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Home , Lepton, Muon neutrino, Neutrino

LEPTON UNIVERSALITY by J. RITCHIE PATTERSON

“You know, it would be sufficient to really understand the .” —Albert Einstein

LL OF US ARE FAMILIAR with the electron as the small companion of Athe . It is the atomic nucleus that gives ordinary its weight, but the electron is responsible for the space that matter fills, for while it is small, the electron rules vast territories. The electron was first discovered thanks to the that it carries, and we rediscover the electron whenever we happen to get an electrical shock.

BEAMLINE 7 Indeed, the root “electro-” comes As far as we know, the electron, It tells us that the electron is small from the Greek word meaning am- and are fundamental indeed: the agreement would be ber, so named because amber tends , and unlike or spoiled if the radius of the elec- − to accumulate electric charge when , they cannot be broken down tron were greater than about 10 14 rubbed, just as we do when we shuf- into smaller components. This is not centimeters. fle across a wool rug. the first time that we have thought Colliders provide an even more While the electron is unique in the we have found the fundamental powerful microscope into the elec- role that it plays in ordinary mat- building blocks of , and in the tron’s structure. By studying the ter, it is actually part of a trio. The past we have often been wrong. Or- deflections of and anti- electron’s partners, called the muon dinary matter turned out to be made electrons when they collide with one and the tau, have the same electric of molecules, molecules of atoms, another, experimenters in Japan charge and seem to share its other atoms of nuclei and electrons, nuclei and Europe have probed the electron − properties as well. All its properties, of and , and, most on a scale as small as 10 17 cm, but that is, except one: their are recently, protons and neutrons of they see no sign of substructure. very different. The muon is heavier . But the electron, muon and Equally precise studies of the muon than the electron by a factor of about tau appear to be indivisible: we see also yield null results. We know 200, while the tau is heavier by the no sign of constituents and they are much less about the tau, but so far, whopping factor of 3500. These three minuscule in size. In fact, surprising it too appears to be free of smaller particles are three of twelve known as it may seem, they may be point- constituents. fundamental particles of nature. like, with no spatial extent at all. The muon, which we write as µ, How do we measure the size of HE ELECTRON, MUON and was discovered in 1947 in cosmic the electron? Just as the electron tau are not alone. Each has a rays. At the time of its discovery, has electric charge, it also has an in- Tpartner called a , ν ν ν physicists had untangled quantum trinsic angular that is written as e, µ and τ respectively. mechanics and understood atomic constant in time. We call this angu- The are massless or near- structure and the nucleus. The puz- lar momentum “” and measure ly so, are electrically neutral, and are zles of nature had apparently been it in units of ’s constant h, insensitive to the strong that solved, and many physicists were which, like the , is a binds atomic nuclei. As a result they ready to declare victory and retire. fundamental constant of nature. The are rarely detected. When they were The appearance of a new spin of the electron (and the muon first proposed by came as a surprise, and not an en- and tau) is h/2 . Like all circulating in order to explain the apparent tirely welcome one. As I. I. Rabi said electric charges, the electron’s spin loss of and momentum in at the time, “Who ordered this?” generates a magnetic field similar to radioactive decays, he apologized, “I The tau (τ) is a much more recent the one around the earth, the have done a terrible thing, I have discovery. It was found in 1975 by strength of which depends on the postulated a particle that cannot be a group led by Martin Perl at the spatial distribution of its charge. Cal- detected.” SPEAR at SLAC. culations of this magnetic field have Neutrinos may be hard to detect, This group observed that the col- been carried out for the electron us- but they are not rare. In fact, they are lision of an electron with an anti- ing the theory of quantum electro- produced abundantly in the , and electron (or “”) sometimes dynamics (QED) with the assumption more than 1013 pass through your produced particles in a configuration that the electron is pointlike. The re- body each second, and then contin- inconsistent with all known process- sults agree with the experimental ue though the earth and out the other es. Instead, it was exactly what one value within one part in one billion. side. Of these, only one per year would expect if new particles were This is the most precise test of the- interacts in your body, leaving a being produced that were similar to ory and experiment in , and brief ripple in its wake. Like the elec- electrons, but much heavier. the agreement is a triumph for QED. tron, muon and tau, the neutrinos

8 SPRING 1995 are believed to be carbon copies of “up” quarks and one “down” one another. All have the same spin while a is made of one “up” νe νµ ντ and the same , and like quark and two “down” quarks). All µ τ the electron, muon and tau, they are other particles that we have observed ( e ) ( ) ( ) believed to be fundamental particles. (other than the ), such as the ν π What links the e and e as part- , are bound states of the u c t ners? In radioactive decays, we see quarks. d s b that the e is always accompanied by Like the leptons, the six quarks ( ) ( ) ( ) ν ν ν a e, but never, say, by a µ or a τ . seem to come in pairs, and their That the neutrino species are distinct masses range from very small in the The known fundamental particles was first demonstrated in 1962 by first generation to very large in the of nature. The upper six particles are Leon Lederman, Mel Schwartz, Jack third generation. In fact, the top the leptons and the lower six are the Steinberger and their collaborators quark weighs about as much as a quarks. Both the leptons and the quarks in an experiment which earned them gold nucleus. Why there should be are arranged into three generations of the Nobel prize. three generations, and the relation- two particles each (shown in brackets). ship between the and quark HESE SIX PARTICLES, the elec- generations, are mysteries. tron, muon and tau plus their Could there be a fourth generation Tthree neutrinos are known as of quarks or leptons waiting to be “leptons,” a name derived from the discovered? Current evidence sug- Greek word λεπτοσ meaning small gests not. Data from the LEP accel- or light. (Had early particle physicists erator at CERN in Geneva, Switzer- known about the weighty τ, they land have shown that there are only might have chosen a different name!) three species of light (or massless) ν ν High energy physicists like to arrange neutrinos: these are the e, µ, and the leptons in a special way (see fig- ντ. Thus, if an additional generation ure on right), and refer to each lepton exists, its neutrino must be very mas- and its neutrino as a generation. The sive—a marked departure from the generations are ordered by the masses generations that we now know. of the charged leptons. In addition to the leptons, there is ORCES THAT CONTROL the in- another set of particles called quarks. teractions between particles are Quarks are similar to leptons, but un- Fas essential to nature as the par- like leptons, can interact via the ticles themselves. We know of four strong force. We know of six kinds : ; ; (or “flavors”) of quark: “down,” “up,” the “strong” force, which binds to- “strange,” “charm,” “bottom,” and gether quarks into protons, neutrons, “top.” All of these are produced pro- π or other, less common, par- lifically at accelerators except the ticles; and the “weak” force, which “top” quark, for which the first di- is responsible for the decay of ra- rect evidence was reported last year dioactive nuclei and for much of the by particle physicists at the Fermi activity in the sun. National Accelerator Laboratory lo- All of the forces operate in about cated outside Chicago. Quarks are the same way. Associated with each the building blocks of protons and one is a charge: electric charge for neutrons (a is made of two electromagnetism, for gravity,

BEAM LINE 9 A particle physicist’s view of electromagnetic interactions. In the upper figure (a) two (a) charged particles, each represented by a solid line, scatter. A carrying e– e– energy and momentum is emitted by one and absorbed by the other. The lower figure (b) shows the less familiar of an electron and anti-electron. Their energy goes into the photon which then materializes into two new particles—in this case a tau and anti-tau.

Photon

and something called “color” for the photon which then materializes as a strong force (this “color” has noth- new particle and anti-particle pair. τ– τ– ing to do with the usual meaning of This is how particles such as taus Time the word). Similarly, there is a “weak were produced at the SPEAR and PEP (b) charge” associated with the weak storage rings at SLAC and are cur- e– τ+ force. Electrons, , and taus car- rently produced at the CESR electron- ry this charge as do the neutrinos and anti-electron collider at Cornell Uni- quarks. versity. Note that this diagram is When Newton developed the con- identical to figure (a) rotated by 90 cept of force, he viewed it as “action degrees except that some particles + τ– e at a distance.” We now know that have been changed from particle into Time special particles travel between the anti-particle and vice versa. As the interacting objects, deflecting them similarity of the diagrams suggests, with the momentum and energy the same physics is responsible for which they carry. In the case of both processes. (c) gravity, this special particle is the Weak interactions work in the e– ν e , while for electromagnetic same fashion as the electromagnet- interactions it is the photon. ic and gravitational interactions. It is handy to diagram these When two particles carrying the processes as shown in the adjacent weak charge approach one another, W– figures. In these diagrams, each par- they can exchange a particle, in ticle is represented by a line, and this case either a “W-” or time increases from the left hand “Z-boson.” The W-boson was first side of the diagram to the right. In observed directly in 1983 by two – νµ µ diagram (a), the electrical repulsion groups at CERN, one led by Carlo Time of two charged particles, the particles Rubbia, and is now produced rou- are well-separated at the left-hand tinely at the proton collider at Fer- (d) – e side of the diagram, they approach milab, and the Z-boson is produced each other, and exchange a photon, and studied in detail at the LEP col- which is represented by the wiggly lider at CERN and at the SLC at SLAC. W– line, and recoil. These diagrams are Just as the photon materializes ν– e µ– convenient for high energy physicists into a pair of particles, so does the W. because they can be translated into For the W, this can be either a pair of

νµ mathematical formulae for the prob- leptons or a pair of quarks. When- Time ability that the scattering will occur: ever the W decays into leptons, it al- using a method first introduced by ways chooses two leptons from the ν µ A particle physicist’s view of the weak , one simply writes same generation: an e and e, a and ν τ ν interactions. In the upper figure (c) an down a mathematical factor for each µ, or a and τ. In fact, experiments electron and exchange line and intersection point. have searched for decays in which a W-boson, and a muon and electron The repulsion or attraction of two the W (or some unexpected exotic neutrino emerge. In the lower figure (d) charged particles is the most famil- particle) produces leptons from two a muon decays by emitting a W-boson iar electromagnetic interaction, but different generations, and none have and neutrino, and the W then it is not the only possibility. As been observed, even though some materializes as an electron and electron shown in diagram (b), a particle and of these experiments, such as one anti-neutrino. This process is very similar to as well as much its anti-particle can meet and an- known as SINDRUM at the Paul of the activity in the sun. nihilate one another to produce a Scherrer Institute in Switzerland and

10 SPRING 1995 another at the Los Alamos National Lab in New Mexico known as MEGA, have searched among nearly 1 tril- MODERN PARTICLE lion decays. Interestingly, when the W decays into quarks, it disregards ETECTORS the ban on cross-generational mix- D ing. This cross-generational quark mixing has some fascinating conse- TYPICAL MODERN particle digitized and recorded for later quences, some of which motivate the detector is really a collec- analysis whenever something construction of the new accelerators tion of many devices each interesting happens in the known as B-factories. A with its own capabilities. These detector. We can now explain why we find electrons rather than muons inside devices are layered around one The figure shows the signals in atoms. The allows another like the peels of an onion, the CLEO detector at Cornell a muon to decay into an electron plus all centered on the point University when an two neutrinos, as shown in diagram where the two accel- electron and anti- (d), in a process that is very similar erated particle electron annihi- to the radioactive decay of an - beams collide. late to produce ic nucleus. Muons typically survive − In most a pair of 2 × 10 6 seconds before decaying in detectors, taus. The this way. The reverse process, the de- the inner- µ− taus decay cay of the electron into a muon plus γ − neutrinos, is forbidden by energy most layers π π+ before conservation because the muon is display the π+ reaching the heavier than the electron. trajectories detector, so The mass of the lightest lepton has of charged we see only a major impact on our day to day particles. By the particles lives. What would happen if the elec- placing these that they decay tron were as heavy as a muon? For devices in a large into. In this event, starters, atoms would be much more compact because the large centri- magnetic field, we learn one tau traveling up fugal force of these heavy electrons not only the particle’s direction of and to the left decays into a muon could be overcome only if they travel, but also its momentum, and two neutrinos, while the other, hugged the atomic nucleus. As a which is inversely proportional to traveling down and to the right, result, an apple would weigh about its curvature in the magnetic field. decays into three plus a what it does now, but it would be the Other devices measure the parti- neutrino. We recognize the muon size of a grain of sand. Similarly, a cle’s , which combined by the signal that it leaves in the typical person, if she or he could sur- with the momentum measurement outermost layers of the detector— vive at all, which is doubtful given reveal its mass and therefore its it is the only particle that pene- the consequences of heavy electrons for chemistry, would stand only one identity. Yet other devices detect trates that far. The neutrinos third of an inch tall. and other electrically escape undetected. Like the muon, the tau can decay neutral particles. Events like this one can be into an electron plus neutrinos, but Each of these devices pro- used to measure the tau lifetime. its large mass gives it a multitude duces electrical signals which are of other possibilities. For example, it

BEAM LINE 11 0.9 1.1 may decay into a muon plus neu- will reveal whether this discrepancy trinos in an analogous process or is significant. even into quark pairs plus a neutrino. The ratios of weak charges gµ /ge Because it has so many decay op- and ge/gτ have also been measured at g /g g /g µ e e τ tions, the tau is very shortlived. The TRIUMF in Vancouver and at Fermi- 1.0 1.0 average lifetime of the tau is 0.3 tril- lab and CERN respectively, and the lionths of a second. This has been results are summarized in the adja- measured at accelerators where taus cent figure. If the weak charge of one are produced. At the SLC collider at of the leptons were larger than the 1.1 0.9 SLAC and the LEP collider in Europe, others, the overlap band shown in the 0.9 1.0 1.1 taus travel about 2 cm in their life- figure would move from the center gτ/gµ time, which is easily measurable of the triangle toward one of the with modern particle detectors. corners. In fact, as far as we can tell This “Universality Triangle” helps What evidence do we have that now, their weak charges are identi- physicists assess the weak charges of the leptons. The two vertical lines the electron, muon and tau are iden- cal. Even a minute difference, how- indicate the band of values of gτ /gµ tical apart from their masses? This ever, would signal a profound dis- compatible with recent measurements. is a topic of current research. Cur- covery, so future research will The bands of allowed values of the other rently, physicists test whether the attempt to determine these charges two ratios of weak charges are indicated weak charges of the muon and tau even more precisely. by lines perpendicular to the other two are identical by comparing their de- It may seem surprising that parti- sides of the triangle. If g , g , and g e µ τ cay rates into an electron plus neu- cles whose properties and interac- are identical, then the bands will overlap at the center of the triangle. On the other trinos. Calculations of the Feynman tions seem to be identical should hand, if one charge differs from the diagrams indicate that the ratio of have such different behavior: the other two, then the overlap region (which the tau to muon decay rates should electron, a principal player in every- 5 2 is shaded) will march toward one corner be proportional to (mτ/mµ) (gτ /gµ) , day matter; the muon, found pri- of the triangle. where gµ and gτ are the weak charges marily in cosmic rays; and the tau, of the τ and µ and mτ and mµ are confined to particle accelerators. their masses. Recent measurements But as far as we know, the tau and of the tau mass done at BEPC in Bei- the muon are simply heavy replicas jing, China, and decay rate done of the electron—only their vastly most precisely at LEP (the muon has different masses are responsible for been around for years and its mass the very different roles that they play and decay rate are very precisely in our . known) show that the ratio of weak Many questions remain. Why are charges is gτ /gµ = 0.994 ±0.004, there three generations? Where do where ± 0.004 indicates the experi- they get their masses, and why do mental uncertainty. We see that the they differ so drastically from one ratio of the muon and tau weak another? Why do the weak interac- charges is nearly unity, and that tions always produce leptons with- if they differ, it is only by a tiny in a single generation? Is it a coin- fraction. In fact, the ratio of weak cidence that there are also three charges differs by an amount slight- generations of quarks? Someday we ly larger than the experimental error. may know the answers to these Future, more precise, experiments questions.

12 SPRING 1995