenkov light created by the neutrino interactions is detected by an array of 9500 photomultiplier tubes (PMTs), each capable of regis- tering a single photon of light. This sensitivity to single photons is nec- essary because a neutrino event typ- ically produces only 500 photons. The PMTs are supported by a stainless steel geodesic sphere 17.8 m in diameter. Nested inside the PMT support sphere is a second sphere 12 m in diameter, built from 5.5-cm thick transparent acrylic. This vessel and its contents are what makes SNO unique: rather than holding ordinary water, which has two hydrogen atoms bound to one oxygen atom

(H2O), the acrylic vessel holds 275,000 gallons (1000 metric tons) of heavy water, which has two deuterium atoms bound to one oxygen atom

(D2O). Deuterium is the same as hydrogen in all respects except that it has a proton and a neutron in its nucleus (called a deuteron). It is this extra neutron that allows SNO to dis- tinguish different types of neutrinos and thus determine whether there are neutrinos other than electron neu- trinos reaching Earth from the Sun. Cosmic rays are not the only source of false signals that SNO has been designed to avoid. Just about any material one can find on Earth— water, steel, mine dust—has a tiny amount of naturally occurring radio- Outside view of the SNO detector after construction was complete. Supported by a active contamination. These radio- geodesic sphere, 9500 photomultiplier tubes view the acrylic sphere within. active nuclei produce charged par- (Courtesy Lawrence Berkeley National Laboratory) ticles that can generate the same

20 FALL 2001 Cerenkov light as neutrino interac- second, only about five of them pro- p tions or cosmic rays. For this reason, duce a signal in the entire detector nearly everything used in the con- in any given day. For those few neu- e SNO d struction of the detector—even the trinos that do interact, ’s heavy νe p glass used in the photomultiplier water allows three possible scenar- n tubes—was designed specifically for ios. For one, an electron neutrino can SNO out of materials that are low be absorbed by a neutron inside a p in radioactivity. Locating the detec- deuteron; the neutron then becomes Absorption of a neutrino by a neutron in tor in an active mine adds an extra a proton and emits an electron (see a deuteron. The neutron changes into a challenge, since the dust and mud top diagram at right) which then pro- proton, emitting an energetic electron. have relatively high levels of radio- duces Cerenkov light that is detect- active contamination. Just a table- ed by the PMTs. This neutrino-ab- spoon of mine dust dropped into the sorption reaction is special because 275,000 gallons of heavy water would it only occurs for electron neutrinos. have enough radioactivity in it to SNO’s second reaction is the same ν e e mask the neutrino signals. electron-scattering interaction as wit- The entire underground lab is nessed by the Kamiokande and ν therefore operated as a clean room, Super-Kamiokande experiments. A the air is continuously filtered and solar neutrino collides with an Scattering of an electron by a neutrino. everything and every person enter- electron and kicks it out of its D2O ing it must be cleaned off to remove molecule. As with the neutrino- This process occurs most often with any telltale remnants of the outside absorption reaction, the scattered electron neutrinos, but can also occur for muon and tau neutrinos. mine. The heavy water in the detec- electron produces Cerenkov light tor is also purified, removing not just that is detected by the photomulti- dust but radon, a radioactive gas. And plier tubes. SNO can distinguish as a last line of defense, ordinary these electrons from those created in water fills the space outside the the neutrino-absorption reaction by acrylic vessel, shielding the heavy the fact that, like a billiard ball struck n water from radioactive signals orig- dead on by the cue ball, the scattered inating in the surrounding rock. This electron emerges close to the direc- H O shielding, the great depth, and tion of the incoming neutrino. There- d 2 n the insistence on ultrapure materi- fore knowledge of the electron’s di- ν ν p als as well as dust-free air and very rection and the Sun’s position when clean people, has made the center the event occurred are required to p of the SNO detector the point of low- identify it as an electron-scattering Breakup of a deuteron by a neutrino. est radioactivity on Earth. event. Although this reaction can This process happens with equal occur for either muon neutrinos or probability for any neutrino type. DETECTING NEUTRINOS tau neutrinos, it happens most often (6.5 times more often) for the elec- When a neutrino enters the SNO tron neutrinos. detector, several things can happen. In the third reaction, which is also By far the most likely is that it sensitive to muon and tau neutrinos, sails straight through unobserved; the neutrino breaks the deuteron up although about five million solar into a separate neutron and proton. neutrinos pass through each square The neutron itself cannot produce centimeter of the detector every Cerenkov light, but eventually it is

BEAM LINE 21 the Sun. For this to be true, neutrinos born as electron neutrinos in the solar center must be changing their flavor somewhere on their way to the Earth.

THE FIRST RESULTS

The SNO experiment therefore has two good ways of demonstrating the presence of muon and tau neutrinos coming from the Sun. The deuteron- breakup reaction has the advantage that it occurs for all neutrino flavors equally, and thus it directly measures the Sun’s total neutrino output. Because of this, we would expect to find a big difference between the number of neutrinos measured through deuteron breakup (all kinds) and the number measured through neutrino absorption (just electron neutrinos). Using the electron-scattering reac- 10/16/1999 14:31:22 Run: 5566 GTID: 375971 tion is not as easy. Because it occurs less than one sixth of the time for Computer reconstruction of a typical captured by one of the other deu- muon and tau neutrinos as for elec- solar neutrino event in the SNO detector. terons in the heavy water, producing tron neutrinos, the difference be- The filled-in dots correspond to photo- a gamma ray that scatters an electron tween the total number of interac- multiplier tubes that have detected in the water. This secondary electron tions measured with this reaction photons; the resulting cone of Cerenkov then produces Cerenkov light. In this and the number measured with the light produces a ring when projected deuteron-breakup reaction, all neu- neutrino-absorption reaction will not onto the sphere. trino flavors are on an equal footing— be very large unless most of the neu- a muon neutrino or a tau neutrino has trinos coming from the Sun have the same probability of interacting changed from electron neutrinos into this way as has an electron neutrino. muon or tau neutrinos. Using these three reactions, prov- But the electron-scattering reac- ing that neutrino oscillations tion does have one great advantage: explain the solar neutrino problem the Super-Kamiokande experiment becomes very simple: if the number has already used this reaction to of neutrinos measured with either observe neutrinos from the Sun. Over the electron-scattering reaction or several years, this experiment has ob- the deuteron-breakup reaction is served that the number of neutrinos greater than the number measured producing electron-scattering reac- with the neutrino-absorption reac- tions in the detector was 45 percent tion, their must be neutrinos other of what had been required by theory. than electron neutrinos coming from If the theory of the Sun is correct,

22 FALL 2001 then the missing 55 percent are muon long search begun over 30 years ago Gazing at sunshine from the or tau neutrinos that the electron- by Ray Davis. The additional neutri- bottom of Creighton mine, the scattering reaction has failed to nos that Super-Kamiokande observes Sudbury Neutrino Observatory has detect. What is needed, therefore, is with the electron-scattering reaction already learned two important a separate determination of the flux are the first confirmation that the things: that neutrinos born in the of electron neutrinos alone; once this theory of the Sun is indeed correct! Sun do change into other kinds on is known, we can conclude that any Recall that the electron-scattering their journey toward Earth, and that additional flux is due to these other reaction only detects 1/6.5 of the this change is responsible for the neu- kinds of neutrinos. muon and tau neutrino flavors, so the trino deficit first noticed by Ray SNO’s first order of business was 10 percent extra flux that Super- Davis. He started out trying to learn thus to use the neutrino-absorption Kamiokande sees must mean that something about the Sun, but what reaction to measure the number of actually about 65 percent of neu- he and the rest of us discovered was electron neutrinos and to compare trinos coming from the Sun are that we first needed to learn some- this to Super-Kamiokande’s mea- muon or tau neutrinos. Adding this thing more about the neutrinos surement of the number of neutrinos 65 percent to the 35 percent worth of themselves. And while there are still observed with the electron-scatter- electron neutrinos which SNO mea- many ways yet in which we will use ing reaction. In 241 days of taking sures gives us 100 percent of the the Sun to help us understand neu- data, SNO detected 1169 neutrino expected total. After 30 years, we find trinos, we can now begin anew the events, about 950 of which were that there really aren’t any neutrinos project that was postponed after determined to have been due to “missing” after all. They were just Davis’s first observations—and start neutrino-absorption reactions. The more difficult to detect! Our theory using neutrinos to understand the theory of the Sun predicted that there of the Sun therefore appears to be Sun itself. should have been over 2700 electron more correct than our fundamental neutrinos detected this way (assum- theory of m atter. ing no oscillations) during this time—in other words, SNO only THE FUTURE observed 35 percent of the expected number. The fact that Super- There is still much left for SNO to do. Kamiokande measures 45 percent of In particular, measurements with the the expected number using a reac- deuteron-breakup reaction will tion that can detect all neutrino provide a much more accurate flavors, while SNO measures just 35 determination of the total number of percent of the expected number using neutrinos coming from the Sun. a reaction that detects only electron Comparing the number of neutrinos neutrinos, can mean just one thing: measured with the deuteron-breakup there are other kinds of neutrinos reaction to those that are measured coming from the Sun. with the neutrino-absorption reac- For the first time, therefore, there tion will provide a much better is clear evidence that neutrinos from determination of the degree to which For Further Information the Sun can change from one type neutrinos oscillate. In addition to into another. The Standard Model, this, SNO should be able to look for To learn more about the Sudbury which predicts that the three neutri- changes over time in the Sun’s out- Neutrino Observatory, go to: no flavors are separate and distinct, put of neutrinos, observe their must be altered to accommodate this energies, and measure other char- http://owl.phy.queensu.ca effect. But there is an added bonus, acteristics that can tell us about one that starts to bring to an end the properties of the solar core.

BEAM LINE 23 Neutrino Experiments at by PAUL NIENABER

ARTICLE PHYSICS EXPERIMENTS often get lumped under the general heading of “big science.” Particle accelerators are gargantuan, Neutrino physics detectors are huge and complex, and today’s multi-institutional collaborations generate is thriving P social dynamics whose unraveling would baffle the most sophisticated supercomputer. Neutrino in the experiments are no exception to this observation; the detectors come in three sizes: large, extra-large, and Nation’s positively Bunyanesque. Yet, like the graceful hippopotami in Walt Disney’s Fantasia, a “little” neutrino experiment can on occasion dance briskly in, do some beautiful tour- heartland. de-force physics, and take a bow—all for a relatively modest ticket price. Two premier danseurs of this sort on the Fermilab stage are the DONUT and BooNE experiments. DONUT (Direct Observation of the Nu-Tau) has completed its run and is now accepting bravos for being the first experiment to detect the tau neutrino. BooNE (Booster Neutrino Experiment) is waiting in the wings, warming up to delve more deeply into the peculiar pas de deux of neutrino oscillations. According to the Standard Model, our current best working theory of elementary particles and fundamental forces, a neutrino can interact and change into its electrically charged lepton partner by emitting or absorbing the charged carrier of the weak force, the W boson. This type of exchange is called a “charged-current” interaction (to distinguish it

24 FALL 2001 Fermilab

from the exchange of a neutral Z boson). The weak force also obeys a two- fold lepton-number conservation rule: both the number of leptons and their kind remains unchanged. Interacting electron neutrinos yield only electrons, muon neutrinos only muons, tau neutrinos only taus. Neutrinos and their charged partners can switch back and forth within a Standard Model genera- tion, but they cannot jump between generations. This conservation rule makes the charged-current interaction a useful “generation tagger”: if you see a muon zipping through your neutrino detector, and you can ascertain that it came directly from a neutrino interaction, you can be sure it was produced by a muon neutrino. Because neutrinos are immune to the strong and electromagnetic interactions, they provide a unique “weak force scalpel” for investigating the properties of particles. Electron neutrinos are difficult to concentrate into a beam whose direction and energy can be varied, so muon neutrinos have be- come the scalpel of choice. The recipe is fairly straightforward: take a proton beam, smack it into a block of material, magnetically focus and direct the charged pions and kaons spewing forth from the collision, allow them to de- cay into muons and muon neutrinos, filter out the charged decay products and any other unwanted debris, and voila! You have a beam of muon neutrinos. But because neutrinos interact so weakly, the chances of any single one interacting in a detector are minuscule. The probability of witnessing an interaction depends on the number of neutrinos, their energy, and the number of other particles available for them to hit. Getting a reasonable number of events therefore requires lots of neutrinos, lots of energy, and lots and lots of detector material—hence the Brobdingnagian scale of neutrino experiments.

BEAM LINE 25 Outgoing ITH THE DISC OVERY which interact almost immediately Lepton of the top quark at Fer- in detector material and produce a Incoming milab in 1995, the set of distinctive shower of particles. Muon Neutrino Charged W Weak Boson six fundamental constituents of the neutrinos engender muons, which Target strongly interacting particles in the travel relatively undisturbed through Standard Model was complete. The matter before decaying and thus lepton six-pack, however, still had leave long tracks in detectors. Tau one gaping hole—the tau neutrino re- neutrinos make taus—and there’s the mained the most elusive of its stand- rub. The tau lifetime is less than one A neutrino charged-current interaction. offish cousins. Experiments at SLAC third of a trillionth of a second; mul- and CERN had determined that there tiply this brief instant by the speed were at most three conventional, of light and you get a decay length lightweight neutrinos, and there was literally the size of a gnat’s whisker, indirect evidence from the missing about 90 microns. If a tau is ex- momentum observed in decays of the tremely relativistic, it may travel a tau lepton, but no definitive sight- few millimeters before decaying, but ings (see “Pauli’s Ghost,” by Michael tracing these microscopic paths to Riordan, this issue). discover any kinks in them demands Understanding why the tau neu- extremely high-precision tracking. trino is so camera shy takes us back This requirement, coupled with the to the charged-current interaction. small likelihood of tau neutrinos ever DONUT detector for direct observation of Electron neutrinos make electrons, interacting, makes detecting them tau neutrinos. an extraordinary challenge. To rise to that challenge came the DONUT experiment, which ran at the Fer mi- Muon Detector lab Tevatron in 1997. There are two hurdles to over- Calorimeter come if you want to bag a tau neu- trino: first you gotta make ‘em, and Drift Chambers then you gotta take ‘em. To make tau neutrinos, you employ a variation on the muon-neutrino recipe, but in- Magnet stead of using pions and kaons as the parents, you must employ much Emulsion Target heavier mesons. The lepton-number conservation rule says you can only make a tau or its neutrino in tandem Steel Shield with an antilepton of the third gen- eration. To get tau neutrinos, there- fore, you have to produce a parent 1 5 m e t e particle that includes a tau (whose r s mass is a whopping 1777 MeV, almost twice the mass of a proton) in its Neutrino decay chain. DONUT’s tau neutrinos Beam come from the decay of DS mesons, produced by slamming the Tevatron’s

26 FALL 2001 800 GeV protons into a tungsten tar- get. These mesons are also heavy (1970 MeV) and short-lived, so no attempt to focus them is made. The aim of the experiment is to look for charged-current scattering of the resulting tau neutrinos. This process will make final-state taus, but how does one corral these evanescent heavyweights? The key to this experiment, the factor that makes it both beautiful and rather difficult, is the use of a large target composed of photographic emulsion plates. These plates are just standard black-and-white film medium, silver bromide, suspended in a gel. Instead of a thin layer of this substance coat- ing a 35 mm piece of plastic film, however, DONUT’s plates contain the bulk emulsion deposited on square sheets about the size of a large pizza box (50 cm square). Charged particles traversing the emulsion DONUT was also in the unusual One of the four tau neutrino events leave tracks of exposed grains, and position of having to deal with neu- recorded by the DONUT detector. The finding a vertex (where the tau neu- trino backgrounds, since the proton track with a kink is a tau lepton decaying trino interacted) that spawns a interactions in the tungsten produce into an invisible tau neutrino and another particle. kinked track (where the resulting tau plenty of electron and muon neutri- lepton subsequently decayed) pro- nos, too. These extra neutrinos can vides the “smoking gun,” evidence interact in the emulsion as well and for a tau neutrino. render the stack awash in tracks. Isolating such a vertex with a To help pick out a few specific track that kinks a scant few milli- bent needles from this huge haystack meters downstream of it is no easy of tracks, the experimenters posi- feat. The emulsion stack is only 36 tioned a series of scintillating-fiber meters downstream of the tungsten tracking detectors among the emul- target, which means that extensive sion plates. These trackers, together shielding must be used to keep the with other detectors downstream of emulsion from being swamped by the emulsion array, helped identify superfluous tracks. The usual pas- the particles and trace out their sive shielding (which filters out paths. The vertex and track recon- unwanted particles by letting them struction enabled a tiny, precisely interact in material) is not enough; selected region of the emulsion to be DONUT’s emulsion stack needed ac- pinpointed for closer scanning and tive elements (magnetic fields to measurement. The actual digitiza- divert charged detritus) as well. tion and scanning of the emulsion,

BEAM LINE 27 section by tiny section, was done at direct sightings of the tau neutrino, a unique facility in Nagoya, Japan, these events confirmed at last that it one of the few of its kind in the indeed exists. world. In July 2000, after data reduc- tion and a careful analysis to screen TABOUT THE TIME that out backgrounds, the DONUT team the DONUT collaboration showed the world four events with a Awas looking for telltale distinctive kink in one track, indi- kinks, evidence was accumulating cating a tau lepton decay. The first from other quarters that the Standard Model might not be quite right. Sev- eral experiments had seen hints that two fundamental features thought to hold for neutrinos—the expectation Neutrino Oscillations that they are massless and that they cannot change from one kind to another—were in fact incorrect. ERE’SAGOODWAY to think about neutrino oscillations. Sup- Rather loose upper bounds had been pose you could only hear a single pitch (or frequency) of sound at a set on the neutrino masses by care- Htime: That is, your “sound detector” could only tell that a note was a G, fully adding up all the visible energy or a B, or a D; it could not detect a mixture of these notes. If a note (you hear) in certain particle decays that pro- starts out as a G (or a B or D for that matter), it would stay that way forever. duce neutrinos. And the prohibition This is the way that charged leptons behave. on leptons jumping from one gen- Neutrinos play by different rules, however, and admit the possibility that a eration to another had been probed note originating as a G (an electron by searches for exotic processes such neutrino, say) can “detune” as it trav- as µ → eγ. els, developing B or D components One way to establish more strin- (a muon or tau neutrino). This kind of gent limits is to examine the curious 1 behavior is analogous to that of the possibility of neutrino oscillations strings in a guitar, which—once G (see box at left). Understanding y t

plucked—can excite lesser vibrations i l

i how such oscillations might oc- b

on the other strings. a

b All G Mostly G cur requires us to step into the o

Does this mean that one would r No B Some B

P sometimes-counterintuitive world hear chords in neutrino beams? No— of quantum mechanics. According to remember, you can detect only one B the Standard Model, neutrinos are “pitch” at a time. What it does mean, massless and conserve lepton num- however, is that if you create a pure 0 Distance Traveled ber. But if this is not in fact the case, beam of 1000 Gs, say, and set up a then neutrinos can have different listening post some distance away, Oscillation probability: even though the masses, and a neutrino created as one you might get 990 Gs and 10 Bs. neutrino beam starts out as 100 per- kind (say an electron neutrino, What fluctuates in neutrino oscilla- cent G’s, as it moves through space, produced in the nuclear reactions tions is the probability that a neutrino the beam can gain (and lose, and then occurring in the Sun’s core) could created as one specific kind will be regain) some B’s. What oscillates is the evolve, or oscillate, into another kind detected as that same kind (or probability. as it zooms along through space. another kind) of neutrino. Quantum mechanics tells us that the characteristics of any neutrino oscillation—its amplitude and repeat

28 FALL 2001 length—are governed by four factors. Two of them are experiment-specific: the neutrino energy E and the source- to-detector distance L. The other two are intrinsic properties of how the neutrinos fluctuate back and forth from one kind to another: the oscil- lation strength itself (the amplitude of the wave) and the absolute differ- ence between the squares of their 2 2 2 two masses ∆m = |m1 − m2|. One of the current problems in neutrino-os- cillation physics has to do with this last parameter, ∆m2. Since there are three known kinds of neutrino, there can only be two independent differ- ences between their mass states. The three classes of experiments cur- rently yielding positive indications of neutrino oscillations—those studying neutrinos from the Sun, those observing neutrinos from muons produced in Earth’s atmos- phere, and two accelerator-based experiments—do not yield a coher- of about 1–2 eV2. A result of no small Some of the 1200 photomultiplier tubes ent set of ∆m2 values (see article by importance, it cries out for indepen- on the inside of the BooNE detector. Boris Kayser, this issue). This dis- dent confirmation. Enter the second crepancy could point to a new, fourth of Fermilab’s lissome leviathans, the kind of neutrino (called a “sterile” Booster Neutrino Experiment. neutrino because it cannot interact BooNE’s main raison d’etre is to with ordinary matter). Or it could test the LSND result. It will either imply that one (or more) of these provide confirming evidence (and get experiments is not really seeing enough events to establish the νµ to oscillations but another effect. νe transformation once and for all) or The one accelerator-based exper- prove LSND wrong—and set more iment that reported a neutrino- stringent limits on this particular oscillation signal is the Liquid Scin- type of oscillation. There are some tillator Neutrino Detector (LSND), surface similarities between LSND which detected neutrinos from pions and BooNE: both use large tanks filled produced by a medium-energy pro- with mineral oil and lined with ton beam at the Los Alamos National photomultiplier tubes. But the detec- Laboratory. The collaboration ob- tor geometries are different (LSND is served an excess of events above cylindrical, while BooNE is spheri- background that can be explained by cal), the source-to-detector distance the oscillation of muon neutrinos L and beam energy E are different into electron neutrinos, with a ∆m2 (although the ratio L/E for the two

BEAM LINE 29 experiments is the same), and the The BooNE detector is a 12 m backgrounds for BooNE will be dif- diameter steel sphere filled with 770 ferent from those of LSND. tons (about 250,000 gallons) of min- The BooNE experiment will fol- eral oil (see illustration on this page). low the standard recipe for making a A second light-tight inner wall muon neutrino beam, starting with divides the tank into an active cen- 8 GeV protons from the Fermilab tral sphere and an outer “veto” shell Booster, a low-energy synchrotron in used to flag exiting or extraneous in- the Tevatron acceleration chain used coming particles. Charged particles to boost protons from 400 MeV to 8 produced by neutrino interactions in GeV before passing them on to the the oil are detected by means of the Main Injector. Protons extracted blue Cerenkov light they generate, from the Booster will strike a beryl- which will be detected by 1280 photo- lium target; pions generated by these multiplier tubes lining the inner sur- collisions will be focused by a mag- face of the central sphere or the 330 Artist’s conception of the BooNE netic horn, and their decay will pro- tubes in the veto shell. Again, the detector, showing some of the photomul- duce the muon neutrinos speeding charged-current interaction will tiplier tubes lining its 250,000-gallon tank. off toward the detector. allow physicists to identify the kind of neutrino involved; a long muon

track (from the interaction of a νµ Signal Region in the mineral oil) will generate a pat- tern of Cerenkov light distinct from that produced by an electron shower

Veto Region (from a νe interaction). As this article goes to press, con- struction of the BooNE detector is nearing completion, and the proton- extraction beam line and target-horn systems are well underway. Data taking is expected to commence in early 2002, after which the experi- ment should run for one to two years. If a signal is observed, a second detector can be built downstream of the first—to make further detailed measurements of the parameters for

νµ → νe oscillations. Such a result would add to the growing body of evidence that neutrinos do indeed have mass, and point to the excit- ing possibility of new physics beyond the Standard Model.

EUTRINO PHYSICS has long been a featured N performer in Fermilab’s

30 FALL 2001 multifaceted particle physics pro- Peering still further into the gram. DONUT has made a major con- future, one can glimpse the in- For Further Information tribution, and the upcoming BooNE triguing possibility of a “neutrino DONUT home page: experiment bids fair to continue that factory”—a storage ring for muons http://www-donut.fnal.gov history. Future neutrino-beam op- whose decay would provide a copi- portunities might include upgrading ous source of muon and electron neu- BooNE home page: http://www-boone.fnal.gov/ the Booster to allow it to provide trinos. Such a high-energy source of even greater fluxes of neutrinos. electron neutrinos would open whole Juha Peltoniemi’s “ultimate There are also the exciting prospects new vistas for further exploration neutrino” page: http://cupp.oulu.fi/neutrino/ of “long-baseline” neutrino experi- of the weak interaction. But what- ments where the distance between ever scenarios the future contains, the source and detector is hundreds neutrinos will surely play leading Members of the MINOS collaboration be- of kilometers rather than hundreds roles in them. fore one of its enormous steel plates. of meters. One such Bunyanesque experi- ment stretches all the way from Fer- milab to the Soudan mine in north- ern Minnesota, where the detector for the MINOS (Main Injector Neu- trino Oscillation Search) experiment will be located. The MINOS beam starts with 120 GeV protons from the Main Injector, which feeds the Fer mi- lab Tevatron. The neutrino beam pro- duced by these protons hitting a tar- get will be directed north and slightly downward, to intercept a six-kiloton steel-and-scintillator detector in an iron mine some 750 km away— making MINOS one of the largest neutrino experiments on the planet. When this football field-length detec- tor is complete, it will capture neu- trinos beamed from Fermilab and further probe the mysteries of neu- trino oscillations. The combination of neutrino energies and the long baseline between source and detec- tor make MINOS sensitive to values of ∆m 2 a hundred times smaller than BooNE. This is the same range of values that has been observed for oscillations of neutrinos in the up- per atmosphere (see article by John Learned in the Winter 1999 Beam Line, Vol. 29, No. 3).

BEAM LINE 31 The Enigmatic World of by BORIS KAYSER

Trying EUTRINOS ARE AMONG THE MOSTABUNDANT elementary particles in the Universe. They are a billion times to N more abundant than nucleons or electrons. If we would like to understand the Universe, we must therefore understand neutrinos. Learn- discern ing about the world of neutrinos is not easy, however, because their inter- action with other matter is extremely feeble. In the last few years, however, dramatic progress has been made. An the experiment has now confirmed (see article by Paul Nienaber, this issue) that neutrinos come in three different varieties, or “flavors”: νe , νµ, and ντ. These three neutrino flavors correspond to the three charged leptons: patterns the electron e, its heavier cousin the muon µ, and its still heavier cousin called the tau lepton τ. Neutrinos of different flavor are distinctly different objects, but it has been convincingly demonstrated that a neutrino can of spontaneously change its flavor! This metamorphosis is known as neutrino flavor oscillation, or simply neutrino oscillation. The evidence neutrino that neutrinos oscillate between flavors is particularly compelling in the case of atmospheric neutrinos—neutrinos made in the Earth’s atmosphere by cosmic rays. In 2001, the evidence also became rather strong in the case masses of solar neutrinos—neutrinos made in the Sun by nuclear processes (see article by Joshua Klein, this issue). Any neutrino oscillation necessarily implies that neutrinos have mass. and Thus, even though neutrino masses are extremely tiny, the observation that neutrinos oscillate tells us that these masses are not zero. These mixing. neutrino masses could have very interesting consequences for both particle physics and cosmology. How big are the neutrino masses? Unfortunately, oscillation experi- ments cannot tell us, because oscillation depends only on the differences— or “splittings”—between the squared masses of different neutrinos. Of course,

32 FALL 2001 Neutrinos

these splittings can teach us about the pattern of neutrino masses but not about the absolute scale of this pattern. To discover this scale, we will have to carry out very different kinds of experiments. While finding experimental evidence for neutrino masses has been a challenge, many theoretical physicists have strongly suspected that these masses do indeed exist. Neutrinos are close relatives of the other con- stituents of matter—the quarks and the charged leptons—and interact with them via the weak nuclear force. Moreover, the quarks, charged leptons, and neutrinos have very similar weak interactions. The very successful theory of the elementary particle interactions known as the Standard Model relates the quarks, charged leptons, and neutrinos to one another by placing them into families. Each family contains two quarks, or else one neutrino and one charged lepton, and the weak force can turn one member of any family into the other. With these hints that neutrinos are related to the charged leptons and quarks, and the observation that all of the charged leptons and all of the quarks have nonzero masses, one expects that the neutrinos probably have nonzero masses as well. The masses of the charged leptons and quarks are incorporated, although not explained, in the Standard Model—which may also be able to include neutrino masses. But because the neutrino masses are very tiny, any such inclusion would have to involve some extremely small numbers that are not terribly likely to be ingredients of a genuine physical explanation. Thus, the complete explanation of neutrino masses will almost certainly take us to new physics beyond the Standard Model. Such physics is almost universally believed to exist because of the many questions this theory leaves open. How- ever, this new physics has proved remarkably elusive. By opening a window on the new physics, neutrino masses could become very interesting indeed.

BEAM LINE 33 Owing to their great abundance, Furthermore, when a neutrino the neutrinos might well play a sig- interacts with matter and gives birth nificant role in shaping the evolution to a charged lepton, that lepton and destiny of the Universe (see always has the same flavor as the article by Joel Primack, this issue). neutrino. A νe always begets an e, a Exactly what role they play depends νµ a µ, and a ντ a τ. This is how we on their masses, and on how big know that νe, νµ and ντ must be dis- these masses are. tinctly different objects. How is it possible for neutrinos to oscillate between different flavors, HE MOST convincing sin- and why does this phenomenon im- gle piece of evidence for neu- ply that neutrinos have mass? To T trino oscillation in Nature help explain the physics of neutrino comes from the behavior of the oscillation (see box on next page), let atmospheric neutrinos. These neu- me first sharpen the concept of neu- trinos have been detected by several trino flavor. By the neutrino ν of fla- underground detectors and studied vor (= e, µ, or τ), physicists mean most extensively by the Super- the neutrino produced together with Kamiokande detector in a zinc mine a particular charged lepton in any in Japan. Atmospheric neutrinos process where there was no neutrino studied by an underground detector Atmospheric neutrinos incident on an or charged lepton to begin with; one can originate in the atmosphere underground detector (labeled D). The neutrino plus one charged lepton are directly above the detector, on the neutrinos originate from all around the world. Upward-going neutrinos (colored created. For example, the νµ is the opposite side of the Earth, or above lines) have much more time to oscillate neutrino produced together with a any other point on Earth (see illus- + + than downward-going ones (black lines). muon in the pion decay π → µ + νµ. tration on the left). By detecting muons created in the Super- Kamiokande detector by incoming atmospheric muon neutrinos, the Super-Kamiokande collaboration has determined how the flux of them depends on the direction from which D they are coming. The upward-going flux, coming up from all directions below the horizon at the location of the detector, is only about half the corresponding downward-going flux. Of course, the upward-going neutri- Neutrino Made Earth nos originated in distant parts of the in the Atmosphere atmosphere, while the downward- going ones started much closer and traveled a much smaller distance. The flux of cosmic rays energetic enough to produce atmospheric neu- trinos with energies above a few bil- Atmosphere lion electron volts is known to be isotropic. The atmospheric muon

34 FALL 2001 Neutrino Mass and Mixing

IFNEUTRINOS HAVE MASSES, then the neutrino, ν , produced together with a specific charged lepton, , need not be a particle with a definite mass. Instead, it can be a quantum-mechanical superposition of such particles. This perplexing sleight-of-hand is called neutrino mixing. Let us call the neutrinos that do have definite masses—that is, the neutrinos in this energy range are neutrino “mass eigenstates”—ν1, ν2 and ν3, and so on. Then ν can be a therefore being produced at the same superposition of these eigenstates. Quantum mechanically, the ν at its rate everywhere around the Earth. moment of birth is described by the wave labeled “At Birth” in the figure Now one can easily formulate a below, where we have made the simplifying assumption that ν has only two significant components of definite mass, ν and ν . Each of these neutrino-flux version of the Gauss’s 1 2 mass-eigenstate components is represented by a traveling wave. When Law familiar to any student of elec- ν is born, the peaks of the waves coincide, and the two waves then add tricity and magnetism. From this law up to a resultant wave with the and the equality of neutrino produc- properties of ν . If ν1 and ν2 have tion rates around the Earth, it follows different masses M1 and M2 and that as long as atmospheric muon our neutrino has a certain average ν2 ν2 neutrinos neither appear nor disap- momentum, then its ν1 and ν2 ν ν pear within the Earth, the upward pieces will travel at different 1 1 and downward ν fluxes studied by speeds. Consequently, after the µ neutrino has traveled awhile, the At Birth Later Super-Kamiokande should be equal. peak of the wave corresponding A propagating neutrino wave consisting Thus, independent of any theoreti- to the lighter mass-eigenstate of ν1 and ν2 pieces. The vertical line cal assumptions, the observed in- component (ν say) will have 1 passes through the peak of the ν2 wave. equality of these two fluxes implies forged ahead of the peak of the When the neutrino is born (At Birth), this that something is adding or taking wave corresponding to the line coincides with the peak of the ν1 away muon neutrinos within the heavier component. wave, but at a later time (Later) it does Earth. The dominant candidate for Now, at any given time, the not. the culprit is neutrino oscillation. wave representing our neutrino is Since the upward neutrinos come the sum (obtained using the usual rules for adding waves characterized by both amplitudes and phases) of the ν and ν contributions. Clearly, from more distant parts of the at- 1 2 this sum is not the same “Later” as it was “At Birth.” After the neutrino mosphere than the downward ones has traveled for awhile, it therefore cannot be a pure ν anymore. If one and consequently have more time to were to analyze this neutrino in terms of its flavor content, one would oscillate, the upward muon neutri- quickly find that it has developed components bearing flavors other than nos can oscillate away into neutri- its original flavor. Thus, if the neutrino interacts in a detector and pro- nos of another flavor, while the duces a charged lepton, there is some probability that this charged lep- downward ones simply do not get the ton is of another flavor. For example, our neutrino can be born together chance. This mechanism would with a muon, so that it starts life as a νµ, but after developing compo- explain why fewer muon neutrinos nents with other flavors, it can interact to produce a τ. We describe this sequence of events by saying that our ν had turned into a ν . This is are observed going upward than µ τ how a neutrino can spontaneously change flavor. downward. From upper limits on the The possibility of this flavor change depends intrinsically on having oscillation of electron neutrinos gen- neutrino mass eigenstates with unequal masses, which in turn requires erated by nuclear reactors, we know that at least one of these masses be nonzero. Thus, neutrino oscillation that the oscillating atmospheric νµ implies neutrino mass. A calculation shows that when only two neutrino are not (or at least not appreciably) mass eigenstates ν1 and ν2 play a significant role, the probability for a 2 2 turning into ν . Thus they must be neutrino to change flavor is proportional to sin [(δM 21/4)(L/E)]. Here, e 2 2 2 δM 21 = M 2 − M 1 is the splitting between the squared masses of the con- turning into ντ—or perhaps into some new kind of neutrino not seen before. tributing mass eigenstates, L the distance traveled by the neutrino be- tween its birth and detection, and E its energy. Note that the probability High-precision experiments at CERN for neutrino oscillation really does oscillate, as the name implies. This and SLAC have taught us that only probability depends only on the splitting between the neutrino squared three species of neutrinos—νe, νµ and masses, and not on the individual masses. This remains true when more ντ—interact with matter through the than two mass eigenstates (hence more than one splitting) are involved. weak nuclear force. Consequently, any new, fourth kind of neutrino

BEAM LINE 35 would be even more of a ghost, since from which the neutrinos are com- it would not interact with matter ing—extremely well. The Super- through any known force save grav- Kamiokande results are supported by ity. Such a neutrino is called a “ster- those of the MACRO experiment in ile” neutrino, νs. the Gran Sasso underground labo- The hypothesis that the atmos- ratory in Italy, and by those of the The Super-Kamiokande detector during pheric muon neutrinos are oscillat- Soudan-2 experiment in the Soudan filling in 1996. Physicists in a rubber raft are polishing the 20-inch photomultipli- ing into tau neutrinos fits a host of mine in Minnesota. The neutrino ers as the water slowly rises. (Courtesy data—including the detailed depen- squared-mass splitting δM2 required ICRR, University of Tokyo) dence of the νµ flux on the direction by the data is approximately 0.003 eV2. This value allows neutrinos with typical atmospheric neutrino energies to oscillate significantly if they travel upward to the detector from the other side of the Earth, but not if they travel downward from just above. The neutrino mixing required by the atmospheric oscillation data is

quite large. That is, a νµ is a super- position of mass eigenstates in which no single one of them dominates.

Rather, the νµ is an almost 50–50 mixture of two mass eigenstates. And while it is still possible that

some of the atmospheric νµ are os- cillating into sterile neutrinos, Super- Kamiokande analyses tell us that no more than 25 percent of them are doing so. Oscillation into tau neu- trinos is the favored path. Compelling as the evidence for atmospheric muon neutrino oscilla- tion is, one would like to confirm this observation by showing that muon neutrinos produced by a par- ticle accelerator also undergo the same oscillation. Several experi- ments aim to do just that, and one of them, K2K, has already reported pre- liminary results (see article by Koichiro Nishikawa and Jeffrey Wilkes, this issue). These results are not yet conclusive, but they do seem to indicate that muon neutrinos are oscillating into something else.

36 FALL 2001 N INDEPENDENT strong This past June, the Sudbury Neu- piece of evidence for oscil- trino Observatory (SNO) reported a Alation is the behavior of so- new result that, when compared to The heart of the solar neutrino detector lar neutrinos, which are generated in an earlier Super-Kamiokande result, operated by Brookhaven/University of huge numbers by the nuclear implies that ν or ν are indeed ar- Pennsylvania chemist Raymond Davis µ τ and his collaborators. The vessel con- reactions that power the Sun. At riving here from the Sun! The SNO tains about 615 tons of perchlorethylene birth, all of these neutrinos are elec- collaboration has succeeded in mea- and is located almost a mile under- tron neutrinos, since they are cre- suring the solar νe flux arriving at ground in the Homestake Gold Mine in ated in association with electrons (or, Earth, independently of whether any Lead, South Dakota. (Courtesy more precisely, positrons). Solar neu- other flavors of neutrinos might be Brookhaven National Laboratory) trinos have been detected here on Earth by several underground detec- tors, all of which are sensitive largely—or exclusively—to electron neutrinos. Interestingly, all these detectors report solar neutrino fluxes that are only 30 to 60 percent of those expected from the theoretical calcu- lations of neutrino production in the Sun. Valiant attempts to modify the theory of the Sun, without invok- ing neutrino mass or oscillation, have been notably unsuccessful in explain- ing this deficit. Hence, it has been hypothesized that the deficit is caused by the oscillation of electron neutrinos into another, hard-to-see flavor. Perhaps this oscillation is occurring in outer space between the Sun and the Earth. Or perhaps a matter-enhanced neutrino oscilla- tion, known as the Mikheyev- Smirnov-Wolfenstein (MSW) effect after the theorists who first explored its physics, is occurring within the Sun itself as the neutrinos produced by the solar core stream outward. Ei- ther way (if we neglect the possibil- ity of sterile neutrinos), νµ or ντ (or both) are eventually produced. To confirm that the solar neutrinos are undergoing oscillation, one would therefore like to verify that νµ or ντ, which are not being forged in the so- lar furnace, are nevertheless arriving here on Earth from the Sun.

BEAM LINE 37 smaller than what is called for by the atmospheric-neutrino oscillation. This means that the solar oscillation does not involve the same two neu- trino mass eigenstates. To a certain extent, the solar-neutrino data sug- gest that the MSW effect is indeed involved, that δM2 falls in the range 10−5 eV2 to 10−4 eV2, and that the neu- trino mixing that is playing a role is large—like its analogue in the atmospheric-neutrino oscillation. The MSW effect, like neutrino oscillation in empty space, requires neutrino mass. Thus, whether or not

the solar νe are turning into νµ or ντ through the MSW effect, the obser- vation that they are also undergo- ing a metamorphosis is another pow- erful piece of evidence that neutrinos indeed have masses.

HEREISA THIRD piece of evidence that neutrinos The LSND experiment was designed to arriving as well (see article by Joshua T oscillate, but it is uncon- search for the presence of electron anti- Klein, this issue). In contrast, the ear- firmed. This evidence comes from neutrinos. Over 1200 photomultiplier lier Super-Kamiokande result (from the Liquid Scintillator Neutrino tubes line the inner surface of the the same detector that studied the Detector (LSND) experiment at Los mineral oil tank, shown above with LSND Alamos, which has recorded the physicist Richard Bolton of Los Alamos. atmospheric neutrinos) measured (Courtesy Los Alamos National this same νe flux plus an admixture appearance of electron antineutrinos Laboratory) of one-sixth of the νµ and ντ fluxes in a neutrino beam produced via the arriving from the direction of the decays of positively charged muons. Sun. By subtracting from this total As these decays do not yield electron

the νe flux measured by SNO, physi- antineutrinos directly, their appear- cists can extract the νµ+ ντ flux. They ance has been interpreted as result- find that, at a strong—albeit not yet ing from the oscillation of the muon completely conclusive—level of con- antineutrinos that µ+ decays do fidence, the νµ + ντ flux arriving at create. Earth from the Sun is not zero. This The squared-mass splitting de- is an important result. manded by the reported LSND oscil- The neutrino squared-mass split- lation is around 1 eV2, much larger ting δM2 required by the solar-neu- than what is called for by the trino data falls somewhere between atmospheric and solar oscillations, 10−11 eV2 and 10−4 eV2, depending on which gives rise to an interesting sit- whether or not the MSW effect is uation. Suppose there are only three involved. Either way, the splitting is different flavors of neutrino: νe, νµ,

38 FALL 2001 and ν . Then there are only three cor- flavors? Equivalently, how many dif- τ ν responding neutrino mass eigen- ferent neutrinos of definite mass— 3 states: ν1, ν2, and ν3. To accom mo- that is, mass eigenstates—are there? date the solar and atmospheric • What are the masses of these oscillations, the neutrino squared- eigenstates? From neutrino oscilla- 2 2 (Mass) δMatm mass spectrum must be as pictured tion experiments, we already know in the illustration on the right. There something about the squared-mass must be a closely-spaced pair of neu- splittings between the different ν 2 2 δM trinos we shall call ν1 and ν2, whose eigenstates, but what are their indi- ν1 solar 2 splitting δM solaris the small one vidual masses? called for by the solar data. This pair • Is each neutrino mass eigen- must be separated from the third state identical to its antiparticle, as A three-neutrino (Mass)2 spectrum that can accommodate both the solar and eigenstate, ν3, by the bigger splitting is a photon, or distinct from it like 2 atmospheric neutrino oscillations. An δMatm required by the atmospheric an electron? 2 2 equally possible spectrum places the data. Since both δM solar and δMatm Alone among all the spin-1/2 con- 2 closely-spaced pair of neutrinos at the are much smaller than 1 eV , this stituents of matter—the quarks and top rather than at the bottom. three-neutrino spectrum cannot leptons—neutrinos may be their own include any pair of neutrinos whose antiparticles. No quark or charged splitting is the large 1 eV2 demanded lepton can be identical to its anti- by LSND. No three-neutrino spec- particle because it is electrically trum that fits both the solar and charged, so its antiparticle must be atmospheric oscillations can also fit oppositely charged. However, a neutral the LSND oscillation. If the LSND lepton—that is to say, a neutrino— oscillation is confirmed by future is electrically neutral and as far as we experiments, then Nature must con- know does not carry any other tain at least four different kinds of charge-like attribute either. Thus, a neutrinos. As Nature contains only neutrino could well be identical to three kinds of neutrinos that inter- its antiparticle. act with matter via the weak nuclear An important question is whether force, any additional kinds would the properties of neutrinos are related have to be of the almost totally non- in a deep way to the fact that we interacting sterile variety. Thus, it is exist. That existence depends on the very interesting, to say the least, to fact that the Universe contains find out whether the oscillation re- nucleons and electrons—the parti- ported by LSND is genuine. cles of which we are made—but very few of the corresponding antiparti- OW THAT physicists cles. That is, the Universe currently have ascertained that neu- contains far more matter than anti- N trinos almost certainly matter. Since the Big Bang presum- have nonzero masses, there are some ably produced equal amounts of mat- major questions we would like to see ter and antimatter, the present answered: asymmetry must have arisen after • How many different neutrino the Big Bang. But for this to have

flavors are there in all? Just νe, νµ and occurred, matter particles and their ντ? These three plus one sterile fla- corresponding antiparticles must vor? These three plus many sterile behave differently, at least under

BEAM LINE 39 Thanks to the

dramatic discoveries

of the past few years,

we now know that

certain conditions. Otherwise, a experiment known as KARMEN, there is a rich world matter-antimatter symmetrical Uni- carried out at the Rutherford Labo- verse would have remained that way. ratory in England, does not see any One possibility is that there was a of massive neutrinos such oscillation. However, the two time in the early Universe when experiments have different sensitiv- electrons were produced more waiting to be explored, ities and are not necessarily in con- copiously than positrons by the flict. To find out whether or not the decays of some very heavy parent LSND oscillation signal is genuine, particles. Subsequently, the electrons but we have only begun an experiment known as BooNE, participated in processes that created presently under construction at nucleons but not antinucleons. to explore it. Fermilab, will be carried out (see If indeed there was an early epoch article by Paul Nienaber, this issue). in which electrons were produced The splittings δM2 among the dif- more abundantly than positrons, ferent neutrino eigenstates will be then neutrinos may exhibit related determined through a broad program behavior. For example, it may be that The most popular explanation of of neutrino-oscillation studies. the probability for an antineutrino to the fact that the neutrinos are so Together, these splittings will tell us oscillate is different from that for the much lighter than the charged lep- the M2 spectrum, but not its base- corresponding neutrino. If so, this tons and quarks is called the “see- line. Determining this baseline—that would be a clear difference between saw mechanism.” This mechanism is, the absolute scale of neutrino the behavior of antimatter and that can apply only to the neutrinos—it mass—will not be easy. One ap- of matter. Any such difference would cannot be the physics behind the proach will be to study very care- be extremely interesting to observe. masses of quarks or charged leptons. fully the high-energy end of the elec- Essentially this same difference The see-saw mechanism requires tron energy spectrum in tritium beta 3 3 − 3 can still occur even if a neutrino and that the neutrinos are their own anti- decays: H → He + e + νm. Here, H its antineutrino are the same particle. particles. It predicts that a typical is the nucleus of a tritium atom (one If so, an interacting neutrino whose neutrino will have a mass of order proton plus two neutrons), 3He is the 2 spin is antiparallel to its momentum Mquark/MBig. Here Mquark is a typical nucleus of a heliu m-3 atom (two pro- − − will make a charged lepton (e , µ , or quark (or charged lepton) mass, and tons plus one neutron), and νm − τ ), while one whose spin is parallel MBig is a very large mass far beyond (m = 1,2,3. . .), is any one of the neu- to its momentum will make an the reach of present-day accelerators. trino mass eigenstates. Through the antilepton (e+,µ+, or τ+). The oscillation The new physics responsible for the kinematics of these decays, the

patterns of the lepton- and antilepton- neutrino masses and other phenom- masses of the neutrinos νm will be producing neutrinos can differ. If they ena supposedly resides at the high, reflected in the energy spectrum of do, we will once again have a distinc- presently-inaccessible mass scale rep- the electron. However, this effect

tion between the behavior of matter resented by MBig. Obviously, the big- will be experimentally invisible and that of antimatter. ger MBig is, the smaller the neutrino unless at least one νm has a mass big 2 The biggest question of all about mass Mquark/MBig will be. Hence, the enough to be within range of the the neutrinos is: What is the under- name “see-saw mechanism.” mass sensitivity of the experiment,

lying physics responsible for the neu- and this particular νm is emitted in a trino masses, mixings, and other OW CAN WE answer all substantial fraction of all the tritium properties? And is this physics also these neutrino questions? decays. responsible for the masses of the H The evidence that there are A tritium experiment sensitive to charged leptons and quarks, or can it more than three neutrinos is the neutrino masses as small as 0.3 eV is only be glimpsed by studying the neutrino-oscillation signal observed now in the planning stages. Known neutrinos? by the LSND experiment. A similar as KATRIN, it will be performed in

40 FALL 2001 Karlsruhe, Germany. If the LSND observation of this process would searches for neutrinoless double beta neutrino oscillation is genuine, then yield information about this scale. decay and astrophysical studies of the the LSND data tell us that there are The proposed searches should be sen- neutrinos that, eons ago, helped shape two definite-mass neutrinos, νm and sitive to a mass scale an order of the Universe that we see today. If the νn, separated from one another by magnitude smaller than the planned recen t past is any guide, this future ex- 2 2 2 2 δMmn = Mm – Mn > 0.2 eV . It follows tritium experiment can reach. ploration of the enigmatic world of that Mm, the mass of νm, is no To probe the possible connection neutrinos will be very exciting indeed. smaller than 0.4 eV, which is within between neutrinos and our existence, range of the experiment. we will need very intense neutrino It is amusing that the search for beams produced by accelerators. the absolute scale of neutrino mass These beams will probably have to brings us back to take a very detailed be “bright” enough to remain rela- look at the energy spectra in nuclear tively intense even after traveling beta decay. It was the observation several thousand kilometers to a dis- that these spectra are continuous, tant detector. Then it will be possi- rather than discrete, that originally ble to use them to see whether neu- led Pauli to propose the existence trino oscillations depend on whether of neutrinos! the neutrinos in the beam are of the To test whether the electron neu- kind that produce particles of mat- trino is identical to its antiparticle, ter or of the kind that produce par- we can search for neutrinoless dou- ticles of antimatter. ble beta-decay—in which one nu- cleus decays into another with the SFOR the big question— emission of two electrons, and noth- what’s behind it all—who ing else. Since there are no electrons Acan say how we will answer before the decay, and two electrons that? The answer may include in- after it, such a process clearly does gredients presently being considered, not conserve the hypothetical, and it may well include physics that charge-like “lepton number” that nobody has yet dreamed of. would distinguish charged and neu- Thanks to the dramatic discover- − tral leptons like e and νe on the one ies of the past few years, we now know hand from charged and neutral an- that there is a rich world of massive + − tileptons like e and νe on the other neutrinos waiting to be explored, but hand. Indeed, the occurrence of this we have only begun to explore it. − decay would indicate that νe = νe, Major, fundamental questions about so that there is no conserved lepton the neutrinos need to be answered. To number to distinguish leptons from attempt to answer them, a diverse their antiparticles. Several groups experimental program will occur in around the world are hoping to carry the coming years. This program will out sensitive searches for this po- include experiments on accelerator- tentially very informative decay. In generated neutrinos that travel to both principle, these searches could yield near and far detectors, and others on a bonus: The rate for neutrinoless neutrinos produced by tritium decay, double beta-decay depends, albeit in nuclear reactions in power plants, cos- a nontrivial way, on the absolute mic rays, the Sun, and perhaps super- scale of neutrino mass. Thus the nova explosions. It will also entail

BEAM LINE 41 The K2K Neutrino E by KOICHIRO NISHIKAWA & JEFFREY WILKES

HREE YEARS AGO, it was already evident that our standard picture of neutrinos needed work. The world’s first T Unexpected shortfalls in the fluxes of solar and atmospheric neutrinos were not about to go away; these deficits had primed many physicists to accept the clear evi- long-baseline dence for neutrino mass that the Super-Kamiokande Collabo- ration presented in 1998. But the Super-Kamiokande results indicated only that at least one kind of neutrino indeed has neutrino experiment mass. Many other questions remain unanswered. What is the complete pattern of neutrino masses? How are the three is beginning known kinds of neutrinos connected to the mass eigen- states? Do we really need a fourth neutrino, which would have to be “sterile”? And how does the new information fit to produce results. within the framework of the Standard Model, which has worked so well for us for so long? The next steps are to confirm and extend the Super- Kamiokande results and then try to map out, at least in out- line, the pattern of neutrino masses. Groups in the United States, Europe and Japan are now working to develop the neutrino beams and detectors needed for this next generation of neutrino experiments, in which beams of well-known composition are directed over a distance of hundreds of kilo- meters from an accelerator to a remote neutrino detector. One group, however, has a big head start on the others. Since 1999, the K2K (KEK to Kamioka) experiment, the world’s first long-baseline neutrino experiment to begin operations, has slowly but surely been logging data.

42 FALL 2001 Experiment

PREHISTORY OF THE EXPERIMENT

In the early 1980s, underground water Cerenkov detectors were built in Japan and the United States by Masatoshi Koshiba, Frederick Reines, and others to search for nucleon decay. A few thousand tons of water contained the requi- site nucleons, and the water itself also served as the track-sensitive medium, generating Cerenkov light from relativistic particles. To reduce cosmic ray backgrounds to manageable levels, the detectors had to be located deep within the earth in underground mines. Kamiokande (Kamioka Nucleon Decay Experiment), built in the Kamioka-Mozumi mine in the Japanese Alps, was essentially a 10 m diameter by 10 m tall cylindrical tank of ultra- pure water, viewed by hundreds of large photomultiplier tubes (PMTs). The IMB (Irvine-Michigan-Brookhaven) experiment, built in the Morton Salt mine under Lake Erie, near Cleveland, had a similar arrangement, with a cubic tank of water about 10 m on a side. Neither experiment found convinc- ing evidence for nucleon decay, but they were fortunately sensitive to neutrinos from extraterrestrial sources. Thus nucleon-decay seekers became astrophysical neutrino observers, and found plenty to write home about. Previous solar neutrino observations, con- ducted by Ray Davis and collaborators since 1967 at the Homestake gold mine in South Dakota, had reported a significant deficit compared to theoretical predictions. The Kamiokande and IMB experiments confirmed this result and added a new puzzle to the picture. While the Homestake experiment was insensitive to high-energy neutrinos, the two water Cerenkov detectors could easily observe neutrinos created in the upper atmosphere by cosmic rays. But the experimental observations of these “atmospheric” neutrinos fell well short of theoretical expectations, adding another deficit to the growing list of neutrino puzzles.

BEAM LINE 43 It became clear that a new, much in April 1996, two separate analysis larger detector could be of great value groups began to reduce the data and in solving both the solar and provide independent checks upon atmospheric neutrino puzzles. Thus one another. One group took as its Yoji Totsuka and collaborators began starting point the analysis methods planning the next-generation detec- and software from IMB, and the other tor called Super-Kamiokande, even as from Kamiokande. Such careful pro- the original experiments continued cedures helped ensure the prompt collecting data. Super-Kamiokande acceptance of Super-Kamiokande was designed on a truly gargantuan results by the physics community. scale, with a 40 m diameter, 40 m tall By June 1998, collaboration mem- cylindrical tank holding 50,000 tons bers had enough data and confidence of water that is monitored by over in their analyses to present a truly 11,000 giant phototubes, each 50 cm historic result at the Neutrino-98 in diameter (see photo on page 36 and Conference, held in nearby Taka- drawing below). To construct and yama, Japan: the first convincing operate such a huge detector required evidence for neutrino oscillations, pooling resources, so members of which emerged from their atmos- both the Kamiokande and IMB groups pheric neutrino data (see John joined forces. Learned’s article in the Winter 1999 The two groups each brought their issue of Beam Line, Vol. 29, No. 3). own independent ideas and ap- Distributions of neutrino flux versus Artist’s conception of the Super- proaches to the experiment. When incidence angle showed a clear deficit Kamiokande detector. Super-Kamiokande began taking data (relative to expected fluxes) for up- ward-going muon neutrinos coming from the other side of the Earth, while downward-going muon neu- trinos (and all electron neutrino dis- tributions) agreed with theoretical predictions. The difference could not be explained by the fact that upward- going neutrinos had to pass through a lot more matter, since the Earth is essentially transparent to these neutrinos. But the deficit could be explained by neutrino oscillations, since the upward-going neutrinos must have trekked 10,000 km or so from their point of birth, while down- ward going neutrinos traveled only 15 or 20 km. In fact, the hypothesis that particles produced as muon neu- trinos oscillate into tau neutrinos fits the data very well, since the Super- Kamiokande detector cannot effi- ciently identify tau neutrinos—

44 FALL 2001 leading to an apparent deficit of muon neutrinos. Such oscillations can occur only if at least one of these neutrinos has mass. Neutrino Hall

HISTORY OF K2K

The idea of directing a neutrino beam toward a distant detector had long been discussed by Koshiba, Reines, Al Mann, and others. Until recently, Proton Beam Decay however, the high cost of construct- Pipe ing a far detector as well as a new, downward-directed beam line at any Target Hall suitable accelerator was prohibitive. But Super-Kamiokande could serve as a perfect detector for a neutrino beam from KEK, the Japanese na- tional particle-physics laboratory at Tsukuba, 150 miles from Tokyo. The 12 GeV proton synchrotron at KEK could produce a neutrino beam with beam toward Super-Kamiokande (see Aerial view of the KEK neutrino beam the energy range required to test the photo above). line. The extracted 12-GeV proton beam oscillation parameters suggested by With remarkable rapidity, plans enters at left, goes through a 90-degree bend and then enters the target hall. the early Kamiokande results; a rel- for the K2K experiment were ap- Neutrinos from the 200 m decay pipe atively modest 1 degree downward proved in 1995. It was organized as pass through the neutrino hall at right angle was required for this beam. a scientific collaboration distinct before heading toward Super- And many components of past ex- from Super-Kamiokande, although Kamiokande. periments could be recycled to build there is substantial overlap in mem- a near detector on the KEK site, to bership. Civil construction of the sample the outgoing neutrino beam. neutrino beam line and experimental In particular, a 1000-ton water tank hall began in 1996, with an extremely was already available, as well as a tight project timeline. This was muon detector system (iron plates especially true when KEK was a and proportional tubes) and a lead- prime contender to be the future site glass detector. The only major equip- of the Japan Hadron Facility (now ment needed was a scintillating-fiber officially named the High Intensity tracking detector plus new photo- Proton Accelerator Facility), which multiplier tubes for the water tank. would have required upgrading the The KEK synchrotron was steadily proton synchrotron from 12 to 50 upgraded, and a significant increase GeV and left K2K only two years of in its luminosity was achieved. Bend- running time. The decision to site ing magnets on loan from SLAC were the facility at Tokai relieved much used to bend the primary proton of this pressure—but only after the beam about 90 degrees into the orien- construction and commissioning tation needed to direct the neutrino phases were completed.

BEAM LINE 45 designed. Thus K2K may take longer to reach its experimental goals. The SciFi Tracker shakedown process was completed Lead Muon by April 1999, and data-taking runs Chamber Glass Array began in earnest. Since then, K2K has 1 kt Water Cerenkov been operating for about six months µ Detector a year, as the accelerator is shut down during the summer months and Neutrino Beam other experiments take beam in the To Super-Kamiokande autumn. p

THE K2K BEAM LINE

K2K uses a fairly conventional wide- band neutrino beam design. The

Fine-Grained primary beam is composed of 12 GeV Detector (kinetic energy) protons from the KEK proton synchrotron. Every 2.2 sec- onds, approximately 6 trillion pro- Schematic view of the K2K near Civil construction was rapidly tons are extracted and focused onto detectors, showing (right to left) the completed, and experimental areas a 66 cm long aluminum target. A 1 kiloton water Cerenkov detector, the became available to scientists by the double-horn magnet system focuses SciFi tracker, lead-glass array, and the summer of 1998. All beam line and positive pions produced by p-Al muon detector. target area components were quickly collisions, while sweeping negative installed and tested. Meanwhile, the secondary particles out of the beam experimenters took a few months to line. Between the horn system and install the detector elements in the the 200 m long decay volume (where + + new Neutrino Hall, a cylindrical pit the π decay to νµ and µ ) the pion 24 m in diameter located about 300 m monitor—a gas-Cerenkov detector of downstream of the production tar- novel design—is periodically insert- get. Filling the tank and water puri- ed in the beam to measure the pion fication began in December 1998, kinematics before they decay. With with commissioning of other detec- these kinematic distributions tor elements completed during the known, it is possible to predict the next few months. neutrino spectrum at any distance Tuning of the neutrino beam from the source. began in early 1999, followed by a Following the decay volume, an series of engineering runs to test the iron-and-concrete beam dump filters beam and near detectors together. out essentially all charged particles The 20 millimeter diameter alu- except muons with energy greater minum target used to produce neu- than 5.5 GeV. Downstream of the trinos failed mechanically under the dump there is a muon monitor sys- severe thermal stresses and was tem, which measures the residual replaced by a 30 millimeter target, muons to check beam centering and which meant that the neutrino in- muon yield. Steering and monitoring tensity is a bit lower than originally of the neutrino beam are carried out

46 FALL 2001 based on data from this system. The near detector provide independent + νµ and µ in the decay volume orig- checks on the event rates, as well inate from the same pion decays, so as supplementary data on inter- View from ground level into the Neutrino the measured central position of the actions and rates. Hall at KEK, showing near detector components: 1 kiloton detector (at top muon profile is well correlated with Following the 1 kt detector, the in this view); SciFi tracker; lead-glass the neutrino beam direction. The scintillating-fiber detector (SciFi) array; and muon detector (bottom). beam line was aligned by an engi- neering survey using the Global Positioning System (GPS) to deter- mine the line from the target to the Super Kamiokande far detector to better than 0.01 mrad; the construc- tion precision for the beam align- ment is better than 0.1 mrad, which is much better than required.

THE NEAR DETECTOR

The near detector array (see illust- tration on opposite page and photo- graph at right) is located in an under- ground experimental hall about 300 m from the pion production target, after an earth berm about 70 m thick that eliminates virtually all prompt beam products other than neutrinos. The one kiloton (1 kt) water Cerenkov detector uses the same techniques and analysis algorithms as the far detector, with 680 20-inch diameter hemispherical PMTs lining a cylindrical volume 8.6 m in diam- eter and 8.6 m high. Because this detector has the same target mater- ial, PMTs, and basic geometry as Super-Kamiokande and uses the same event identification and analy- sis methods, many potential biases and systematic errors cancel between the two detectors. Thus the 1 kt detector is the primary source of data on neutrino event rates at the near site. A beam simulation based on these data is then used to estimate the expected νµ signal at the far detector. Other components of the

BEAM LINE 47 103 provides detailed tracking capability, allowing for the 250 km flight path. and allows identification of different Duplicate GPS clock systems are 101 types of interactions such as quasi- used at the near and far sites to gen-

s elastic or inelastic scattering. It con- erate time data with 20 nanosecond t

n –500 –250 0 250 500

e sists of 19 planes of water target precision. Beam-induced neutrino v E 8 tanks, interleaved with scintillating interactions at Super-Kamiokande fiber modules. Downstream of the are selected by comparing these two 4 SciFi detector is the lead-glass array, independent time measurements,

0 for tagging electromagnetic showers. TKEK for the starting time of the KEK –4 –2 0 2 4 The muon detector then measures beam spill, and T for the Super- ∆T (µs) SK the energy, angle, and production Kamiokande event-trigger times. The

Top plot: ∆T distribution over a ±500 µsec point of muons from charged-current time difference ∆T = TSK − TKEK − window for events remaining a) after neutrino interactions. The charac- TOF, where TOF is the time of flight

requiring that the total collected charge teristics and stability of the νµ beam at the speed of light between the near in a 300 nsec time window is equivalent are directly monitored at the near site and far detectors, should be distrib- to more than 20 MeV deposited energy by this detector, whose large trans- uted within an interval of 1.1 µsec, (hatched area); b) after all cuts except verse area and mass make it possible matching the duration of the pro- timing (white bars). Bottom plot: ∆T to measure the beam direction, sta- ton beam spill at KEK. Using a con- distribution over a ±5 µsec window for events that originate in the fiducial bility, and intensity profile over a rea- servative 200 nanosecond estimate volume of Super-Kamiokande. sonable angular range. The neutrino of the synchronization accuracy beam direction has been stable between the two sites, we establish within ±1 mrad throughout all data- a 1.5 µsec long time interval inside taking periods. which candidate neutrino interac- tions must occur. All neutrino events PRESENT RESULTS occurring within the time window are carefully examined. Cuts and The basic strategy in a long base- analysis procedures similar to those line experiment is to sample the used in atmospheric neutrino analy- beam at a near detector, thus deter- ses in Super-Kamiokande are applied mining its characteristics before any to select events that are fully con- oscillation effects can occur. Then tained within the same 22.5 kiloton one uses these observations to deter- fiducial volume. The fiducial-volume mine the expected beam characteris- and timing cuts have a powerful tics at the far detector in the absence effect in separating beam-induced of oscillations. We use a beam sim- events from cosmic-ray and other ulation to make this extrapolation; background events (see graph on this it is validated by comparing its pre- page). dictions with the pion monitor mea- Up until April 2001, 44 fully- surements in the beam line, before contained beam-induced neutrino the decay volume. The agreement be- events were observed in Super- tween the simulation and the pion Kamiokande. This number should be monitor data has been excellent. compared with the number of events Candidate events observed in expected in the absence of any neu- Super-Kamiokande must be syn- trino oscillations, which is 60–68 chronized with the arrival of protons events based on the number of neu- at the production target at KEK, trino interactions observed in the

48 FALL 2001 1 kt near detector. The expected Super-Kamiokande as the far detec- numbers of Super-Kamiokande tor, this time about 300 km distant For Further Information events based on analyses of interac- from the production target. Narrow- tions in the other near detectors are band neutrino beams will be used, To learn more about the K2K and consistent with this value. These with mean energy below 1 GeV— Super-Kamiokande experiments, results so far are consistent with the an energy range well matched to go to: oscillations of muon neutrinos into the detection efficiency of Super- some other kind of neutrino, as Kamiokande. Official K2K home page: reported by the Super-Kamiokande With such a neutrino beam, we http://neutrino.kek.jp/ experiment, but more events are should be able to achieve the fol- K2K FAQ (for the layman): needed before the K2K experiment lowing goals: http://www.phys.washington.edu/ can claim to have reached definitive • Observations of muon neutrino ~superk/k2k/k2k_faq.html conclusions to this question. disappearance will allow the para- Super-Kamiokande official home 2 page: meter ∆m µx for νµ → νx oscillations http://www-sk.icrr.u-tokyo.ac.jp/ FUTURE PLANS to be determined with an accuracy and SK-USA home page: of 10−4 eV2 in one year; a 1 percent http://www.phys.washington.edu/ It will take another two years to measurement of the corresponding ~superk/ achieve the original K2K goal, which mixing parameter can be obtained in was to provide a statistically signif- five years. Maury Goodman’s Neutrino Oscil- lation Industry page: icant test of neutrino oscillations • Although recent results from http://www.hep. anl.gov/ndk/hyper- in a long-baseline experiment. It Super-Kamiokande and SNO make text/nuindustry.html appears likely that the experiment the existence of sterile neutrinos Next Generation Long-baseline will have time to accumulate at most seem unlikely, observations of Neutrino Experiment (JHFnu): a few hundred events in Super- neutral-current neutrino interactions http://neutrino.kek.jp/jhfnu/ Kamiokande before the KEK syn- will allow improved limits to be set chrotron is shut down in 2005. This on possible sterile neutrino contri- number is sufficient to provide butions. improved confidence limits on oscil- • The possibility of neutrino decay lation parameters, which will give can be definitely tested by searching valuable guidance for future long- for a clear dip in the νµ disappearance baseline experiments. Meanwhile, rate as a function of energy. KEK and the Japan Atomic Energy Measurement of CP violation in Research Institute will be jointly the lepton sector may even be fea- building a very high-intensity proton sible, but it will require a much larger accelerator in Tokai, scheduled for far detector, with mass of about a completion in 2006. After K2K ends, megaton. There are no fundamen- our physics goals will move from tal engineering showstoppers, and exploratory to precision measure- the design of a megaton detector ments of the two sets of ∆m 2 and (dubbed “Hyper-Kamiokande”) is mixing angles (representing the currently under study. The discov- squared-mass difference and the ery of CP violation in the lepton sec- degree of mixing between each pair- tor, and a great improvement of lim- wise combination of the three under- its on—or the possible observation lying neutrino-mass states). A next- of—proton decay may finally reveal generation experiment using the the secret of the baryon-antibaryon Tokai accelerator will again use asymmetry in the Universe.

BEAM LINE 49 Whatever Happened to H by JOELR.PRIMACK

HE LIGHTESTOFTHE FUNDAMENTAL matter particles, neutrinos may play important roles in determining the structure of the Universe. They helped to control the expansion rate of the Universe during its first few minutes, when deu- Neutrinos T terium and most of its helium were formed. Neutrinos and photons each accounted for nearly half the energy in the Universe during its first few thousand years. The fact that may have had neutrinos are so ubiquitous—with hundreds of them occupy- ing every thimbleful of space today—means that they can a significant have an impact upon how matter is distributed in the Uni- verse. Over the past two decades, the likelihood of this possi- bility has risen and fallen as more and more data has become impact on available from laboratory experiments and the latest tele- scopes. We now know from the Super-Kamiokande and SNO experiments that neutrinos indeed have mass. Current esti- the structure mates suggest that the total neutrino mass in the Universe may even be comparable to that of the visible stars. By 1979 cosmologists had become convinced that most of of the Universe. the matter in the Universe is completely invisible. This grav- itationally dominant component of the Universe was named “Dunkelmaterie,” or “dark matter,” by the astronomer Fritz Zwicky, who in 1933 first described evidence for dark matter in the Coma cluster (see article on Fritz Zwicky by Stephen Maurer, Winter 2001 Beam Line, Vol. 31, No. 1). Its galaxies were moving much too fast to be held together by the gravity of its visible matter. This phenomenon was subsequently seen in other galaxy clusters, but since the nature of this

50 FALL 2001 ot Dark Matter?

dark matter was completely unknown, it was often ignored. During the 1970s, it became clear that the motion of stars and gas in galaxies—and of satellite galaxies around them—required that this dark matter must greatly outweigh the visible matter in galaxies. The data gathered since then provides very strong evidence that most of the matter in the Universe is invisible. The Coma cluster of galaxies, which is about 50 million light years distant from the Milky Way. ARK MATTER made of light neutrinos, with masses of a few electron volts (eV) or less, is called D “hot dark matter” (HDM) by cosmologists, because those neutrinos would have been moving at nearly the speed of light in the early Universe. (See the tables on the next page for a summary of dark-matter types and associated cos- mological models.) For a few years in the late 1970s and early 1980s, hot dark matter looked like the best dark-matter can- didate. But HDM models of cosmological structure forma- tion led to a “top-down” formation scenario, in which superclusters of galaxies are the first objects to form after the Big Bang, with galaxies and clusters forming through a subsequent process of fragmentation. Such models were abandoned by the mid-1980s after cosmologists realized that

BEAM LINE 51 if galaxies had formed early enough on nearby galaxies and clusters only to agree with observations, their dis- if the average density of matter in the tribution would be much more Universe were at or close to the crit- inhomogeneous than is the case. ical density (Ωm = 1). But like all such Since 1984, the most successful critical-density models, CHDM structure-formation models have required that galaxies and clusters been those in which most of the must have formed fairly recently, mass in the Universe comes in the which disagrees with observations. form of cold dark matter (CDM)—par- The evidence now increasingly favors ticles that were moving sluggishly in ΛCDM models, in which cold dark the early Universe. But the HDM matter makes up about a third of the stock rose again a few years later, and critical density, with a cosmological for a while in the mid-1990s it ap- constant Λ or some other form of peared that a mixture of mostly CDM “dark energy” contributing the with 20–30 percent HDM gave a bet- remainder. This model also helped ter fit to the observations than either resolve the crisis regarding the age of one or the other. This “cold plus hot the Universe (see box on next page). dark matter” (CHDM) theory fit data The question has now become one of how much room is left for neu- trinos in these cosmological models. Types of Dark Matter To describe the possible role of neutrinos as dark matter, I must now 2 3 Ωi represents the fraction of the critical density ρc = 10.54 h keV/cm needed to explain how structures such as galax- close the Universe, where h is the Hubble constant H0 divided by 100 km/s/Mpc. ies formed as the Universe expanded. The expansion itself is described by Dark Matter Fraction of Type Critical Density Comment our modern theory of gravity and spacetime, Einstein’s theory of gen- Baryonic Ωb ~ 0.04 about 10 times the visible matter eral relativity. In order for structure Hot Ων ~ 0.001–0.1 light neutrinos to form, there must have been some Cold Ωc ~ 0.3 most of the dark matter in galaxy halos small fluctuations in the initial den- sity of matter, or else some mecha- nism had to generate such fluctua- Dark Matter and Associated tions afterward. The only such mechanisms that have been investi- Cosmological Models gated are “cosmic defects” such as cosmic strings, and the pattern of Ωm represents the fraction of the critical density in all types of matter. fluctuations produced by such Ω is the fraction contributed by some form of “dark energy.” Λ defects is inconsistent with the tem- Acronym Cosmological Model Flourished perature fluctuations observed in the cosmic microwave background (CMB) radiation. On the other hand, HDM hot dark matter with Ωm = 1 1978–1984 SCDM standard cold dark matter with Ω = 1 1982–1992 “adiabatic” fluctuations—in which m all components of matter and energy CHDM cold + hot dark matter with Ω ~ 0.7 and Ω = 0.2–0.3 1994–1998 c ν fluctuate together—occur naturally ΛCDM cold dark matter Ω ~ 1/3 and Ω ~ 2/3 1996–today c Λ in the simplest cosmic inflation mod- els and are in excellent agreement

52 FALL 2001 The Age of the Universe

IN THE MID-1990S there was a crisis in cosmology, because the age of the old globular-cluster stars in the Milky

Way, then estimated to be tGC = 16±3 with the latest CMB results an- across contained the amount of mat- billion years (Gyr), was higher than the nounced at the American Physical ter (both ordinary and dark) in a large expansion age of the Universe, which for

Society meeting in April 2001. galaxy like our own Milky Way. But Ωm = 1 is texp = 9±2 Gyr. Here I have The evolution of adiabatic fluc- the temperature was then about 100 assumed that the Hubble parameter has tuations into galaxies and clusters is million degrees, so each particle had the value h = H0/(100 km/s/Mpc) easy to understand if you just think a thermal energy far higher than the = 0.72±0.07, the final result from the of gravity as the ultimate capitalist rest energy of light neutrinos. As they Hubble Space Telescope project principle: the rich always get richer would therefore have been moving measuring H0. and the poor get poorer. A “rich” at nearly the speed of light, these But when the data from the region of the Universe is one that has neutrinos would have rapidly spread Hipparcos astrometric satellite became more matter than average. Although out, and any fluctuations in density available in 1997, it showed that the dis- the average density of the Universe on the scale of galaxies would soon tances to the globular clusters had been steadly decreases due to its expan- have been smoothed back to the aver- underestimated, which implied in turn sion, those regions that start out with age density. that their ages had been overestimated; a little higher density than average The first scales to collapse in such now tGC = 13±3 Gyr. The age of the expand a little slower than average a HDM scenario would therefore cor- Universe is consequently tU ~ tGC + 1 and become relatively more dense, respond to the mass inside the cos- = 14±3 Gyr. while those with lower density mic horizon when the temperature Several lines of evidence now show expand a little faster and become rel- dropped to a few eV and the neutri- that the Universe does not have Ωm = 1 atively less dense. When any region nos inside it became nonrelativistic. but rather Ωm + ΩΛ = 1.0±0.1 and Ωm has achieved a density about twice This mass turns out to be about = 0.3±0.1. Taken together, these yield 10,000 the average, it stops expanding and times the mass of our galaxy, texp = 13±2 Gyr, in excellent agreement begins to collapse—typically first including its dark halo. Evidence was with the revised globular cluster age. The in one direction, forming a pancake- just then becoming available from high-redshift supernova data alone give shaped structure, and then in the the first large-scale galaxy surveys tSN = 14.2±1.7 Gyr. other two directions. that the largest cosmic structures— Moreover, a new type of age I can now explain why the first superclusters—have masses of ap- measurement based on radioactive hot-dark-matter boom occurred proximately this size, which at first decay of Thorium-232 (half-life 14.1 Gyr) about two decades ago. Improving glance appeared to be a big success measured in a number of very old stars upper limits on CMB anisotropies for the HDM scenario. gives a completely independent age were ruling out the previously Superclusters of roughly pancake tdecay = 14±3 Gyr. A similar measure- favored cosmological model, which shape were observed to surround ment, based on the first detection in a included only ordinary matter. There roughly spherical voids (regions star of Uranium-238 (half-life 4.47 Gyr), was also evidence from a Moscow where few galaxies are found), in reported this past February, gives tdecay experiment suggesting that the elec- agreement with the first cosmologi- = 12.5±3 Gyr. Work in progress should tron neutrino mass was about cal computer simulations, which soon improve the precision of these 20–30 eV, which would have corre- were run for the HDM model. In this measurements. sponded to a nearly critical-density picture superclusters should have All the recent measurements of the Universe in which neutrinos would formed first, since any smaller-scale age of the Universe are therefore in have dominated the total mass (see fluctuations in the dominant hot excellent agreement. It is reassuring that box on page 56). In such a cosmology, dark matter would have been erased three completely different cosmological the primordial fluctuations on galaxy by free streaming. Galaxies then had clocks—stellar evolution, expansion of scales are erased by “free streaming” to form by fragmentation of the the Universe, and radioactive decay— of the relativistic neutrinos in the superclusters. But it was already agree so well. early Universe. One year after the Big becoming clear from observations Bang, a region about one light year that galaxies are much older than

BEAM LINE 53 The Andromeda Galaxy, which together superclusters, contrary to what the predicted (to within a normalization with the Milky Way and several dwarf HDM scenario implies. And the uncertainty factor of about 2) the galaxies, forms the Local Group of apparent detection of electron neu- magnitude of the CMB temperature galaxies. trino mass by the Moscow experi- fluctuations, which were discovered ment was soon contradicted by in 1992 using the COBE satellite. But results from other laboratories. Hot the simplest CDM model, standard matter fell into decline. CDM (or SCDM) with the matter den-

sity equal to the critical value (Ωm HELPED TO DEVELOP the = 1), had already begun to run into cold dark matter model in trouble. I1982–1984 just as the problems Cosmological theories predict sta- with the hot dark matter model were tistical properties of the Universe— becoming clear. Proto-galaxies form for example, the size of density fluc- first in a CDM cosmology, and galax- tuations on various scales, described ies and larger-scale objects form by mathematically by a power spec- aggregation of these smaller lumps— trum. Sound or other fluctuation although the cross-talk between phenomena can be described in the smaller and larger scales in the CDM same way—for example, low fre- theory naturally leads to galaxies quencies might be loud, correspond- forming earlier in clusters than in ing to relatively high power at long lower-density regions. In this and wavelengths. Put simply, SCDM had other respects, CDM models appeared difficulty fitting the full spectrum of to fit observations much better than density fluctuations at both short and HDM. The first great triumph of cold long wavelengths—or at small and dark matter was that it successfully large scales. With a given amount

54 FALL 2001 The detailed studies

of cosmological

structures now of fluctuation power on the large models. The main problem with Ωm scales probed by COBE (billions of = 1 cosmologies containing only cold light years), that is, SCDM has a lit- going on or about dark matter plus a small admixture tle too much power on small scales of ordinary baryonic matter is that relevant to galaxies and clusters (mil- the galaxy-scale inhomogeneities are lions of light years and less). It pro- to begin may too big compared to those on larger duces too many of them. But the fact scales. Adding a little hot dark mat- that the SCDM theory could work ter appeared to be just what was fairly well across such a wide range eventually reveal needed to solve this problem. of size scales suggested that it had And there was even a hint from a kernel of truth. Cosmologists be- an accelerator experiment that neu- gan to examine whether some vari- something about trino mass might lie in the relevant ant of SCDM might work better. range. The experiment was the Although these COBE results did Liquid Scintillator Neutrino Detector not come in until 1992, I became wor- (LSND) experiment at Los Alamos ried that SCDM was in trouble when neutrino mass itself. National Laboratory (see photograph the large-scale flows of galaxies were on page 38), which recorded a num-

first observed by my UC Santa Cruz ber of events that appear to be νµ → νe colleague Sandra Faber and her “Sev- oscillations. Comparison of the LSND en Samurai” group of collaborators. data with results from other neutrino Earlier studies had established that experiments allows two discrete 2 the local group of galaxies (including values of ∆mµ,e (see box, next page) the Milky Way and the “nearby” small scales better than SCDM.Both around 10.5 and 5.5 eV2, or a range Andromeda galaxy) is speeding at a of these variants had been proposed of values between 0.2 and 2 eV2. If velocity of about 600 kilometers per in 1984, when cold dark matter was true, this means that at least one neu- second with respect to the CMB ref- still a new idea, but their detailed trino has a mass greater than 0.5 e V, erence frame. But the Seven Samu- consequences were not worked out which would imply that the contri- rai and others found that bulk until the problems with SCDM began bution of hot dark matter to the cos- motions of other galaxies often had to surface. mological density is much greater similar high velocities across regions than that of all the visible stars. Such several tens of millions of light years VENIFMOST of the dark an important conclusion requires in- wide. It soon became clear that this matter is cold, a little hot dark dependent confirmation. The KArl- result was inconsistent with the Ematter can still have dramat- sruhe Rutherford Medium Energy expectations of the “biased” SCDM ic effects on the small scales relevant Neutrino (KARMEN) experiment model that seemed to fit the prop- to the formation and distribution of results exclude a significant portion erties of galaxies on smaller scales. galaxies. In the early Universe, the (but not all) of the LSND parameter In the late 1980s Jon Holtzman, then free streaming of fast-moving neu- space, and the numbers quoted above Faber’s graduate student, did a the- trinos would have washed out any take into account the current KAR- oretical dissertation with me in spatial inhomogeneities on the scales MEN limits. The Booster Neutrino which he calculated detailed expec- that later became galaxies, just as Experiment (BooNE) at Fermilab (see tations for 96 variants of cold dark in the HDM scenario. Consequently, article by Paul Nienaber, this issue) matter. When we compared these CDM fluctuations did not grow as should attain greater sensitivity and predictions with the data available fast on these scales, and at the rela- help to resolve this issue. in early 1992, it was clear that the tively late times when galaxies By 1995 simulation techniques best bets were CHDM and ΛCDM, formed there was less flutuation and supercomputer technology had each of which could fit the data on power on small scales in CHDM advanced to the point where it

BEAM LINE 55 was possible to do reasonably high- resolution cosmological-scale simu- lations including the random veloc- Neutrino Mass and Cosmological Density ities of a hot dark matter component. The results at first appeared very THEATMOSPHERIC-NEUTRINO DATA from the Super- favorable to CHDM. Indeed, as late as Kamiokande underground neutrino detector in Japan provide strong 1998 a CHDM model (with Hubble

evidence of muon to tau neutrino oscillations, and therefore that these parameter h = 0.5, mass density Ωm neutrinos have nonzero mass (see the article by John Learned in the = 1, and neutrino density Ων = 0.2) was Winter 1999 Beam Line, Vol. 29, No. 3). This result is now being confirmed found to be the best fit of any cos- by results from the K2K experiment, in which a muon neutrino beam from mological mode to the galaxy dis- the KEK accelerator is directed toward Super-Kamiokande and the number tribution in the nearby Universe. But of muon neutrinos detected is about as expected from the atmospheric- cosmological data was steadily im- neutrino data (see article by Jeffrey Wilkes and Koichiro Nishikawa, this proving, and even by 1998 it had be-

issue). come clear that h = 0.5 and Ωm = 1 But oscillation experiments cannot measure neutrino masses directly, were increasingly inconsistent with 2 2 2 only the squared mass difference ∆mij = |mi − mj | between the oscillating several observations, and that h ~ 0.7 species. The Super-Kamiokande atmospheric neutrino data imply that and Ωm ~ 1/3 worked much better. −4 2 −3 2 1.7×10 < ∆m τµ < 4×10 eV (90 percent confidence), with a central value For example, CHDM predicts that 2 −3 2 ∆m τµ = 2.5×10 eV . If the neutrinos have a hierarchical mass pattern galaxies and clusters formed rela- m << m << m like the quarks and charged leptons, then this implies νe νµ ντ tively recently, but around 1998 that ∆m2 ≅ m2 so m ~ 0.05 eV. τµ ντ ντ increasing numbers of galaxies were These data then imply a lower limit on the HDM (or light neutrino) discovered to have formed in the first contribution to the cosmological matter density of Ων > 0.001—almost as few billion years after the Big Bang. much as that of all the stars in the disks of galaxies. There is a connection And the fraction of baryons found in between neutrino mass and the corresponding contribution to the cosmo- clusters, together with the reason- logical density, because the thermodynamics of the early Universe speci- able assumption that this fraction is fies the abundance of neutrinos to be about 112 per cubic centimeter for representative of the Universe as a each of the three species (including both neutrinos and antineutrinos). It whole, again gives Ωm ~ 1/3. That follows that the density Ω contributed by neutrinos is Ω = m(ν)/(93 h 2 eV), ν ν there is a large cosmological constant 2 where m(ν) is the sum of the masses of all three neutrinos. Since h ~ 0.5, (or some other form of dark energy) -3 mν ~ 0.05 eV corresponds to Ων ~ 10 . τ yielding ΩΛ ~ 2/3 then follows from This is however a lower limit, since in the alternative case where the any two of the following three oscillating neutrino species have nearly equal masses, the values of the results: (1) Ω ~ 0.3, (2) CMB aniso- individual masses could be much larger. The only other laboratory m tropy data implying that Ω + Ω = approaches to measuring neutrino masses are attempts to detect neutrino- m Λ 1, and (3) high-redshift supernova less double beta decay, which are sensitive to a possible Majorana compo- data implying that Ω – Ω ~ 0.4. nent of the electron neutrino mass, and measurements of the endpoint of Λ m The abundance of galaxies and the tritium beta-decay spectrum. The latter gives an upper limit on the clusters in the early Universe agrees electron neutrino mass, currently taken to be 3 eV. Because of the small well with the predictions of the values of both squared-mass differences, this tritium limit becomes an ΛCDM model. However, the highest- upper limit on all three neutrino masses, corresponding to m(ν) < 9 eV. A resolution simulations of this model bit surprisingly, cosmology already provides a stronger constraint on neu- 1990 trino mass than laboratory measurements, based on the effects of neutri- that were possible in the mid- s nos on large-scale structure formation. gave a dark matter spectrum that had more power on scales of a few million light years than did the

56 FALL 2001 observed galaxy power spectrum, 0.5, the limit on the sum of the neu- although the simulations and data trino masses is m(ν) < 2.4 (Ωm/0.17 – 1) agreed on larger scales. This result eV. Thus for Ωm < 0.5, the sum of all was inconsistent with the expecta- three neutrino masses is less than For Further Reading tions that galaxies would be (if any- 5 eV. This limit on the sum of the neu- For a more technical article on the thing) more clustered than the dark trino masses is much stronger than present subject with extensive matter on small scales, not less. the best current laboratory limit. references, see “Hot Dark Mat- When it became possible to do even Astronomical observations that ter in Cosmology,” by Joel R. higher-resolution simulations that may soon lead to stronger upper Primack and Michael A. K. allowed the identification of the dark limits on m(ν)—or perhaps even a de- Gross, in Current Aspects of matter halos of individual galaxies, tection of neutrino mass—include Neutrino Physics, D. O. Cald- however, their power spectrum data on the distribution of low- well, Ed. (Berlin: Springer, turned out to be in excellent agree- density clouds of hydrogen (the so- 2001); also available on the ment with observations. The galax- called “Lyman-alpha forest”) in the Internet as astro-ph/0007165 ies were less clustered than dark mat- early Universe, large-scale weak grav- and interactively as nedwww. ter because galaxies had merged or itational lensing data, and improved ipac.caltech.edu/level5/ were destroyed in very dense regions measurements of the fluctuations in Primack4/frames.html due to interactions with each other the CMB radiation temperature at The latest published upper limit and with the cluster center. This small angles. These data are sensi- on the sum of neutrino masses explanation turned a troubling dis- tive to the presence of free-streaming from cosmological structure crepancy into a triumph for ΛCDM. neutrinos in the early Universe, formation is Rupert A. C. Croft, Thus ΛCDM is the favorite theory which can lead to fewer galaxies on Wayne Hu, and Romeel Davé, today. But we know from the Super- small scales, depending on the val- Phys. Rev. Lett 83, 1092 Kamiokande evidence for atmos- ues of the neutrino masses. (1999). pheric neutrino oscillations that The hot dark matter saga thus X. Wang, M. Tegmark, and M. Zaldarriaga, astro- there is enough neutrino mass to cor- illustrates once again the fruitful ph/0105091, gives a tighter respond to some hot dark matter, at marriage between particle physics −3 upper limit of 4.4 eV on m(ν). least Ων ~ 10 (see box on opposite and cosmology. While neutrino- page) about one-fourth as much as oscillation experiments can tell us the visible stars. So the remaining only about differences between the question regarding neutrinos in cos- squared masses of the neutrinos, the mology is how much room remains structure of the Universe can give us for a little hot dark matter in ΛCDM. information about the neutrino cosmologies. There could be perhaps masses themselves. In an earlier 10 times the lower limit just quoted, example of this connection, cosmo- but probably not 100 times. The rea- logical arguments based on Big Bang son there is any upper limit at all nucleosynthesis of light elements put from cosmology is because the free a strict limit on the possible number streaming of neutrinos in the early of light neutrino species; this limit Universe must have slowed the was eventually borne out by high- growth of the remaining CDM fluc- energy physics experiments on Z tuations on small scales. Thus to bosons at CERN and SLAC. The have the galaxy structures we see to- detailed studies of cosmological day, there must be much more cold structures now going on or about to than hot dark matter. For the obser- begin may eventually reveal some- vationally favored range 0.2 ≤ Ωm ≤ thing about neutrino mass itself.

BEAM LINE 57 CONTRIBUTORS

MICHAEL RIORDAN is Assis- JOSHUA KLEIN received his PAUL NIENABER has the great tant to the Director at the Stanford Ph.D. from Princeton University in good fortune to be part of two Linear Accelerator Center, Lecturer 1994. He then moved to the Univer- extraordinary neutrino groups at in the History and Philosophy of sity of Pennsylvania where he has Fermilab. He joined the NuTeV col- Science Program at Stanford, and worked on building and making laboration in 1992 and is now work- Adjunct Professor of Physics at the measurements with the Sudbury ing on data analysis from that University of California, Santa Cruz. Neutrino Observatory. He is cur- experiment; in addition, he became A Contributing Editor of the Beam rently Research Assistant Professor a member of the BooNE collaboration Line, he is author of The Hunting of Physics at Pennsylvania. in 2000 and is participating in con- of the Quark, and coauthor of The SNO is in many ways the quin- struction and testing of its detector. Shadows of Creation: Dark Matter tessential particle astrophysics The photograph above shows Paul and the Structure of the Universe experiment featuring the Sun as a inside the BooNE tank, surrounded and Crystal Fire: The Birth of the neutrino laboratory and neutrinos as by the photomultiplier tubes he Information Age. He leads a group of solar probes. Klein’s interest in SNO helped install. He is a Jesuit priest, a historians and physicists researching encompasses both of these features, Guest Scientist at Fermilab, and a and writing the history of the Super- including an interest in neutrinos member of the physics faculty at the conductingSuper Collider. In 1999 he themselves as the Standard Model’s College of the Holy Cross in Worces- received a Guggenheim Fellowship most enigmatic particle. ter, Massachusetts, where he thor- to pursue research on this subject oughly enjoys infecting college stu- at the Smithsonian Institution in dents’ minds with the manifold Washington, DC. While there, he also pleasures of doing physics. This is his got married again. first article for the Beam Line.

58 FALL 2001 BORIS KAYSER is a theoretical physicist who has been particularly interested in the physics of massive neutrinos and the asymmetry be- JEFFREY WILKES is Kenneth K. tween matter and antimatter. He Young Memorial Professor of Physics also enjoys brachiating around in KOICHIRO NISHIKAWA is at the University of Washington in quantum mechanics. An author of Professor of Physics at Kyoto Uni- Seattle. His thesis research at the well over 100 scientific papers, he versity. He received his Ph.D. in 1980 University of Wisconsin involved the is also co-author of a popular slender from Northwestern University, last of many cosmic-ray experiments book on neutrino physics and a fre- where his thesis research involved conducted at Echo Lake, Colorado. quent, enthusiastic speaker on par- di-muon production using the high As a postdoc at the University of ticle physics topics. energy neutrino beam at Fermilab. Washington, he learned the nuclear For nearly three decades, Kayser As a postdoc at the University of emulsion trade as Jere Lord's ap- served as Program Director for Chicago, he was subsequently in- prentice and has been there ever Theoretical Physics at the National volved in an experiment on CP since. Science Foundation, in which capac- violation in the neutral kaon system. After an interlude with the ity he was instrumental in estab- Nishikawa returned to Japan—to quixotic and habit-forming enterprise lishing the Institute for Theoreti- the Institute for Nuclear Study of the called DUMAND, he joined Super- cal Physics at the University of University of Tokyo—and joined the Kamiokande, where he is convener California, Santa Barbara. He has Super-Kamiokande experiment. He of an analysis group. He is US co- now joined Fermilab as a Distin- moved to Kyoto University in 1999. spokesman for the K2K long-baseline guished Scientist and looks forward He serves as the spokesman for the neutrino experiment, to which his to spending full time on his first K2K long-baseline neutrino experi- main contribution seems to be the love—physics research. ment. care and feeding of a wildly fluctu- Kayser enjoys nature, and he is ating shift schedule. Most recently, one of the primates in the photo he has been developing WALTA, a above. school-network cosmic-ray detector project in Seattle.

BEAM LINE 59 A Brief Neutrino Bibliography

John N. Bahcall, Neutrino Astrophysics (Cambridge University Press, New York, 1989) John N. Bahcall, Raymond Davis, et al., Eds., Solar Neutrinos: The First Thirty Years (Addison-Wesley, Reading, 1994). David O. Caldwell, Ed., Current Aspects of Neutrino Physics (Springer Verlag, Berlin, 2001) Boris Kayser, The Physics of Massive Neutrinos (World Scientific, Singapore, 1989) Nickolas Solomey, The Elusive Neutrino (Scientific American Library, New York, 1997) Christine Sutton, Spaceship Neutrino (Cambridge University Press, Cambridge, 1992)

JOEL PRIMACK is Professor of Physics at the University of Cali- fornia, Santa Cruz. He has been working at the interface between par- ticle physics and cosmology since the early 1980s, when he and several col- leagues conceived the idea of cold dark matter, which has since become the “standard model” of cosmologi- cal structure formation. He is cur- rently doing research in Germany under an award from the Humboldt Foundation. Primack has also worked exten- sively on science and public policy issues. He helped initiate the Con- gressional Fellowship program spon- sored by the American Association for the Advancement of Science, as well as the American Physical Society’s Forum on Physics and Society.

60 FALL 2001 DATES TO REMEMBER

Jan 7 - Mar 15 Joint Universities Accelerator School: Courses in the Fundamentals of the Physics, Technologies, and Applications of Particle Accelerators (JUAS 2002), Archamps, France, (J. Delteil, ESI/JUAS, Centre Universitaire de Formation et de Recherche, F-74166 Archamps, France or [email protected] or http://www.esi.cur- archamps.fr)

Jan 14 - 25 US Particle Accelerator School, Long Beach, CA ([email protected] or http://www. indiana.edu/~uspas/programs/ucla.html)

Feb 4 - 8 9th International Workshop on Linear Colliders, Menlo Park, CA (Robbin Nixon, SLAC, MS 66, Box 4349, Stanford, CA 94309 or [email protected])

Mar 3 - 8 International Spring School on the Digital Library and E-Publishing for Science and Technology, Geneva, Switzerland (http://cwis.kub.nl/~ticer/spring02/index.htm)

Mar 7 - 9 IUPAP International Conference on Women in Physics, Paris, France (Dr. Judy Franz, American Physical Society, One Physics Ellipse, College Park, MD 20740 or http://www.if.ufrgs.br/~barbosa/conference.html)

Mar 18 - 22 Annual Meeting of the APS, Indianapolis, IN (American Physical Society, One Physics Ellipse, College Park, MD 20740 or http://www.aps.org/meet/MAR02/)

Apr 20 - 23 Meeting of the APS and the High-Energy Astrophysics Division (HEAD) of the American Astronomical Society, Albuquerque, NM (American Physical Society, One Physics Ellipse, College Park, MD 20740 (http://www.aps.org/meet/APR02/)

May 8 - 17 CERN Accelerator School: Superconductivity and Cryogenics for Accelerators and Detectors, Erice, Sicily (CERN Accelerator School, AC Division, 1211 Geneva 23, Switzerland or [email protected])

May 24 - 28 DPF 2002: Meeting of the Division of Particles and Fields of the American Physical Society, Williamsburg, VA (Marc Sher, Physics Department, College of William and Mary, Williamsburg, VA 23187 or [email protected] or http://www.dpf2002.org)

May 25 - 30 20th International Conference on Neutrino Physics and Astrophysics (Neutrino 2002), Munich, Germany (NEUTRINO 2002 Secretariat, Technische Universitat Munchen, Physik Department E15, D-85747 Garching, Germany or [email protected] or http://neutrino2002.ph.tum.de/)

Jun 20 - 22 Physics in Collisions, Stanford, CA (Maura Chatwell, MS 81, 2575 Sand Hill Road, Menlo Park, CA 94025 or [email protected])