Neutrinoless Double-Beta Decay [On a path of discovery with the neutrino]

Vincente E. Guiseppe University of South Carolina

EBSS2017 Summer School Argonne National Lab. July 27, 2017 March 7, 2012 1:28 WSPC/INSTRUCTION FILE Elliott-mpla

Double Beta Decay 3 measurements of neutrino mass if the neutrino is Majorana. The interpretation of cosmology results would be greatly enhanced by a laboratory neutrino mass result with which to constrain models. An open question in neutrino physics is whether or not the lightest neutrino mass eigenstate is the dominant component of the electron neutrino. If so, we refer to the neutrino mass spectrum as being normal. If not, we refer to it as inverted. Figure 1 shows the e↵ective Majorana neutrino mass as a function of the lightest neutrino mass for these 2 possibilities using the neutrino oscillation parameters from Ref. 13.

3. Cancellation E↵ects of CP Phases Figure 1 seems to indicate that a large fraction of the potential parameter space within the normal hierarchy can resultOutline in a negligible m even if neutrinos are h i Majorana particles. This is a bit misleading because for the expression in Eqn. 2 to result in a small m , specific values of the mixing elements, mass eigenstates h i and phases✦ Neutrino must conspire discoveries to cancel. Barring some symmetry that requires such a cancellation,✦ State this of wouldneutrino be physics a unnatural coincidence. In fact the fraction of the parameter✦ spaceOpen that questions would result- New in discoveries a cancellation is rather small if the parameter values are random. In Fig. 2 one sees that for a given value of m in the region ✦ Neutrinoless double-beta decay 1 of parameter space that can potentially su↵er such cancelations, about 5% of the ✦ - spaceThe results MAJORANA in m Programless than 1 meV. 1 2 h i

θ12 = 33.58 δ

θ13 = 8.33 δ

2 V. E. Guiseppe Fig. 1. The e↵ective Majorana neutrino massEBSS as a 2017 function of the lightest neutrino mass. In the left panel, ✓13 was taken to be zero, whereas in the right panel it is set to the best fit value in recent global fits.

4. Nuclear Physics and The observation of 0⌫ would have profound qualitative physics conclusions. How- ever, the interpretation of those results quantitatively requires knowledge of M0⌫ . Furthermore, an accurate knowledge of M0⌫ has benefits for experimental design. Most nuclear matrix element calculations involve either the quasiparticle random he Reines-Cowan Experiments The Reines-Cowan Experiments

The Missing Energy and the The Desperate Remedy Neutrino Hypothesis 4 December 1930 Beta Decay and the Missing Energy During the early decades of this Gloriastr. entury, when radioactivity was first Zürich Physical Institute of the eing explored and the structure of the Federal Institute of Technology (ETH) In all types of radioactive decay, a radioactive nucleus does not only emit alpha, beta, or gamma radiation, but it also converts tomic nucleus unraveled, nuclear beta Zürich mass into energy as it goes from one state of definite energy (or equivalent rest mass M1) to a state of lower energy (or smaller ecay was observed to cause the trans- Dear radioactive ladies and gentlemen, rest mass M2). To satisfy the law of energy conservation, the total energy before and after the reaction must remain constant, so mutation of one element into another. As the bearer of these lines, to whom I ask you to listen the mass difference must appear as its energy equivalent (kinetic energy plus rest mass energy) among the reaction products. n that process, a radioactive nucleus graciously, will explain more exactly, considering the ‘false’ statistics of N-14 and Li-6 nuclei, as well as the mits an electron (or a beta ray) and Early observations of beta decay suggested that a nucleus continuous b-spectrum, I have hit upon a desperate remedy Two-Body Final State ncreases its positive charge by one to save the “exchange theorem”* of statistics and the energy decays from one state to a state with one additional unit of nit to become the nucleus of another theorem. Namely [there is] the possibility that there could positive charge by emitting a single electron (a beta ray). Helium-3 (1, 2) exist in the nuclei electrically neutral particles that I lement. A familiar example is the beta The amount of energy released is typically several million The Neutrino is Born wish to call neutrons,** which have spin 1/2 and obey the ecay of tritium, the heaviest isotope exclusion principle, and additionally differ from light quan- electron volts (MeV), much greater than the rest mass energy f hydrogen. When it undergoes beta ta in that they do not travel with the velocity of light: of the electron (0.51 MeV). Now, if a nucleus at rest decays Tritium (2, 1) Recoil✦ nucleus and ecay, tritium emits an electron and The mass of the neutron must be of the same order of magni- into two bodies—the final nucleus and the electron—the law electron Pauliseparate postulates the need for a neutrino with equal and urns into helium-3. tude as the electron mass and, in any case, not larger than of momentum conservation implies that the two must separate 0.01 proton mass. The continuous b-spectrum would then become opposite- momentum.Missing energy in continuous β spectrum The process of beta decay was with equal and opposite momentum (see top illustration). understandable by the assumption that in b decay a neutron udied intensely. In particular, is emitted together with the electron, in such a way that Thus, conservation of energy and momentum implied that the e– cientists measured the energy of the the sum of the energies of neutron and electron is constant. electron from a given beta-decay process would be emitted (N,Z) (N 1,Z+ 1) + e+?? (N, Z) (N − 1, Z + 1) + e– , mitted electron. They knew that a Now, the next question is what forces act upon the neu- with a constant energy. efinite amount of nuclear energy was trons. The most likely model for the neutron seems to me to where N = number of neutrons, and be, on wave mechanical grounds (more details are known by Z = number of protons. eleased in each decay reaction and the bearer of these lines), that the neutron at rest is a Moreover, since a nucleus is thousands of times heavier than “Dear radioactive ladies and hat, by the law of energy conservation, magnetic dipole of a certain moment m. Experiment probably an electron, its recoil velocity would be negligible compared with gentlemen, he released energy had to be shared by required that the ionizing effect of such a neutron should that of the electron, and the constant electron energy would ... I have hit upon a desperate he recoil nucleus and the electron. not be larger than that of a g ray, and thus m should prob- carry off just about all the energy released by the decay. Observed Expected ably not be larger than e.10-13 cm. electron The requirements of energy conser- spectrum of remedy to save the `exchange But I don’t feel secure enough to publish anything energies energy ation, combined with those of momen- about this idea, so I first turn confidently to you, dear The graph (center) shows the unexpected results obtained theorem’ of statistics and the energy um conservation, implied that the radioactives, with a question as to the situation concerning from experiment. The electrons from beta decay were not theorem. Namely [there is] the lectron should always carry away the experimental proof of such a neutron, if it has something emitted with a constant energy. Instead, they were emitted like about 10 times the penetrating capacity of a g ray. with a continuous spectrum of energies up to the expected possibility that there could exist in ame amount of energy (see the box Number of electrons I admit that my remedy may appear to have a small a Beta Decay and the Missing Energy” priori probability because neutrons, if they exist, would value. In most instances, some of the energy released in the the nuclei electrically neutral Energy n the facing page). That expectation probably have long ago been seen. However, only those who decay appeared to be lost. Scientists of the time wondered Endpoint of particles that I wish to call wager can win, and the seriousness of the situation of the spectrum eemed to be borne out in some experi- whether to abandon the law of energy conservation when neutrons, ...” ments, but in 1914, to the great conster- continuous b-spectrum can be made clear by the saying of my considering nuclear processes. honored predecessor in office, Mr. Debye, who told me a short ation of many, James Chadwick Pauli, 1930 while ago in Brussels, “One does best not to think about Three-Body Final State howed definitively that the electrons that at all, like the new taxes.” Thus one should earnestly Three-Body Decay and the Neutrino Hypothesis. mitted in beta decay did not have one discuss every way of salvation.—So, dear radioactives, put Pauli’s solution to the energy crisis was to propose that the 3 V. E. Guiseppe Helium-3 (1, 2) EBSS 2017 nergy or even a discrete set of ener- it to test and set it right.—Unfortunately, I cannot nucleus underwent beta decay and was transformed into three personally appear in Tübingen, since I am indispensable here ies. Instead, they had a continuous bodies: the final nucleus, the electron, and a new type of on account of a ball taking place in Zürich in the night pectrum of energies. Whenever the particle that was electrically neutral, at least as light as the Tritium (2, 1) from 6 to 7 of December.—With many greetings to you, also to Electron and lectron energy was at the maximum Mr. Back, your devoted servant, electron, and very difficult to detect (see bottom illustration). neutrino share bserved, the total energy before and Thus, the constant energy expected for the electron alone was the available energy. fter the reaction was the same, that is, W. Pauli really being shared between these two light particles, and the nergy was conserved. But in all other *In the 1957 lecture, Pauli explains, “This reads: exclusion electron was being emitted with the observed spectrum of Electron Antineutrino ases, some of the energy released in principle (Fermi statistics) and half-integer spin for an odd energies without violating the energy conservation law. he decay process appeared to be lost. number of particles; Bose statistics and integer spin for an (N, Z) (N − 1, Z + 1) + e– + ν . In late 1930, Wolfgang Pauli even number of particles.” Pauli made his hypothesis in 1930, two years before Chadwick ndeavored to save the time-honored discovered the neutron, and he originally called the new parti- This letter, with the footnote above, was printed in the September 1978 issue of aw of energy conservation by propos- Physics Today. cle the neutral one (or neutron). Later, when Fermi proposed his famous theory of beta decay (see the box “Fermi’s Theory of ng what he himself considered a Beta Decay and Neutrino Processes” on the next page), he renamed it the neutrino, which in Italian means the “little neutral one.” desperate remedy” (see the box “The **Pauli originally called the new particle the neutron (or the “neutral one”). Later, Fermi Desperate Remedy” on this page)— renamed it the neutrino (or the “little neutral one”).

Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science The Neutrino is Born

✦ Fermi’s theory of beta decay - Explained the existence of the neutrino - A weak interaction

Beta Decay: n p + e +¯ e Electron Capture: e + p n + e Inverse Beta Decay: ¯ + p n + e+ e - Correctly explained all aspects of beta decay ✦ Now, must detect a neutrino to confirm their existence 43 2 typical =1.2 10 cm ✦ Path length of 10 light-years!

4 V. E. Guiseppe EBSS 2017 The Neutrino is Detected

✦ Reines & Cowan (1956) - Exploit the unique signature of inverse beta he Reines-Cowan Experiments decay The Reines-Cowan Experiments + Inverse Beta Decay: ¯e + p n + e surface radioactivity had died away PMT PMT Antineutrino (2) sufficiently) and dig down to the tank, Incident Ionization antineutrino cascade recover the detector, and learn the truth (1) about neutrinos!” Inverse Gamma rays beta decay This extraordinary plan was actually granted approval by Laboratory PMT Neutron e– PMT Proton γ Director Norris Bradbury. Although the Gamma rays experiment would only be sensitive to n e+ neutrino cross sections of 10–40 square uv Neutron capture centimeters, 4 orders of magnitude e+ PMT γ PMT larger than the theoretical value, Terphenyl (3) Visible light Bradbury was impressed that the plan Inverse was sensitive to a cross section 3 orders beta Blue decay light of magnitude smaller than the existing Positron upper limit.1 As Reines explains in annihilation PMT PMT retrospect (unpublished notes for a talk Liquid scintillator and cadmium Liquid scintillator given at Los Alamos), “Life was much simpler in those days—no lengthy proposals or complex Pulse height review committees. It may have been Figure 3. The Double Signature of Inverse Beta Decay analyzer that the success of Operation Green- - LiquidThe new idea scintillatorfor detecting the neutrino wasdetectors to detect both products at ofHanford inverse beta Site Current house, coupled with the blessing given decay, a reaction in which an incident antineutrino (red dashed line) interacts with a Oscilloscope our idea by Fermi and Bethe, eased theandproton Savannah through the weak force. RiverThe antineutrino turns into a positron (e1), and the path somewhat!” proton turns into a neutron (n). In the figure above, this reaction is shown to take As soon as Bradbury approved the place in a liquid scintillator. The short, solid red arrow indicates that, shortly after it igure 2. Liquid Scintillation Counter for Detecting the Positron from Inverse Beta Decay plan, work started on building and- Detectedhas been created, thereactor positron encounters antineutrinos an electron, and the particle with and antiparticle a eines and Cowan planned to build a antineutrino and the proton causes the by emitting visible photons as it returns testing El Monstro. This giant liquid- annihilate each other. Because energy has to be conserved, two gamma rays are emit- ounter filled with liquid scintillator and proton to turn into a neutron and the to the ground (lowest-energy) state (3). scintillation device was a bipyramidal protonted that travel target in opposite directions and will cause the liquid scintillator to produce a ned with photomultiplier tubes (PMTs), antineutrino to turn into a positron (e1). Because the liquid scintillator is trans- tank about one cubic meter in volume. flash of visible light. In the meantime, the neutron wanders about following a random 5 he “eyes” that would detect the The neutron wanders about undetected. parent to visible light, about 20V. percent E. Guiseppe Four phototubes were mounted on each path (longer,EBSS solid 2017 red arrow) until it is captured by a cadmium nucleus. The resulting ositron from inverse beta decay, which The positron, however, soon collides of the visible photons are collected by of the opposing apexes, and the tank nucleus releases about 9 MeV of energy in gamma rays that will again cause the liquid s the signal of a neutrino-induced with an electron (e2), and the particle- the PMTs lining the walls of the was filled with very pure toluene to produce a tiny flash of visible light. This sequence of two flashes of light separated vent. The figure illustrates how the liq- antiparticle pair annihilates into two scintillation counter. The rest are activated with terphenyl so that it by a few microseconds is the double signature of inverse beta decay and confirms the id scintillator converts a fraction of the gamma rays (g) that travel in opposite absorbed during the many reflections would scintillate. Tests with radioactive presence of a neutrino. nergy of the positron into a tiny flash directions. Each gamma ray loses about from the counter walls. A visible sources of electrons and gamma rays f light. The light is shown traveling half its energy each time it scatters photon releases an electron from the proved that it was possible to “see” and its release mechanism. same in both cases. Clearly, the nuclear hrough the highly transparent liquid from an electron (Compton scattering). cathode of a phototube. That electron into a detector of almost any size. But one late evening in the fall of explosion was the best available cintillator to the PMTs, where the The resulting energetic electrons then initiates the release of further Reines and Cowan also began to 1952, immediately after Reines and approach—unless the background could hotons are converted into an electronic scatter from other electrons and radiate electrons from each dynode of the PMT, consider problems associated with Cowan had presented their plans at a somehow be further reduced. ulse that signals the presence of the photons to create an ionization cascade a process resulting in a measurable scaling up the detector. At the same Physics Division seminar, a new idea Suddenly, Reines and Cowan real- ositron. Inverse beta decay (1) begins (2) that quickly produces large numbers electrical pulse. The pulses from all the time, work was proceeding on drilling was born that would dramatically ized how to do it. The original plan had hen an antineutrino (red dashed line) of ultraviolet (uv) photons. tubes are combined, counted, the hole that would house the experi- change the course of the experiment. been to detect the positron emitted in nteracts with one of the billions and The scintillator is a highly transparent processed, and displayed on an ment at the Nevada Test Site and J. M. B. Kellogg, leader of the inverse beta decay (see Figure 2), a illions of protons (hydrogen nuclei) in liquid (toluene) purposely doped with oscilloscope screen. on designing the great vacuum tank Physics Division, had urged Reines process in which the weak interaction he molecules of the liquid. The weak terphenyl. When it becomes excited by and Cowan to review once more the causes the antineutrino to turn into a harge-changing interaction between the absorbing the uv photons, it scintillates 1H. R. Crane (1948) deduced the upper limit of possibility of using the neutrinos from positron and the proton to turn into a 10–37 square centimeters on the cross sections for a fission reactor rather than those neutron. Being an antielectron, the neutrino-induced ionization and inverse beta from a nuclear explosion. positron would quickly collide with an decay. This upper limit was based on null results pproached, we would start vacuum vacuum. For about 2 seconds, the falling detector. When all was relatively quiet, from various small-scale experiments attempting The neutrino flux from an explosion electron, and the two would annihilate umps and evacuate the tank as highly detector would be seeing the antineutri- the detector would reach the bottom of to measure the results of neutrino absorption and would be thousands of times larger than each other as they turned into pure s possible. Then, when the countdown nos and recording the pulses from them the tank, landing on a thick pile of foam from a theoretical limit deduced from the maxi- that from the most powerful reactor. energy in the form of two gamma rays eached ‘zero,’ we would break the while the earth shock [from the blast] rubber and feathers. mum amount of solar neutrino heating that could The available shielding, however, traveling in opposite directions. Each take place in the earth’s interior and still agree uspension with a small explosive, passed harmlessly by, rattling the tank “We would return to the site of with geophysical observations of the energy would make the background noise from gamma ray would have an energy llowing the detector to fall freely in the mightily but not disturbing our falling the shaft in a few days (when the flowing out of the earth. neutrons and gamma rays about the equivalent to the rest mass of the

2 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science Flavors of Neutrinos

✦ More than just electron-type neutrinos ✦ Lederman, Schwartz, Steinberger (1962) - muon-type neutrino detected in a muon beam at BNL - 10 ton Al spark chamber ✦ Fermilab experiment (2000) BNL - tau-type neutrino detected in a mixed neutrino beam at Fermilab

FermiLab

FermiLab 6 V. E. Guiseppe EBSS 2017 Neutrino Problem

✦ Ray Davis - Solar Neutrinos fusion produce neutrinos

- CCl4 target 1500-m underground in Homestake, SD - Inverse beta decay: 37 37 e + Cl e + Ar xorcising Ghosts Exorcising Ghosts e + n e + p n October 1920, Sir Arthur Edding- is particularly puzzling because scien- the result would be evidence for physics Figure 1. The Primary Neutrino- Neutrino endpoint ton, one of the foremost astrophysi- tists have failed to find errors in the beyond the Standard Model. The models Producing Reactions in the Sun 1 Reaction energy (MeV) cists of the century, delivered his standard theoretical framework of the that emerge from elementary particle p + p D + e + νe pp Nearly all the Sun’s energy comes from I 2 pp 0.42 residential address to the British Asso- Sun or in the terrestrial experiments physics, astrophysics, and cosmology p + e + p D + νe pep the fusion of protons into deuterium iation at Cardiff. In his speech, entitled monitoring the neutrinos. would be subject to a new set of con- pep 1.44 nuclei. The deuterium is converted into The Internal Constitution of the Stars,” Where have the solar neutrinos straints and would have to be modified helium-4 by following one of three reaction 7Be 0.86 e referred to a proposal suggested the gone? One intriguing answer may lie with potentially profound implications. pathways (labeled a, b, and c). Of the 8 ear before by the former president of outside our conventional understanding The status of the solar-neutrino BNL p + D 3He + γ B 15 reactions shown, four proceed via charge- he association to bore a hole into the of physics. Whereas a remedy based problem, along with how new experi- changing weak interactions (colored rust of the earth in order to discover the upon modifications in solar models ments propose to solve it, forms the 14 % boxes) and therefore produce electron onditions deep below. Motivated by the appears difficult to construct, scientists central theme of this article. Particular neutrinos. Over 95 percent of the neutri- apid progress in astronomy at the time, are particularly excited about the possi- emphasis is reserved for the Sudbury 3He + 4He 7Be + γ Too few detected!nos are created in the pp fusion reaction. ir Eddington proposed something bility that something profound may hap- Neutrino Observatory, an experiment One proton undergoes inverse beta decay, easier” to penetrate, namely, the Sun. pen to the neutrinos as they make their under construction that promises to 86 % 99 % 0.1 % creating a neutron, positron, and an elec- Eddington could scarcely have antic- way out of the Sun en route to Earth. resolve the question of whether neutrino tron neutrino. The neutron then binds to pated the ramifications of his sugges- We know of three different types, or oscillations, and in particular the MSW 7 7 2 7 7 8 the proton to form a deuteron (labeled D). Be Be + e Li + νe Be + p B + γ on. After more than seventy-five years flavors, of neutrinos—the electron, effect, are responsible for the observed Other neutrino-producing reactions are f study, the scientific community’s muon, and tau neutrinos. We also know shortfall of solar neutrinos. pep (electron capture), 7Be (electron cap- nvestigations of our closest star have that the nuclear reactions that power the ture), and 8B (beta decay). Notice that 7Be ielded a remarkably detailed under- Sun are energetic enough to produce 3 3 4 7 4 8 4 1 8B is needed to produce 8B (dashed box). He + He He + 2p Li + p 2 ( He) B 2( He) + e + νe anding of what makes the Sun shine. only electron neutrinos. Moreover, Neutrinos from the Sun Modern experiments, however, observe (a) (b) (c) We now know that the Sun is powered existing experiments that detect solar neutrinos from the pp reaction and 8B y thermonuclear fusion and that its hot neutrinos are only sensitive to the elec- Given the enormous power produced Los Alamos Science, no. 25 (1997) decay, but hardly any 7from 7Be decay. ore can be considered an immense fur- tron flavor. One can thus speculate that V. byE. the Guiseppe Sun and its twenty-billion-year EBSS 2017 ace producing not only heat and light, some of the electron neutrinos produced lifetime, it is a steadfast conclusion that Figure 2. Solar-Neutrino Spectrum ut also vast numbers of neutrinos. in the Sun have transformed, or the Sun produces energy via thermonu- The total integrated flux of all solar neutri- Because of the Sun’s enormous size, oscillated, into muon and/or tau neutri- clear fusion. During the late 1920s and Range of Sensitivity nos reaching the earth is about 65 billion he light produced deep in its interior nos as they make their way to Earth, early 1930s, theoretical calculations, 0.23 0.8 7.0 per square centimeter per second. In this akes tens of years to reach its surface. thereby escaping our terrestrial detectors. including the seminal work of a young Gallium figure, the neutrino flux and energy are Chlorine During that lengthy journey, the pho- The probability for oscillations to occur Hans Bethe, elucidated our understand- plotted on log scales; so, for example, 1011 Kamiokande ons that rain down upon us as sunlight may even be enhanced in the Sun in an ing of the details of these processes. pp the pp flux is about 50 times greater than nd make our existence on Earth energy-dependent and resonant manner As shown in Figure 1, the fusion of the 7Be flux. Also shown are the spectra ustainable lose all the information as neutrinos emerge from the dense core. protons into helium proceeds via three 7 of neutrinos produced from the CNO

/s) Be oncerning the detailed processes of This phenomenon, an example of the branches. Neutrinos are created in four 2 109 cycle (gray curves). The pp, 8B, and CNO 13 he stellar core. Unlike photons, neutri- Mikheyev, Smirnov, and Wolfenstein different reactions, referred to simply as CNO ( N) neutrinos are created in beta decay reac- os interact so feebly with matter that (MSW) effect, is considered by many the pp, pep, beryllium-7 (7Be), and pep tions. A neutrino so produced shares 8 hey escape from the Sun in about scientists to be the most favored solution boron-8 ( B) reactions. The neutrinos 7 energy with another light particle. Hence, 10 15 seconds. They arrive on Earth a mere to the solar-neutrino problem. flee the Sun and begin their voyage to CNO ( O) all those neutrinos have a broad energy minutes later, and thus the solar Neutrino oscillations, or the periodic Earth. (In Figure 1, we have omitted spectrum. The 7Be and pep neutrinos eutrinos are a unique probe of a star’s changes in neutrino flavor, require that neutrinos that emerge from the carbon- 8B result from electron capture: A proton in 105 CNO (17F) nnermost regions and of the nuclear neutrinos possess mass and that neutrino nitrogen-oxygen, or CNO, cycle. The a nucleus captures an electron from an Neutrino Flux (number /cm eactions that fuel them. flavor not be conserved in nature. No cycle is another, though less important, atomic orbital, turns into a neutron, and During the past thirty years, detailed undebated evidence for neutrino mass set of neutrino-producing reactions a monoenergetic neutrino is created. 3 heoretical and experimental studies exists despite years of painstaking in the Sun.) 10 The sensitivity range of the various solar- ave resulted in very precise predic- research around the world. Indeed, the Figure 2 shows the predictions of the neutrino experiments is also shown here. hep ons about the fluxes and energy spec- Standard Model of elementary particles standard solar model for the flux of The gallium experiments have energy a of neutrinos produced deep within requires that neutrinos be strictly mass- electron neutrinos at the earth’s surface. thresholds around 0.23 MeV and are sen- he Sun. But a problem has emerged. less. Nonetheless, quests for a Grand The flux is the number of neutrinos per 0.1 1.0 10.0 sitive to all solar neutrinos. The chlorine our different experiments have mea- Unified Theory of the fundamental square centimeter per second. (The fig- Neutrino Energy (MeV) experiment detects neutrinos from the ured the flux of solar neutrinos, and forces in nature suggest that neutrinos, ure assumes no electron neutrinos have 7Be and CNO reactions, but is primarily very one of them reports a flux that is like other elementary particles, should oscillated into a different flavor.) The pp sensitive to those from the 8B reaction. gnificantly below theoretical predic- have mass. Consequently, should the reaction is the primary mode of neutrino Kamiokande is a water Cerenkov detector ons. The discrepancy is referred to solar-neutrino problem be resolved by production, and the reaction completely that can detect only the high-energy s the solar-neutrino problem, and it invoking neutrino mass and oscillations, dominates energy production in the Sun. portion of 8B neutrinos.

38 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science xorcising Ghosts Exorcising Ghosts

able I. Summary of Pioneering Solar-Neutrino Experiments Figure 3. The Modern Solar-Neutrino Problem One can deduce how the theoretical neutrino flux needs to be distorted in order to match the experimental results. In their analysis, SAGE + GALLEX Chlorine Kamiokande Hata and Langacker (1994) constructed an arbitrary solar model in which the neutrino fluxes are allowed to vary freely instead of being tied to nuclear physics or to astrophysics. The only constraint is the one imposed by the solar luminosity, namely, that the sum of the pp, 7Be, and CNO fluxes roughly equals 6.57 3 1010 neutrinos per square centimeter per second (the total neutrino flux). Target Material 71Ga 37Cl H O 2 The model is then “fit” to the combined data from all experiments. 71 71 2 37 37 2 2 2 Reaction ne + Ga → Ge + e ne 1 Cl → Ar 1 e n 1 e → n 1 e Source of The model that best fits the data is one in which the pp flux is Detection Method Radiochemical Radiochemical Cerenkov Neutrinos F/FSSM identical with the standard-solar-model (SSM) prediction, the Detection Threshold 0.234 MeV 0.814 MeV 7.0 MeV 7Be flux is nearly absent, and the 8B flux is only 40 percent of Neutrinos Detected All 7Be and 8B 8B pp 1 the SSM prediction. These results are presented in the table 7Be 0 (left) as the ratio of F, the flux derived from the combined fit, to 8B 0.4 FSSM, which is the neutrino flux predicted by the SSM. Predicted Rate 132 6 7 SNU* 9 6 1 SNU 5.7 6 0.8 flux units** Observed Rate 74 6 8 SNU 2.5 6 0.2 SNU 2.9 6 0.4 flux units 1.0 SSM 90% C.L.

*1 SNU = 10–36 captures per target atom per second. The 90 percent confidence level for the combined fit is **In units of 106 neutrinos per square centimeter per second. 0.8 shown in blue on this graph of 8B flux versus 7Be flux (each normalized to the SSM predictions). The 90 percent confi- Tc ach column summarizes an experiment and compares the predicted rate of neutrino interactions (based on the Bahcall- dence level for the Bahcall-Pinsonneault SSM is shown at the insonneault standard solar model) to the observed rate. The radiochemical experiments report their results in SNU, a convenient upper right-hand corner. Filling that contour are the results of

SSM 0.6 nit that facilitates comparison between experiments. Kamiokande reports results in flux units. Every experiment shows a significant Combined fit 1,000 Monte Carlo simulations (green dots) that vary the para- B)

Neutrino Problem Confirmation 8 90% C.L. eficit in the observed versus the predicted rate. ( meters of the SSM. The square markers indicate the results φ of numerous nonstandard solar models, which include, for

B)/ 0.4 example, variations in reaction cross sections, reduced 8

✦ (

Kamioka detector able II. Breakdown of the Predicted Rate by Neutrino-Producing Reaction φ heavy-element abundances, reduced opacity models, and even weakly interacting massive particles. Most of the models - Water Cerenkov Detector, 3,000 tonsNeutrino Reaction SAGE + GALLEX Chlorine Kamiokande Based on the standard solar model, the 0.2 call for a power law relation between the 8B and 7Be fluxes total predicted rate of neutrino events (the curve labeled Tc). As the figure shows, the SSM and all pp 70 SNU 0 SNU 0 can be broken down into contributions nonstandard models are completely at odds with the best fit - Elastic scattering pep 3 SNU 0.2 SNU 0 from each of the neutrino-producing to the combined experimental results. 0.0 7Be 38 SNU 1.2 SNU 0 reactions in the Sun. This information is 0.0 0.2 0.4 0.6 0.8 1.0 8 Predicted rates B 16 SNU 7.4 SNU 5.7 listed in each column (rounded to the φ(7Be)/φ(7Be)SSM + e + e nearest SNU) and is displayed as a bar e e CNO SAGE+10 SNU 0.5 SNU Kamio-0 Chlorine graph. (The bars corresponding to the otal Predicted Rate GALLEX 132 6 7 SNU 9 6 1 SNU 5.7kande6 0.8 total predicted rate have been normalized shown in Figure 3. Compared with the Figure 1). Hence, if there are no 7Be dependent, resonant fashion when to 1.) Each colored segment within a bar solar-model predictions, the pp neutrino neutrinos, why are any 8B neutrinos neutrinos travel through dense matter. ✦ Observed Rate 74 6 8 SNU 2.5 6 0.2 SNU 2.9 6 0.4 Gallium Experiments: SAGE and GALLEX corresponds to a specific reaction. flux, with a maximum neutrino energy observed? While modifications to The muon or tau neutrinos would not pep pep Kamiokande observed approximately of 0.42 MeV, seems to be present in the solar models have been attempted be detected in the existing experiments 1.0 half of the expected flux of 8B neutrinos. - radiochemical experiment using Ga CNO CNO full strength. The intermediate-energy by many authors, it appears extremely on Earth, and hence a deficit would be All the neutrinos detected by the chlorine 7Be neutrinos, however, seem to be difficult to render an astrophysical seen in the solar-neutrino flux. For 7Be experiment can likewise come from the missing entirely, while only 40 percent explanation that would solve this puz- suitable choices of neutrino masses and - inverse beta decay 0.8 7 Be 8B reaction. The solar luminosity of the high-energy 8B neutrinos zle. As seen in Figure 3, no model has mixing angles, experiments would essentially fixes the rate of pp neutrinos are observed. successfully reduced the 7Be flux with- measure the full, predicted flux of pp 0.6 that SAGE and GALLEX must see. This energy-dependent suppression out reducing the 8B flux even more! neutrinos, the 7Be flux would be highly 71 71 8B 8B Those experiments are consistent with of the solar-neutrino spectrum However, this pattern for the solar- suppressed, and the measured flux of + Ga e + Ge 8B e BNL an observation of the full pp flux plus establishes what we now refer to as the neutrino spectrum is perfectly explained 8B neutrinos would be reduced to 0.4 some of the 8B flux. Taken together, modern solar-neutrino problem. It is by the mechanism of matter-enhanced 40 percent! (See Figure 4.) ed v ed ✦ v the experiments indicate that the particularly puzzling given the apparent neutrino oscillations, or the MSW effect. Have three decades of solar-neutrino All three sensitive to separate and pp solar-neutrino deficit results from a lack of 7Be neutrinos. At a glance, this (See the article “MSW” on page 156. ) research culminated in the discovery of ed

0.2 Obser v overlapping regions of the ν spectrum Obser lack of intermediate-energy (CNO, 7Be, might imply that 7Be is not being MSW suggests that the probability for neutrino mass? Our interpretation of the and pep) neutrinos. produced in the sun. But those nuclei neutrino oscillations to occur in vacuo modern solar-neutrino problem relies Obser 0 are needed to produce 8B (refer to can be augmented in an energy- upon our confidence that the standard Los Alamos Science, no. 25 (1997) 42 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science

8 V. E. Guiseppe EBSS 2017 Neutrinos Found

✦ SuperKamiokande (1998) - Water Cerenkov detector, 50,000 tons - atmospheric neutrinos Charged Current: + N l + X l ✦ SNO Detector - Water Cerenkov and neutron detection - Sensitive to all flavors of neutrinos BNL Elastic Scattering: ⌫ + e ⌫ + e i ! i Charged Current: ⌫ + d e + p + p e ! Neutral Current: ⌫ + d ⌫ + n + p i ! i ✦ No longer missing neutrinos (if all flavors counted)

9 V. E. Guiseppe EBSS 2017 2015 Nobel Prize in Physics

"The Nobel Prize in Physics 2015". Nobelprize.org. Nobel Media AB 2014. Web. 25 Jul 2017.

V. E. Guiseppe EBSS 2017 10 he Oscillating Neutrino The Oscillating Neutrino he Oscillating Neutrino The Oscillating Neutrino Component Neutrino” on page 32). dependence predicted by the MSW family-number and muon-family- Figure 13. Three Types of Evidence for Neutrino Oscillations n that theory, particle-antiparticle effect (see the articles “Exorcising number conservation laws, the electron (a) Solar neutrinos—a disappearance experiment. The flux of electron neutrinos produced scillations could not occur. Ghosts” on page 136 and “MSW” antineutrino was detected through its in the Sun’s core was measured in large underground detectors and found to be lower than Component Neutrino” on page 32). dependence predicted by the MSWon page 156family-number). and muon-family-charged-current interaction withFigure matte r13., Three Types of Evidenceexpected. for The Neutrino “disappearance” Oscillations could be explained by the oscillation of the electron neutrino n that theory, particle-antiparticleolar Neutrinos.effect (seeIn 1963, the articles after “Exorcising number conservation laws, the electronthat is, through inverse beta dec(a)ay .Solar neutrinos—a disappearanceinto experiment.another flavor. The flux of electron neutrinos produced scillations could not occur. Lederman, Steinberger,Ghosts” on page and Schwartz136 and “MSW”Atmospheric antineutrino Neutrinos. was In detected1992, through itsRecently, members of the LSNDin the Sun’s core was measured in large underground detectors and found to be lower than howed thaton there page were 156). two distinct another neutrinocharged-current deficit was interaction seen—this withcollaboration matter, reported a second expected.positive The “disappearance” could be explained by the oscillation of the electron neutrino Sun olar Neutrinos. In 1963, afterfl avors of neutrino, the idea of oscilla- time in the thatratio is, of through muon neutrinos inverse betato decay.result. This time, they searched intofor anotherthe flavor. Lederman, Steinberger, and Schwartzon betweenAtmospheric electron neutrinos Neutrinos. and In 1992,electron neutrinosRecently, produced members at the of top the LSNDoscillation of muon neutrinos rather Earth howed that there were two distinctmuon neutrinosanother surfaced neutrino for defithe citfirst was seen—thisof the earth’scollaboration atmosphere. reported When high-a second positivethan muon antineutrinos. The muon flavors of neutrino, the idea of oscillame. This- possibilitytime in the requires ratio of mixingmuon neutrinosenergy to cosmicresult. rays, This mostly time, protons, they searched forneutrinos the are only produced during pion Sun Solar on between electron neutrinos andcross the leptonelectron families neutrinos as well produced as at thestrike top nucleioscillation in the upper of muon atmosphere, neutrinos ratherdecay-in-flight, before the pions reach Earth Underground core Primary neutrino source 8 muon neutrinos surfaced for the fionzerorst neutrinoof the masses.earth’s atmosphere.In 1969, it Whenthey high- producethan pions muon and antineutrinos. muons, which The muonthe beam stop. Therefore, these neutri- ~10 kilometers νe detector me. This possibility requires mixingwas decidedenergy that the cosmic idea ofrays, neutrino mostly protons,then decay neutrinosthrough the are weak only forceproduced and duringnos pionhave a higher average energy than p + p D + e+ + Solar νe cross the lepton families as wellscillation as wasstrike worth nuclei testing. in the The upper Sun atmosphere,produce muondecay-in-fl and electronight, before neutrinos. the pions thereach muon antineutrinos measured in the Underground core Primary neutrino source 8 onzero neutrino masses. In 1969,s known it tothey drench produce us with pions low-energy and muons, Thewhich atmosphericthe beam neutrinos stop. Therefore,have very these earlierneutri- experiment. The muon neutrinos ~10 kilometers νe detector was decided that the idea of neutrinolectron neutrinosthen decay that throughare produced the weak in forcehigh and energies,nos ranging have a higherfrom hundreds average energywere than observed to turn into electron + Other sources of neutrinos: p + p D + e + νe scillation was worth testing. Thehe Sun thermonuclearproduce furnace muon andat its electron core, neutrinos.of million electronthe muon volts antineutrinos (MeV) to measuredtens neutrinos in the at a rate consistent with the Flavor Oscillation Cosmic-ray – 7 7 s known to drench us with low-energys shown inThe Figure atmospheric 13(a). By neutrinos using have ofvery giga-electron-volts,earlier experiment. depending The on muon the neutrinosrate for antineutrino oscillation reported e + Be Li + νe shower 8 4 + lectron neutrinos that are producedtandard in astrophysicshigh energies, models ranging about from hundredsenergy of thewere incident observed cosmic to turn ray intoand electronearlier. Since the two experiments Other sources of neutrinos: B 2 He + e + νe he thermonuclear furnace at its core,tellar processesof million and the electron observed volts value (MeV) onto tenshow thisneutrinos energy is at shared a rate amongconsistent the withinvolved the different neutrino energies and Cosmic-ray π0 ✦ e– + 7Be 7Li + + s shown in Figure 13(a). By usingf the Sun’sof luminosity, giga-electron-volts, theorists candependingfragments on the ofrate the for initial antineutrino reaction. oscillation As reporteddifferent reactions to detectFlavor the oscillation:νe shower π 8B 2 4He + e+ + tandard astrophysics models aboutredict the sizeenergy of theof the neutrino incident flux. cosmic But rayshown and in Figureearlier. 13(b), Since the the decay two experimentsof oscillations, the two results are indeed (b)νe Atmospheric neutrinos—a disappearance tellar processes and the observedmeasurements value on howof the this solar-neutrino energy is shared flux amongpions the to muonsinvolved followed different by the neutrino decay energiesindependent. and The fact that the two experiment. Collisions between high-energy π0 + - a neutrino is born as one flavor ~30 kilometers+ µ f the Sun’s luminosity, theoristsresent can an intriguingfragments puzzle:of the initialA signifi reaction.- of As muons todifferent electrons reactions produces to twodetect the results confirm one another is therefore protons and nuclei in the upper atmosphere can π redict the size of the neutrino flantux. fractionBut shown of those in Figureelectron 13(b), neutrinos the decaymuon of neutrinososcillations, for every the electron two results neu -are indeedmost signifi cant. The complete story(b) Atmospheric of neutrinos—a disappearancecreate high-energy pions. The decay of those andexperiment. detected Collisions between high-energyas another + measurements of the solar-neutrinopparently flux disappearpions to muons before followed reaching by the trino.decay But theindependent. measured ratioThe factof these that the twoLSND can be found in the article pions followed by~30 the kilometers decay of the resulting µ + resent an intriguing puzzle: A signifiur terrestrial- of detectors.muons to electronsRay Davis produces two types isresults much confismallerrm one(see anotherthe arti -is therefore“A Thousand Eyes” on page 92. protons and nuclei in the upper atmospheremuons producescan twice as many muon-type e ant fraction of those electron neutrinosmade the firstmuon observation neutrinos of for a everyneutrino electroncle neu “The- Evidencemost signifi for Oscillations”cant. The complete on storyEach of type of experiment showncreate in high-energy pions. The decay neutrinosof those (blue) as electron-type neutrinos he Oscillating Neutrino - Requires that they have mass The Oscillatingνµ Neutrino pparently disappear before reachinghortfall at thetrino. Homestake But the measured Mine in ratio of pagethese 116). TheLSND oscillation can be found of muon in the articleFigure 13, when interpreted as anpions followed by the decay of the resulting(red). But underground neutrino detectors νe + ν ur terrestrial detectors. Ray Davisouth Dakota,two and types all isexperiments much smaller (see theneutrinos arti- into“A tauThousand neutrinos Eyes” appears on page to 92.oscillation experiment,✦ yields informamuons- produces twice as many muon-typedesigned to measure both types see ae much µ made the first observation of a neutrino cle “The Evidence for Oscillations” on Each type of experiment shown in Verifiedneutrinos (blue) and as electron-type constrained neutrinos by solar, ince have confirmed it. Today, the be the simplest explanation. tion about the oscillation amplitude and smaller ratio than 2 to 1. The oscillation of νµ hortfall at the Homestake Minemost in plausiblepage explanation 116). The oscillationof the solar- of muon Figure 13, when interpreted as anwavelength. One can therefore deduce(red). But underground neutrino detectorsmuon neutrinos into tau neutrinos couldνe νµ Atmospheric neutrino source Component Neutrino”outh Dakota, on and page all 32).experiments eutrinodepen dpuzzleenneutrinosce pliesred ini cintot ethed tau boscillationy tneutrinoshe MS Wof appears Accelerator tofamilyoscillation -Neutrinos.number experiment,an Td hme uloonne- fyieldsamily -informainformation- about the sizesatmospheric, of Figureneutrinodesigned 13. to Threemeasure Types both reactor,types of seeEvidence explaina much that andfor defi Neutrinocit. Oscillations ince have confirmed it. Today, the be the simplest explanation. tion about the oscillation amplitude and smaller ratio than 2 to 1. The oscillation of + + n that theory, particle-antiparticle lectroneffect neutrinos(see the aintortic lotheres “E typesxorci sofing accelenruatmorb-ebra sceodn esxeprveraitmioenn tl awwitsh, the elecmassestron and lepton-family mixing(a) paraSolar- neutrinos—a disappearance experiment. The flux of electron neutrinos produced π µ + νµ most plausible explanation of the solar- wavelength. One can therefore deduce muon neutrinos into tau neutrinos could scillations could not occur. eutrinos.Ghosts ”Although on page the136 measuredand “MS shortW” - evidenacnet ifnoeru ntreiuntori nwoa os sdceiltleactitoends tihsrough imeters.ts The specific relationshipsacceleratorin the are Sun’s core was experimentsmeasured in large underground detectors and found to be lower than Atmospheric neutrino source e+ + ν + ν eutrino puzzle lies in the oscillation of Accelerator Neutrinos. The lone information about the sizes of neutrino explain that deficit. e µ allo isn plargeage 1and56 )the. expected amplitude LSNDc.h Tahrgise de-xcpuerrriemnetn itn uteseras ctthieo nh iwghit-h maexplainedtter, in the next section. expected. The “disappearance” could be explained by the oscillation of the electron neutrino – – + + + π µ + ν olar Neutrinos.lectronIn neutrinos 1963, after into other typesor ofneutrinoaccelerator-based oscillations in a vacuumexperiment withint enstihtya tp irsmasseso,t othn rboeu aandgmh filepton-familyrnovmer sthee b leintae a drmixingecay. para- into another flavor. π µ νµ µ eutrinos. Although the measured short- evidence for neutrino oscillations is meters. The specific relationships are - appearance and disappearance e+ + + e– + + Lederman, Steinberger, and Schwartz s small,Atmospheric neutrino Neutrinos.oscillations canIn 1992, acceleratoRecently,r at the L omemberss Alamo sof N etheutr oLSNDn Underground νe νµ νe νµ all is large and the expected amplitude LSND. This experiment uses the high- explained in the next section. howed that there were two distinct tillanother explain neutrino the shortfall defi citthrough was seen—thisScienccollaboratione Center (LA NreportedSCE) to ag secondenerate positiveThe Mechanics of Oscillation ν , ν , ν–, ν – e e π µ µ µ + νµ or neutrino oscillations in a vacuumhe MSW effect.intensity proton beam from the alinearn intense source of neutrinos with Sun detector flavors of neutrino, the idea of oscilla- time in the ratio of muon neutrinos to result. This time, they searched for the e– + + s small, neutrino oscillations can accelerator at the Los Alamos Neutron Underground νe νµ on between electron neutrinos and electronNamed a fneutrinoster Mikhe yproducedev, Smirn oatv , theand topaveragoscillatione energies ofof amuonbout 5neutrinos0 MeV. rather Oscillation, or the spontaneous peri- Earth till explain the shortfall through Science Center (LANSCE) to generate The Mechanics of Oscillation ν , ν , ν , ν muon neutrinos surfaced for the first Wooflfe thenste earth’sin, the M atmosphere.SW effect d eWhenscribe shigh-In 199than5, th muone LSN Dantineutrinos. collaboration The muonodic change from one neutrino mass (c) LSND—an appearance experiment.e ePositiveµ µ pions decay at rest into positive muons, he MSW effect. ow electronan n eintenseutrinos ,source throug ofh tneutrinosheir withrepo rted positive signs of neutrino state to another, is a spectacular exam- which then decay into muon antineutrinos,detector positrons, and electron neutrinos. Negative pions me. This possibilityNamed after requires Mikheyev, mixing Smirnov,energy and cosmicaverage rays, energies mostly of about protons, 50 MeV. neutrinosOscillation, are only producedor the spontaneous during pion peri- nteractions with electrons in solar mat- oscillations. An excess of 22 electron ple of quantum mechanics. A neutrino decaySolar and produce electron antineutrinos, but that rate is almost negligible. A giant liquid- cross the leptonWolfenstein, families the asMSW well effect as describesstrike nucleiIn 1995, in the the upper LSND atmosphere, collaboration decay-in-flodic changeight, before from one the neutrinopions reach mass (c) LSND—an appearance experiment. Positive pions decay at rest into positive muons, Underground er, can dramatically increase their antineutrino events over background produced through the weak force in, Primary neutrino source scintillatorcore neutrino detector located 30 meters downstream looks for the appearance of onzero neutrino masses. In 1969, it they produce pions and muons, which the beam stop. Therefore, these neutri- which then decay into muon antineutrinos, positrons, and electron neutrinos.~10 Negative8 kilometers pions detector ow electron neutrinos, through theirntrin sic oscreportedillation p positiverobabilit ysigns as th ofey neutrinowa s observestated. T tohe yanother were ,i niste arp spectacularreted as say,exam muon- decay, is described as the electron antineutrinos as the signal that the muon antineutrinos have oscillated into that flavor. νe was decidednteractions that the idea with of electrons neutrino in solar matthen- decayoscillations. through theAn excessweak force of 22 andelectronnos haveple of a quantumhigher average mechanics. energy A neutrino than decay and produce electron+ antineutrinos, but that rate is almost negligible. A giant liquid- ravel from the solar core to the surface. evidence for the oscillation of muon sum of two matter waves. As the p + p D + e + νe scillation waser, can worth dramatically testing. Theincrease Sun theirTh iproduces matter antineutrinoemuonnhanc eandme nelectron tevents of neu overt rneutrinos.ino backgroundantinetheutri nmuonoproduceds, in tantineutrinoso el ethroughctron a nthet imeasuredn eweakutri- force inneutrino thein, travels through space (andscintillator neutrino detector located 30 meters downstream looks for the appearance of Muons and electrons Neutrinos , , and ν detector s known to ntrinsicdrench oscillation us with low-energy probability as theyThe atmosphericwas observed. neutrinos They havewere veryinterpreted earlieras say, experiment. muon decay, The is muondescribed neutrinos as the electron antineutrinos as the signal that the muon antineutrinos have oscillated into that flavor. νe νµ νµ e scillations varies with neutrino energy nos (see Figure 13c). The muon anti- depending on which masses are Pions lectron neutrinosravel from that the are solar produced core to in the surface.ndhigh mat tenergies,er devidenceensity .ranging T hfore n theex fromt oscillationgene hundredsration of muonneutriwerenos h asumobservedd b eofen two p rtoo dmatter uturnced intoawaves.t th eelectron As the measured), these matter waves interfere Other sources of neutrinos: Proton This matter enhancement of neutrino antineutrinos, into electron antineutri- neutrino travels through space (and he thermonuclear furnace at its core, f sofol amillionr-neutri nelectrono experi mvoltsents (MeV)is speci fito- tensacceleneutrinosrator targ eatt t har orateugh consistent antimuon with thewith each other constructively or de- beam Muons and electrons Neutrinos ν , ν , and ν Cosmic-rνe adetectory – 7 7 e µ µ scillations varies with neutrino energy nos (see Figure 13c). The muon anti- depending on which masses are e + Be Li + ν Pions Water Copper beam stopshower s shown in Figure 13(a). By using alofly dgiga-electron-volts,esigned to explore w dependinghether the on thedecayrate-at-r eforst. Aantineutrinos in the exp oscillationeriments reportedstructively. For example, the interfer- Proton e nd matter density. The next generation neutrinos had been produced at the measured), these matter waves interfere 8 4 + target tandard astrophysics models about leenergyctron ne ofutr itheno dincidenteficit ha scosmic the ene rraygy anddescriearlier.bed ear lSinceier to sthetud ytwo ele cexperimentstron- ence causes first the disappearance and B 2 He + e + νe f solar-neutrino experiments is specifi- accelerator target through antimuon with each other constructively or de- beam 30 meters 0 tellar processes and the observed value on how this energy is shared among the involved different neutrino energies and Copper beam stop π ally designed to explore whether the decay-at-rest. As in the experiments structively. For example, the interfer- Water + f the Sun’s luminosity, theorists can 8 fragments of the initial reaction. As different reactions to detect the Los Alamos Science Number 25 1997 target Number 25 1997 Los Alamos Science π lectron neutrino deficit has the energy described earlier to study electron- ence causes first the disappearance and 30 meters redict the size of the neutrino flux. But shown in Figure 13(b), the decay of oscillations, the two results are indeed (b) Atmospheric neutrinos—a disappearance Los Alamos Science, no. 25 (1997) measurements of the solar-neutrino flux pions to muons followed by the decay independent. The fact that the two experiment. Collisions between high-energy + 8 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science ~30 kilometers µ 11 resent an intriguing puzzle: A signifi- of muons to electrons produces two results confirm one another is therefore V. E. Guiseppeprotons and nuclei in the upper atmosphere can EBSS 2017 ant fraction of those electron neutrinos muon neutrinos for every electron neu- most significant. The complete story of create high-energy pions. The decay of those pparently disappear before reaching trino. But the measured ratio of these LSND can be found in the article pions followed by the decay of the resulting + ur terrestrial detectors. Ray Davis two types is much smaller (see the arti- “A Thousand Eyes” on page 92. muons produces twice as many muon-type e made the first observation of a neutrino cle “The Evidence for Oscillations” on Each type of experiment shown in neutrinos (blue) as electron-type neutrinos νµ hortfall at the Homestake Mine in page 116). The oscillation of muon Figure 13, when interpreted as an (red). But underground neutrino detectors νe ν outh Dakota, and all experiments neutrinos into tau neutrinos appears to oscillation experiment, yields informa- designed to measure both types see a much µ ince have confirmed it. Today, the be the simplest explanation. tion about the oscillation amplitude and smaller ratio than 2 to 1. The oscillation of most plausible explanation of the solar- wavelength. One can therefore deduce muon neutrinos into tau neutrinos could Atmospheric neutrino source eutrino puzzle lies in the oscillation of Accelerator Neutrinos. The lone information about the sizes of neutrino explain that deficit. + + lectron neutrinos into other types of accelerator-based experiment with masses and lepton-family mixing para- π µ + νµ eutrinos. Although the measured short- evidence for neutrino oscillations is meters. The specific relationships are e+ + + νe νµ all is large and the expected amplitude LSND. This experiment uses the high- explained in the next section. – – or neutrino oscillations in a vacuum intensity proton beam from the linear π µ + νµ e– + + s small, neutrino oscillations can accelerator at the Los Alamos Neutron Underground νe νµ till explain the shortfall through Science Center (LANSCE) to generate The Mechanics of Oscillation νe, νe, νµ, νµ he MSW effect. an intense source of neutrinos with detector Named after Mikheyev, Smirnov, and average energies of about 50 MeV. Oscillation, or the spontaneous peri- Wolfenstein, the MSW effect describes In 1995, the LSND collaboration odic change from one neutrino mass (c) LSND—an appearance experiment. Positive pions decay at rest into positive muons, ow electron neutrinos, through their reported positive signs of neutrino state to another, is a spectacular exam- which then decay into muon antineutrinos, positrons, and electron neutrinos. Negative pions nteractions with electrons in solar mat- oscillations. An excess of 22 electron ple of quantum mechanics. A neutrino decay and produce electron antineutrinos, but that rate is almost negligible. A giant liquid- er, can dramatically increase their antineutrino events over background produced through the weak force in, scintillator neutrino detector located 30 meters downstream looks for the appearance of ntrinsic oscillation probability as they was observed. They were interpreted as say, muon decay, is described as the electron antineutrinos as the signal that the muon antineutrinos have oscillated into that flavor. ravel from the solar core to the surface. evidence for the oscillation of muon sum of two matter waves. As the This matter enhancement of neutrino antineutrinos, into electron antineutri- neutrino travels through space (and Neutrinos , , and ν detector Muons and electrons νe νµ νµ e scillations varies with neutrino energy nos (see Figure 13c). The muon anti- depending on which masses are Pions nd matter density. The next generation neutrinos had been produced at the measured), these matter waves interfere Proton f solar-neutrino experiments is specifi- accelerator target through antimuon with each other constructively or de- beam ally designed to explore whether the decay-at-rest. As in the experiments structively. For example, the interfer- Water Copper beam stop target lectron neutrino deficit has the energy described earlier to study electron- ence causes first the disappearance and 30 meters

8 Los Alamos Science Number 25 1997 Number 25 1997 Los Alamos Science Neutrino Mixing Flavor states are linear combinations of the mass states e Ue1 Ue2 Ue3 1 Neutrino Neutrino = U U U µ µ1 µ2 µ3 2 Masses Flavors U1 U2 U3 3 ✦ Mixing via the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix ✦ Under the 3 neutrino model,

- the mixing matrix described by 3 angles θij 2 - Oscillation probability determined by mass squared differences mij - CP violating phase and Majorana phase By convention:

m2 = m2 m2 > 0 Small and positive (solar scale) 21 2 1 m2 = m2 m2 Larger and sign unknown (atmospheric scale) 31 3 1

V. E. Guiseppe EBSS 2017 12 Neutrino Mixing

✦ Rich program of measuring the oscillation parameters

✦ Example: the search for θ13 - Data Bay Reactor Experiment ‣ ⌫¯ e disappearance through inverse beta decay in scintillating target

PHYSICAL REVIEW D 95, 072006 (2017)

V. E. Guiseppe EBSS 2017 13 Neutrino Mixing Flavor states are linear combinations of the mass states

iCP ⌫e c12c13 s12c13 s13e 10 0 ⌫1 ↵21 iCP iCP i ⌫µ = s12c23 c12s23s13e c12c23 s12s23s13e s23c13 0 e 2 0 ⌫2 ↵ 0 1 0 iCP iCP 1 0 i 31 1 0 1 ⌫⌧ s12c23 c12s23s13e c12c23 s12s23s13e s23c13 00e 2 ⌫3 @ A @ A @ A @ A (where c12 = cos ✓12,etc)

Global Fit to oscillation parameters

2 2 2 5 2 m = m m =7.37 10 eV 2 1 ⇥ 2 2 2 2 m2 + m1 3 2 m = m =2.525 10 eV | | 3 2 ⇥ 2 sin ✓12 =0.297 2 sin ✓23 =0.425 2 sin ✓13 =0.0215

Capozzi et al. PhysRevD 95, 096014 (2017)

14 V. E. Guiseppe EBSS 2017 Normal Inverted We Have More to Learn About Neutrinos

✦ What we DON’T know about neutrinos - How massive are they? ‣ What is the absolute scale? - Which one is the heaviest? ‣ Which hierarchy is correct? ? ? - Are they their own anti- particle? ‣ Can e =¯e L =0 - Is Lepton # violated =¯ ¯ - Is there Leptonic CP- µ e µ e invariance violation

15 V. E. Guiseppe EBSS 2017 We Have More to Learn About Neutrinos

Beta decay endpoint ✦ What we DON’T know about neutrinos - How massive are they? ‣ What is the absolute scale? - Which one is the heaviest? ‣ Which hierarchy is correct? KATRIN - Are they their own anti- particle? ‣ Can e =¯e - Is Lepton # violated - Is there Leptonic CP- invariance violation

16 V. E. Guiseppe EBSS 2017 We Have More to Learn About Neutrinos

Long Baseline Neutrino ✦ What we DON’T know about Oscillation neutrinos - How massive are they? ‣ What is the absolute scale? - Which one is the heaviest? ‣ Which hierarchy is correct? - Are they their own anti- particle? ‣ Can e =¯e - Is Lepton # violated - Is there Leptonic CP- invariance violation

17 V. E. Guiseppe EBSS 2017 We Have More to Learn About Neutrinos

✦ What we DON’T know about neutrinos Neutrinoless double-beta decay - How massive are they? ‣ What is the absolute scale? - Which one is the heaviest? ‣ Which hierarchy is correct? - Are they their own anti- particle? ‣ Can e =¯e - Is Lepton # violated - Is there Leptonic CP- invariance violation

18 V. E. Guiseppe EBSS 2017 Two Neutrino Double-Beta Decay ✦ An allowed nuclear physics process - Can occur when single β decay not allowed - Lepton number is conserved - Observed in a number of isotopes 76 76 Ge Se + 2e + 2 e e e 76Ge e e 76Se Observed in 48Ca, 76Ge, 82Se, 96Zr, 100Mo, 116Cd,128Te , 130Te , 150Nd, 238U

19 V. E. Guiseppe EBSS 2017 Neutrinoless Double-Beta Decay

✦ No neutrinos emitted ✦ Discovery provides: - Neutrino is own antiparticle 76 76 (Majorana) Ge Se + 2e - Lepton number violation - Neutrino mass e 76Ge ne e n p + e + ne ) (RH ne) 76Se (LH ne) Light Neutrino n + n p + e e ) Exchange

20 V. E. Guiseppe EBSS 2017 Neutrinoless Double-Beta Decay

✦ No neutrinos emitted ✦ Discovery provides: - Neutrino is own antiparticle 76 76 (Majorana) Ge Se + 2e - Lepton number violation - Neutrino mass e 76Ge Nuclear Process e ✦ Could be some other nuclear process 76 other than a light neutrino exchange Se

21 V. E. Guiseppe EBSS 2017 How to Measure ββ Observe double-beta decay by collecting the energy of the 2 e- in a detector

✦ With 2 neutrino double-beta 2ν decay, the electrons share the decay energy with the neutrinos

✦ With neutrinoless double-beta dN/dE 0ν decay, the electrons carry the full decay energy Energy Endpoint Energy

Q=2.039 MeV

V. E. Guiseppe EBSS 2017 22 How ββ Relates to the Neutrino

Measure decay rate of to get neutrino absolute mass scale

= G M 2 m 2 2ν 0 0 | 0 | ⇥

✦ G are calculable phase space dN/dE 0ν factors ✦ M are nuclear physics matrix Energy elements Endpoint - Uncertainties among models Energy ✦ mββ is the effective Majorana mass Q=2.039 MeV

23 V. E. Guiseppe EBSS 2017 Neutrino Mixing Matrix and Mass

e Ue1 Ue2 Ue3 1 Neutrino Neutrino = U U U µ µ1 µ2 µ3 2 Masses Flavors U1 U2 U3 3 Effective Majorana Mass: No Mixing: m = m = m e 1 3 2 Mixing: m = Ueimi h i i=1 X ✦ There is an ambiguity in the sign of one of the larger mass-squared differences - Leads to two alternative mass orderings (or hierarchies)

24 V. E. Guiseppe EBSS 2017 Neutrino Mixing Matrix and Mass

✦ There is an ambiguity in the sign of one of the mass-squared differences - Leads to two alternative mass orderings (or hierarchies)

? ? Normal Inverted

V. E. Guiseppe EBSS 2017 25 Allowed Mass Ranges The standard way the two mass hierarchies are plotted for effective Majorana mass vs lightest ν mass

✦ Inverted and normal refer to unknown Reach of current mass hierarchy experiments - Current understanding of ν oscillation parameters ✦ Experiments only

sensitive to effective Cosmology ββ mass

3 Mixing: m = U 2 m h i ei i i=1 X 26 V. E. Guiseppe EBSS 2017 Experimental Approach How to best detect neutrinoless double-beta decay? ✦ Source of decay = detector - minimizes extra mass and unneeded backgrounds ✦ Good energy resolution - maximize signal to background. Always facing background from 2νββ ✦ Extremely low backgrounds in the region of interest - requires ultra-pure materials - analysis techniques to discriminate backgrounds from signal - large Q value to exceed many natural backgrounds - backgrounds of <0.1 event per year for next generation sensitivities ✦ Tag decay daughter - smoking gun of decay, but also 2νββ ✦ Slow 2νββ rate - to minimize its background contribution ✦ Some isotopes have better nuclear theory 27 V. E. Guiseppe EBSS 2017 Current Experiments

CUORE: TeO2 bolometers EXO-200, KamLAND-Zen LXe

MAJORANA/ GERDA : 130 NEMO 3 76Ge diodes SNO+: Te liquid scintillation 28 V. E. Guiseppe Tracking EBSS 2017 Highlights from Current Experiments

✦ CUORE: 24 - T1/2 > 4.0 x 10 yr [Physical Review Letters 115, 102502 (2015) ] ✦ EXO-200 25 - T1/2 >1.1 x 10 yr [J.B.Albertet al., Nature 510 (2014) 299 ✦ GERDA Phase II 25 - T1/2 > 5.3 x 10 yr [M. Agostini et al., Nature 554, 47–52 (2017)] ✦ KamLAND 26 - T1/2 > 1.07 x 10 yr [PRL 117, 082503 (2016)] ✦ NEMO 3 24 - T1/2 > 1.1 x10 [R. Arnold et al., Phys. Rev. D 89, 111101(R) (2014)]

Age of the universe: 1.38 x 1010 yr

V. E. Guiseppe EBSS 2017 29 Experimental Progress

Current best limits on 0νββ

Isotope T1/2 [yr] [eV] Experiment

48Ca > 5.8 x 1022 < 3.1 - 15.4 CANDLES 76Ge > 5.3 x 1025 < 0.15 - 0.33 GERDA 82Se > 3.6 x 1023 < 1 - 2.4 NEMO-3 96Zr > 9.2 x 1021 < 3.6 - 10.4 NEMO-3 100Mo > 1.1 x 1024 < 0.33 - 0.62 NEMO-3 116Cd > 1.9 x 1023 < 1 - 1.8 AURORA 128Te > 1.5 x 1024 < 2.3 - 4.6 geochemical 130Te > 4 x 1024 < 0.26 - 0.97 CUORE 136Xe > 1.07 x 1028 < 0.06 - 0.16 KanLAND-Zen 140Nd > 2 x 1022 < 1.6 - 5.3 NEMO-3

adapted from Barabash, arXiv:1702.06340 (2017) 30 V. E. Guiseppe EBSS 2017 Current and Planned Experiments

Current and near-term sensitivities

Barabash, arXiv:1702.06340 (2017) Plus several others: AMORE, CAMEO, CANDLES, CARVEL, COBRA, DCBA, GEM, GSO, HPXETPC, KamLAND, MOON, NEXT, LUCIFER, LUCINEU, XMASS, ... 31 V. E. Guiseppe EBSS 2017 Current and Planned Experiments

Future Experiments: ✦ These are large and expensive projects - Will require international cooperation - Should only move forward with the best designs worldwide

Barabash, arXiv:1702.06340 (2017)

V. E. Guiseppe EBSS 2017 32 Nuclear Science Long Range Plan

1. Summary and Recommendations The 2015 Nuclear Science Advisory Committee recommended the developmentin some cases, we areof only neutrinoless now poised to reap double-beta the The decay ordering ofsearches these four bullets follows the priority benefits of these initiatives. In other cases, anticipated ordering of the 2007 plan. upgrades were achieved at a small fraction of the cost estimated in 2007, and we are harvesting the benefits RECOMMENDATION II earlier REACHINGthan expected. FOR All of THE our current HORIZON four national The excess of matter over antimatter in the universe is user facilities, the Continuous Electron Beam Accelerator one of the most compelling mysteries in all of science. Facility (CEBAF), the Relativistic Heavy Ion Collider The observation of neutrinoless double beta decay (RHIC), the Argonne Tandem Linac Accelerator System in nuclei would immediately demonstrate that neutrinos (ATLAS), and the NSF-supported National Supercon- are their own antiparticles and would have profound ducting Cyclotron Laboratory (NSCL), were significantly implications for our understanding of the matter- upgraded in capability during this period. A fifth national antimatter mystery. user facility, the DOE-supported Holifield Radioactive Ion We recommend the timely development and BeamThe SiteFacility, of the Wright Brothers’was First closed Airplane Flight down. Care was always taken to leverage U.S. investments in an international context deployment of a U.S.-led ton-scale neutrinoless while maintaining a world-leadership position. double beta decay experiment. A ton-scale instrument designed to search for this as-yet Here are the recommendations ofT thee 20152015 Long Range unseen nuclear decay will provide the most powerful Plan. LONG RANGE PLAN for NUCLEAR SCIENCE test of the particle-antiparticle nature of neutrinos ever performed. With recent experimental breakthroughs RECOMMENDATION I pioneered by U.S. physicists and the availability of deep The progress achieved under the guidance of the 2007 underground laboratories, we are poised to make a Long Range Plan has reinforced U.S. world leadership major discovery. in nuclear science. The highest priority in this 2015 Plan V. E.is Guiseppeto capitalize on the investments made. EBSS 2017This recommendation flows out of the targeted 33 investments of the third bullet in Recommendation I. It ! With the imminent completion of the CEBAF 12-GeV must be part of a broader program that includes U.S. Upgrade, its forefront program of using electrons to participation in complementary experimental eforts unfold the quark and gluon structure of hadrons and leveraging international investments together with nuclei and to probe the Standard Model must be enhanced theoretical eforts to enable full realization of realized. this opportunity. ! Expeditiously completing the Facility for Rare Isotope Beams (FRIB) construction is essential. RECOMMENDATION III Initiating its scientific program will revolutionize our Gluons, the carriers of the strong force, bind the quarks understanding of nuclei and their role in the cosmos. together inside nucleons and nuclei and generate nearly ! The targeted program of fundamental symmetries all of the visible mass in the universe. Despite their and neutrino research that opens new doors to importance, fundamental questions remain about the physics beyond the Standard Model must be role of gluons in nucleons and nuclei. These questions sustained. can only be answered with a powerful new electron ion ! The upgraded RHIC facility provides unique collider (EIC), providing unprecedented precision and capabilities that must be utilized to explore the versatility. The realization of this instrument is enabled properties and phases of quark and gluon matter in by recent advances in accelerator technology. the high temperatures of the early universe and to explore the spin structure of the proton. We recommend a high-energy high-luminosity polarized EIC as the highest priority for new facility construction Realizing world-leading nuclear science also requires following the completion of FRIB. robust support of experimental and theoretical research at universities and national laboratories and operating The EIC will, for the first time, precisely image gluons in our two low-energy national user facilities—ATLAS and nucleons and nuclei. It will definitively reveal the origin NSCL—each with their unique capabilities and scientific of the nucleon spin and will explore a new quantum instrumentation. chromodynamics (QCD) frontier of ultra-dense gluon 4 Do we need this many experiments? Nuclear matrix models

The range of decay rates due to competing nuclear models This spread introduces a large uncertainty in the effective Majorana neutrino mass

Observing 0νββ in several isotopes can help pin down the likely underlying physics

[Engel and Menendez, Rep. Prog. Phys. 80 (2017) 046301]

34 V. E. Guiseppe EBSS 2017 Reach of Next Generation Experiments

✦ Next generation experiments aim to Reach of current push through through the inverted experiments mass ordering. ✦ But then what? Can the normal mass region ever be fully explored?

- Better to ask, what are the Bayesian Cosmology posterior distributions?

Normal Inverted

>50 % 100 % discovery discovery potential; potential;

[Agostini, Benato, Detweiler (2017) arXiv:1705.02996v3 V. E. Guiseppe EBSS 2017 35 Reach of Next Generation Experiments

✦ 3σ discovery potential for current (red dots) and future (black dots) experiments - bands are due to uncertainties in nuclear matrix elements

[Agostini, Benato, Detweiler (2017) arXiv:1705.02996v3]

V. E. Guiseppe EBSS 2017 36 Heidelberg-Moscow & IGEX

0ν 25 35.5 kg y: T½ > 1.9 x 10 y

D. Gonzales et al., Nucl. Phys. B. Proc. Suppl. 87, 278 (2000) H. V. Klapdor-Kleingrothaus et al., Eur. Phys. J. A 12, 147 (2001).

37 V. E. Guiseppe EBSS 2017 Subset of the H-M group

claiming 0νββ

71.7 kg y exposure pulse shape selected events 0ν 25 T½ = 1.2 x 10 y significance: 4.2σ ✦ Problem: - Claimed signal is weak and a repeat test takes a long time - Some uncertainty in background model - Some lines not identified H. V. Klapdor-Kleingrothaus et al., Phys. Lett. B 586, 198 (2004). 38 V. E. Guiseppe EBSS 2017 GERDA

Phase 1 & II Combined Results

25 T1/2 > 5.3 x 10 yr

[arXiv:1703.00570v2 & M. Agostini et al., Nature 554, 47–52 (2017)]

39 V. E. Guiseppe EBSS 2017 EXO & KamLAND-Zen

KamLAND-ZEN 26 EXO-200 T1/2 > 1.07 x 10 yr 25 T1/2 >1.1 x 10 yr (90% CL)

J.B.Albertet al., Nature 510 (2014) 299

A. Gando et al. Phys. Rev. Lett. 117, 082503 40 V. E. Guiseppe EBSS 2017 COURE

CUORE-0 + Cuoricino limit: 24 T½ > 4.0 x 10 yr

[Physical Review Letters 115, 102502 (2015)]

41 V. E. Guiseppe EBSS 2017 The MAJORANA DEMONSTRATOR

Funded by DOE Office of Nuclear Physics, NSF Particle Astrophysics, NSF Nuclear Physics with additional contributions from international collaborators. Goals: - Demonstrate backgrounds low enough to justify building a tonne scale experiment. - Establish feasibility to construct & field modular arrays of Ge detectors. - Searches for additional physics beyond the standard model.

Operating underground at 4850’ Sanford Underground Research Facility Background Goal in the 0νββ peak region of interest (4 keV at 2039 keV) 3 counts/ROI/t/y (after analysis cuts) Assay U.L. currently ≤ 3.5 44.1-kg of Ge detectors - 29.7 kg of 88% enriched 76Ge crystals - 14.4 kg of natGe - Detector Technology: P-type, point-contact. 2 independent cryostats - ultra-clean, electroformed Cu - 22 kg of detectors per cryostat - naturally scalable Compact Shield - low-background passive Cu and Pb shield with active muon veto

N. Abgrall et al. Adv. High Energy Phys 2014, 365432 (2014) V. E. Guiseppe EBSS 2017 42 3σ Discovery vs. Exposure for 76Ge

76Ge (87% enr.) J. Detwiler 1030

Inverted Ordering (IO) 1029

Minimum IO mββ=18.3 meV, taken from using the PDG2013 central values of 28 10 the oscillation parameters, and the most pessimistic NME for the corresponding isotope 100 kg-year exposure among QRPA, SM, IBM, PHFB, 1027 and EDF

DL [years] 3.5 counts/ROI-t-y σ 26

3 T1/2 = 1.2 x 10 y

1/2 min T 26 IO m range 10 ββ Note : Region of Background free Interest (ROI) 0.1 counts/ROI-t-y can be single or 25 10 1.0 count/ROI-t-y multidimensional (E, spatial, …) 10 counts/ROI-t-y

1024 10−3 10−2 10−1 1 10 102 103 Exposure [ton-years]

Assumes 75% efficiency based on GERDA Phase I. Enrichment level is accounted for in the exposure

V. E. Guiseppe EBSS 2017 43 The MAJORANA Collaboration

! OSAKA!UNIVERSITY!

V. E. Guiseppe EBSS 2017 44 MAJORANA DEMONSTRATOR Implementation

In shield Operation ✦Module 1: 16.9 kg (20) enrGe May – Oct. 2015, 5.6 kg (9) natGe Final Installation, Dec. 2015 — ongoing

✦Module 2: 12.9 kg (15) enrGe July 2016 — ongoing ✦ 8.8 kg (14) natGe

V. E. Guiseppe EBSS 2017 45 DEMONSTRATOR Background Model Background+Rate+(c/ROI3t3y) 0 0.2 0.4 0.6 0.8 1

Electroformed7Cu 0.23 OFHC7Cu7Shielding 0.29 Pb7shielding 0.63 Cables7/7Connectors 0.38 Front7Ends 0.60 Ge7(U/Th) 0.07 Plastics7+7other 0.39 GeM68,7CoM607(enrGe) 0.07 CoM607(Cu) 0.09 External7γ,7(α,n) 0.10 Electroformed7CuNatural Radioactivity Rn,7surface7α 0.05 GeCosmogenic7ActivationM68,7CoM607(enrGe) Ge,7Cu,7Pb7(n,7n'γ) 0.21 External7γ,7(α,n)External, Environmental Ge(n,n) 0.17 Ge,7Cu,7Pb7(n,7n'γ)μMinduced Ge(n,γ) 0.13 ν7backgroundsneutrinos direct7μ7+7other 0.03 ν7backgrounds <0.01 Total:7<3.57c/ROIMtMy Background based on assay of materials. Where an upper limit exists, use upper limit as contribution NIMA 828 (2016) 22–36 arXiv:1601.03779 [physics.ins-det] V. E. Guiseppe EBSS 2017 46 Background Sources

Our background sources are primarily naturally occurring radioactivity or cosmogenic-induced reactions Perspective: 200,000 β decays/min in you body from 40K Expect < 3.5 cts/(ROI ton yr)

Natural Th and U decay chains

Cosmogenics, muon-induced neutrons 76 76 Ge(n, n0) Ge 63Cu(n, ↵)60Co

Radon and its long-lived progeny

V. E. Guiseppe EBSS 2017 47 MAJORANA Approach to Backgrounds

✦ The detector: P-type point contact - enrGe metal zone refined and pulled into a crystal that provides purification - Limit above-ground exposure to prevent cosmic activation - Slow drift velocity and localized weighting potential: separation of multi-site events ✦ Rejection of backgrounds - Granularity: multiple detectors hit - Pulse shape discrimination: multiple hits in a detector - Alpha events near surface: based on response Multiple scatters Single-site event ββ γ γ

V. E. Guiseppe EBSS 2017 48 Pulse Shape Analysis Rising edge “stretched” in time ⇒ ✦Use a pulse shape analysis (PSA) improved PSA rejection of multi-site gamma events ✦Benefit of P-type Point-Contact (PPC) detectors for background rejection: - Slow drift time of the ionization charge cloud - Localized weighting potential gives excellent multi-site rejection

Hole vdrift [mm/ns] with paths and isochrones

Luke et al., IEEE trans. Nucl. Sci. 36 , 926 (1989) Barbeau, Collar, and Tench, J. Cosm. Astro. Phys. 0709 (2007). V. E. Guiseppe EBSS 2017 49 Majorana Approach to Backgrounds

✦ Ultra-pure materials - Low mass design - Custom cable connectors and front-end boards - Carefully selected plastics & fine Cu coax cables - Underground Electro-formed Cu ‣ 10 baths at SURF, 6 baths at PNNL ‣ 2474 kg of electroformed copper produced. ‣ Th decay chain (ave) ≤ 0.1 Bq/kg ‣ U decay chain (ave) ≤ 0.1 Bq/kg ✦ Machining and Cleaning - Cu machining in an underground clean room - Cleaning of Cu parts by acid etching and passivation - Nitric leaching of plastic parts

V. E. Guiseppe EBSS 2017 50 Detector Units and Strings

Detector parts stored and assembled inside radon-reduced, dry N2 environment storage and glove boxes.

All parts are uniquely tracked through machining, cleaning, and assembly by a custom- built database.

V. E. Guiseppe EBSS 2017 51 Assembled Detector Unit and String

AMETEK (ORTEC) fabricated enriched-Ge PPC detectors - 35 enriched detectors: 29.7 kg, 88% 76Ge. Canberra fabricated natural-Ge BEGe detectors PFA + fine Cu Electroformed coaxial cable Copper Front-End Elec. PTFE insulator

String Assembly

V. E. Guiseppe EBSS 2017 52 Detector Readout Components

Shipping Custom low mass front-end boards Restraint Feedback Resistor Epoxy Clean Au+Ti traces on fused silica Amorphous Ge resistor FET mounted with silver epoxy EFCu + low-BG Sn contact pin

FET Spring Clip

Fine Cu coaxial cable and clean connectors

Connectors reside on top of cold plate. In-house machined from Vespel. Axon’ pico co-ax cable. Low background solder and flux.

V. E. Guiseppe EBSS 2017 53 Detector Module

- A self contained vacuum and cryogenic vessel - Contains a portion of the shielding - Can be transported for assembly and deployment

Module mated to the glovebox for string installation

Module moving to/from transporter

V. E. Guiseppe EBSS 2017 54 Module and Shield

Loading of enrGe in Cryostat 2

Pb and outer Cu shield Muon Veto Poly Shield Module deployment Radon Panels Enclosure

Pb Bricks Inner Cu Shield

Outer Cu Shield

V. E. Guiseppe EBSS 2017 55 Background Spectrum (DS3 & DS4)

Lowest background configuration with both modules in shield. Enriched detectors in Modules 1 & 2 , before and after PSD cuts

1 No PSD cuts

AvsE cut

AvsE & DCR cuts 10−1 Q Spectrum is dominated by 2νββ −2

Counts/40keV/kg/day 10

10−3 500 1000 1500 2000 2500 3000 Energy (keV)

V. E. Guiseppe EBSS 2017 56 Background Spectrum (DS3 & DS4)

After cuts, 1 count in 400 keV window centered at 2039 keV (0νββ peak) +8.9 - Projected background rate is 5 . 1 3 . 2 c /(ROI t y) ‣ using a 2.9 (M1- DS3) & 2.6 keV (M2 - DS4) keV ROI (68% CL). - Background index of 1.8 x 10-3 c/(keV kg y) Analysis cuts are still beingDS3 optimized. & DS4 (Enriched - High Gain) 10−1 DS3 & DS4 Enriched: 1.39 kg y exposure M1 + M2 (All cuts, 1937 kg-days)

−2 10x more exposure when we 10 Q Counts/keV/kg/day include all data sets. New results to be released later this summer.

10−3

10−4 0 500 1000 1500 2000 2500 3000 V. E. Guiseppe EBSS 2017 Energy (keV) 57 Low Energy Spectrum

✦ Controlled surface exposure of enriched material to minimize cosmogenics ✦ Significant reduction of cosmogenics in the low-energy region. - Low-energy rate is improved in subsequent data sets ✦ Enriched Detectors: ~0.04 cts/(kg-keV-d) near 20 keV ✦ Efficiency below 5 keV is under study.

Phys. Rev. Lett. 118, 161801 (2017).

Permits Low-Energy physics Pseudoscalar dark matter Vector dark matter 14.4-keV solar axion e- ⇒ 3ν Pauli Exclusion Principle

V. E. Guiseppe EBSS 2017 58 Large Enriched Germanium Experiment for Neutrinoless ββ Decay

(LEGEND) The collaboration aims to develop a phased, Ge-76 based double-beta decay experimental program with discovery potential at a half-life significantly longer than 1027 years, using existing resources as appropriate to expedite physics results - Combine the strengths of the GERDA and the MAJORANA DEMONSTRATOR

First phase: Subsequent stages: • (up to) 200 kg • 1000 kg (staged) • modification of • timeline connected to existing GERDA U.S. DOE down select infrastructure at LNGS process • BG goal (x5 lower) • BG: goal (x30 lower) 0.1 0.6 c /(FWMH t y) c /(FWHM t y) • start by 2021 • Location: TBD • Required depth under investigation

V. E. Guiseppe EBSS 2017 59 Outlook

✦ The neutrino history is filled with important discoveries ✦ The future is even more exciting ‣ What is the absolute mass scale? ‣ Which hierarchy is correct? ‣ Are they their own anti-particle? ‣ Is Lepton # violated ‣ Is there Leptonic CP-invariance violation ‣ Leptogenesis ✦ Neutrinoless double-beta decay experiments are poised to help unlock the remaining secrets of the neutrino

60 V. E. Guiseppe EBSS 2017