Reaching for the half-life horizon: neutrino less double beta decay Giorgio Gratta Stanford With thanks to many colleagues who provided (whether they are aware or not) material for this talk

Last 20 yrs: the age of ν physics Double-beta decay - Gratta Discovery of ν flavor change - Solar neutrinos (MSW effect) - Reactor neutrinos (vacuum oscillation) - Atmospheric neutrinos (vacuum oscillation) - Accelerator neutrinos (vacuum oscillation) We also found that: ν masses are non-zero there are 2.981±0.008 light neutrinos (Z lineshape) 3 ν flavors were active in Big Bang Nucleosynthesis The Sun emits neutrinos as expected Supernovae emit neutrinos Jlab user group, Jun 2016 Double-beta decay - Gratta

Example: Normalized flux of νe from reactor sources in KamLAND νe νe nuclear reactor νe νe νx ? νe νe νe detector νe L L0/E (km/MeV, with L0=180 km) Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta If mν ≠ 0 neutrinos show a different image depending on our vantage point: this is the state of definite flavor: interactions couple to this state “Weak interaction eigenstate” this is the state of definite energy: propagation happens in this state “Mass eigenstate” Jlab user group, Jun 2016 Double-beta decay - Gratta

Neutrino oscillations imply non-zero neutrino masses Yet, they provide little information on the absolute mass scale ~20 eV ~2 eV ∑~0.25 eV ~0.1 eV Time of flight from SN1987A (PDG 2002) From tritium endpoint (Maintz and Troitsk) solar ~ 7.6·10-5eV2 ~2.4·10-3 eV2 ~2.4·10-3 eV2 From Cosmology From 0νββ if ν is Majorana solar ~ 7.6·10-5 eV2 Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Fermion mass spectrum 1st 2nd 3rd TeV t b c t GeV s m d u MeV e keV eV Neutrinos meV Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Fermion mass spectrum 1st 2nd 3rd TeV Higgs! t b c t GeV s m d u MeV e keV Something else? eV Neutrinos meV Jlab user group, Jun 2016 Double-beta decay - Gratta

Neutrinos have other peculiarities: They are the only electrically neutral fermions Generation 1st 2nd 3rd 1 e+ + + 2/3 u c t d s b 1/3 Neutrinos do not carry charge What about lepton number? e   Charge -1/3 d s b -2/3 u c t -1 e- - - Jlab user group, Jun 2016 Double-beta decay - Gratta

Could it be that the mass and charge peculiarities are somehow related? Say that for neutrinos , since they have no charge… But… isn’t there a lepton number to conserve? No worries: lepton number conservation is not as “serious” as –say- energy conservation Lepton number conservation is just an empirical notion. Basically lepton number is conserved “because”, experimentally, But, in the neutral case, the distinction could derive from the different helicity states. Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta We have two possible ways to describe neutrinos: “Dirac” neutrinos (some “redundant” information but the “good feeling” of things we know…) “Majorana” neutrinos (more efficient description, no total lepton number conservation, new paradigm…) Which way Nature has chosen to proceed is an experimental question But the two descriptions are distinguishable only if mν≠0 (and the observable difference  0 for mν 0) Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay: a second-order process only detectable if first order beta decay is energetically forbidden Candidate nuclei with Q>2 MeV Candidate Q Abund. (MeV) (%) 48Ca→48Ti 4.271 0.187 76Ge→76Se 2.040 7.8 82Se→82Kr 2.995 9.2 96Zr→96Mo 3.350 2.8 100Mo→100Ru 3.034 9.6 110Pd→110Cd 2.013 11.8 116Cd→116Sn 2.802 7.5 124Sn→124Te 2.228 5.64 130Te→130Xe 2.533 34.5 136Xe→136Ba 2.458 8.9 150Nd→150Sm 3.367 5.6 Jlab user group, Jun 2016 Double-beta decay - Gratta

There are two varieties of ββ decay Double-beta decay - Gratta 0ν mode: a hypothetical process can happen only if: Mν ≠ 0 ν = ν |ΔL|=2 |Δ(B-L)|=2 2ν mode: a conventional 2nd order process in nuclear physics Standard mechanism Jlab user group, Jun 2016 Double-beta decay - Gratta

There are two varieties of ββ decay 0ν mode: a hypothetical process can happen only if: Mν ≠ 0 ν = ν |ΔL|=2 |Δ(B-L)|=2 2ν mode: a conventional 2nd order process in nuclear physics 0νββ is the most sensitive probe of the Majorana/Dirac nature of neutrinos Standard mechanism Jlab user group, Jun 2016 Double-beta decay - Gratta

Standard Model 2νββ decay Background due to the Standard Model 2νββ decay σ/E=1.6% (EXO-200 initial resolution) The two can be separated in a detector with sufficiently good energy resolution Topology and particle ID are also important to recognize backgrounds

If 0νββ is due to the standard mechanism can be calculated within particular nuclear models and a known phasespace factor is the quantity to be measured is the effective Majorana ν mass (εi = ±1 if CP is conserved) Jlab user group, Jun 2016 Double-beta decay - Gratta

Connecting the observable (T0νββ1/2) to parameters of Lepton Number Violating interactions (<mν>, …) requires a mechanism-dependent nuclear matrix element (here below for the “standard mechanism”) Petr Vogel, 2014 The uncertainties on individual isotopes are related to nuclear structure. In addition there is an overall uncertainty (not shown) on the effective value to be used for gA Jlab user group, Jun 2016 Double-beta decay - Gratta

A useful way to relate different isotopes and compare results (assumes the standard mechanism) EDF: T.R. Rodríguez and G. Martínez-Pinedo, PRL 105, 252503 (2010) IBM-2: J. Barea, J. Kotila, and F. Iachello, PRC 91, 034304 (2015) SkyrmeQRPA: M.T. Mustonen and J. Engel PRC 87 064302 (2013) ISM: J. Menendez et al., Nucl Phys A 818, 139 (2009) QRPA: F. Šimkovic et al., PRC 87 045501 (2013) Jlab user group, Jun 2016 Double-beta decay - Gratta

A useful way to relate different isotopes and compare results (assumes the standard mechanism) Jlab user group, Jun 2016 Double-beta decay - Gratta

The state of the art for 136Xe and 76Ge Double-beta decay - Gratta (assumes the standard mechanism) GERDA: M.Agostini et al., Phys. Rev. Lett. 111 (2013) 122503 KLZ: A.Gando et al., arXiv:1605.02889 (May 2016) EXO-200: J.B.Albert et al., Nature 510 (2014) 299 Limits are 90% CL Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta …and measured half lives can be related to a parameter space with input from oscillation and cosmological (or direct) mass measurements (with the usual caveats about NME and gA) Cosmology arXiv:1502.01589 Planck, Cosmology Jlab user group, Jun 2016 Double-beta decay - Gratta

While it is convenient to think in terms of the standard mechanism other mechanisms can produce the lepton number violation SUSY-inspired example: ppeejj+X (LHC@14TeV) u d Peng, Ramsey-Musolf, Winslow, PRD 93, 093002 (2016) Note that even in this case the decay implies Majorana masses, except the relationship to the half-life is now a different one. Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta In fact there is a theorem: “0νββ decay always implies new physics” There is no scenario in which observing 0νββ decay would not be a great discovery Majorana neutrinos Lepton number violation Probe new mass mechanism up to the GUT scale Probe key ingredient in generating cosmic baryon asymmetry Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta A historical note Early experiments: Geochemical or Radiochemical experiments (search for trace amounts of element A in a large amount of B after a long time).  Can’t discriminate between 0ν and 2ν decays Counting experiments with gram quantities of candidate isotopes. “Previous generation” experiments: Counting experiments with kg quantities of enriched material Running experiments: 100kg-class counting experiments (mainly enriched) Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta 07/01/09 Four fundamental requirements for modern experiments: 1) Isotopic enrichment of the source material (that is generally also the detector) 100kg – class experiment running or completed. Ton – class experiments under planning. 2) Underground location to shield cosmic-ray induced background Several underground labs around the world, next round of experiments 1-2 km deep. Jlab user group, Jun 2016 Double-beta decay - Gratta 24

Double-beta decay - Gratta 07/01/09 Four fundamental requirements for modern experiments: 3) Ultra-low radioactive contamination for detector construction components Materials used ≈<10-15 in U, Th (U, Th in the earth crust ~ppm) 4) New techniques to discriminate signal from background Non trivial for E~1MeV But this gets easier in larger detectors. Jlab user group, Jun 2016 Double-beta decay - Gratta 25

Double-beta decay - Gratta The last point deserves more discussion, particularly as the size of detectors grows… The signal/background discrimination can/should based on four parameters/measurements: Energy measurement (for small detectors this is ~all there is). Event multiplicity (γ’s Compton scatter depositing energy in more than one site in large detectors). Depth in the detector (or distance from the walls) is (for large monolithic detectors) a powerful parameter for discriminating between signal and (external) backgrounds. αs can be distinguished from electrons in many detectors. Powerful detectors use most (possibly all) these parameters in combination, providing the best possible background rejection and simultaneously fitting for signal and background. Signal identification based mostly on energy + multiplicity Compton scatters for gammas Find our actual position resolution Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Recent results (>1025 yr half life) Isotope Experiment Exposure (kg yr) Reference 76Ge Gerda 21.6 2.4 >2.1 >1.5x1015 200-400 Agostini et al., PRL 111 (2013) 122503 136Xe EXO-200 100 1.9 >1.1 >8.0x1014 190-450 Albert et al. Nature 510 (2014) 229 KamLAND-ZEN 504** 4.9 >11 (run 2) >8.0x1015 60-161 Gando et al., arXiv:1605. 02889 (2016) * Note that the range of “viable” NME is chosen by the experiments and uncertainties related to gA are not included ** All Xe. Fiducial Xe is more like ~150 kg yr Jlab user group, Jun 2016 Double-beta decay - Gratta

The next step: ton-scale detectors entirely covering the inverted hierarchy Testing lepton number violation with >100x the current T1/2 sensitivity Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta “RECOMMENDATION II The excess of matter over antimatter in the universe is one of the most compelling mysteries in all of science. The observation of neutrinoless double beta decay in nuclei would immediately demonstrate that neutrinos are their own antiparticles and would have profound implications for our understanding of the matter-antimatter mystery. We recommend the timely development and deployment of a U.S.-led ton-scale neutrinoless double beta decay experiment.” Initiative B “We recommend vigorous detector and accelerator R&D in support of the neutrinoless double beta decay program and the EIC.” Jlab user group, Jun 2016 Double-beta decay - Gratta

It is essential that action is taken in a timely fashion the US leadership position in the field should not be wasted. Jlab user group, Jun 2016 Double-beta decay - Gratta

It is essential that action is taken in a timely fashion the US leadership position in the field should not be wasted. The discovery of neutrino-less double-beta decay would make world-wide headlines! This has implications well beyond the subfield and the Nuclear Physics community now owns this spectacular opportunity! Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Many isotopes have comparable sensitivities (at least in terms of rate per unit neutrino mass) R.G.H. Robertson, MPL A 28 (2013) 1350021 There is an “empirical” anticorrelation between phasespace and NME. Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta A healthy neutrinoless double-beta decay program requires more than one isotope. This is because: There could be unknown gamma transitions and a line observed at the “end point” in one isotope does not necessarily imply the 0νββ decay discovery Nuclear matrix elements are not very well known and any given isotope could come with unknown liabilities Different isotopes correspond to vastly different experimental techniques 2 neutrino background is different for various isotopes The elucidation of the mechanism producing the decay requires the analysis of more than one isotope Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta A healthy neutrinoless double-beta decay program requires more than one isotope. This is because: There could be unknown gamma transitions and a line observed at the “end point” in one isotope does not necessarily imply the 0νββ decay discovery Nuclear matrix elements are not very well known and any given isotope could come with unknown liabilities Different isotopes correspond to vastly different experimental techniques 2 neutrino background is different for various isotopes The elucidation of the mechanism producing the decay requires the analysis of more than one isotope Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta One (my own) possible classification of technologies Low density trackers - NEXT, PandaX (136Xe gas TPC) - SuperNEMO (foils and gas tracking, 82Se) Pros: Superb topological information Cons: Very large size Liquid (organic) scintillators - KamLAND-ZEN (136Xe) - SNO+ (130Te) Pros: “simple”, large detectors exist, self-shielding Cons: Not very specific, 2ν background Crystals - GERDA, Majorana (76Ge) - CUORE, CUPID (130Te) Pros: Superb energy resolution, possibly 2-parameter measurement Cons: Intrinsically fragmented Liquid TPC - nEXO (136Xe) Pros: Homogeneous with good E resolution and topology Cons: Does not excel in any single parameter Jlab user group, Jun 2016 Double-beta decay - Gratta

Evolution of various technologies/experimental programs: a (very) terse summary Isotope 100kg-scale detector Main principle Ton scale detector Name Misotope (kg) ~ Status 76Ge Gerda ~35 Ran/Running E res, Segmented GeMa/MaGe ~1000 MJD Running 130Te SNO+ ~265 Commissioning Size/shielding “Phase II” ~2000 CUORE ~206 E res, Segmented Cupid ~500 82Se SuperNEMO ~7100 Construction Tracking 136Xe KamLAND-Zen ~350 750 Ran/commissioning KamLAND2-ZEN 1000+ NEXT ~10 100 Tracking, E res 3x350 PandaX ~200 5x200 EXO-200 ~150 Topology, Energy res. nEXO 5000 Jlab user group, Jun 2016 Double-beta decay - Gratta

Shielding a detector from MeV gammas is difficult! Typical ββ0ν Q values Gamma interaction cross section Example: γ interaction length in Ge is 4.6 cm, comparable to the size of a germanium detector. Shielding ββ decay detectors is much harder than shielding Dark Matter ones We are entering the “golden era” of ββ decay experiments as detector sizes exceed interaction length Jlab user group, Jun 2016 Double-beta decay - Gratta

Diameter or length (cm) Double-beta decay - Gratta LXe mass (kg) Diameter or length (cm) 5000 130 150 40 5 13 2.5MeV γ attenuation length 8.5cm = 5kg 150kg 5000kg This works best for a monolithic detector Jlab user group, Jun 2016 Double-beta decay - Gratta

0νββ searches are quite different from Dark Matter ones 0νββ decay searches Dark Matter searches Optimize for energy resolution at O(2MeV) Optimize for <100keV threshold Gamma ray shielding essential Neutron shielding essential Signal is electron-like Signal is nuclear recoil-like Isotopic enrichment 2νββ is a background But there is synergy in the detector development for some of the technologies Jlab user group, Jun 2016 Double-beta decay - Gratta

Example #1: the high resolution approach Next Generation ton scale 76Ge 0νββ Goal: background of 0.1 c / ROI-t-y 5 yr 90% CL sensitivity: T1/2 > 6.1·1027 yr 5 yr 3σ discovery: T1/2 ~ 5.9·1027 yr Moving forward predicated on demonstration of projected backgrounds by GERDA and/or MJD, with further reductions extrapolated to the ton scale. Envision a phased, stepwise implementation; e.g. 200 → 500 → 1000 kg Anticipate down-select of best technologies, based on what has been learned from GERDA and the Majorana Demonstrator, as well as contributions from other groups and experiments. SNOLAB Design using existing Cryo pit. Generic Design using Jinping style cavity

Beyond? Example #2: the very large Liquid scint approach Beyond KamLAND-Zen 800: Higher energy resolution = lower 2ν background: KamLAND2-ZEN Light collection gain Winston cones x1.8 Higher q.e. PMTs x1.9 LAB-based liquid scint x1.4 Overall x4.8 expected σ(2.6MeV)= 4% → ~2％ target sensitivity 20 meV 1000+ kg xenon But eventually 2ν background becomes dominant Beyond? Super-KamLAND-Zen in connection with Hyper-Kamiokande target sensitivity 8 meV

Double-beta decay - Gratta Example #3: The nEXO detector A 5000 kg enriched LXe TPC, directly extrapolated from EXO-200 130 cm nEXO TPC 46 cm EXO-200 TPC Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Preliminary artist view of nEXO in the SNOlab Cryopit 13m 14m 14 m Ø13 m Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Combining Ionization and Scintillation Anticorrelation between scintillation and ionization in LXe known since early EXO R&D E.Conti et al. Phys Rev B 68 (2003) 054201 By now this is a common technique in LXe 228Th source SS Qββ (@ 2615 keV γ line) Qββ Rotation angle chosen to optimize energy resolution at 2615 keV ( nEXO will do ≤1%) Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Using event multiplicity to recognize backgrounds (from EXO-200 data) single cluster multiple cluster Low background data 2νββ 228Th calibration source γ γ Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Particularly in the larger nEXO, background identification and rejection fully use a fit that considers simultaneously energy, multiplicity and event position.  The power of the homogeneous detector, this is not just a calorimetric measurement! 4.78 ton LXe, 5 yr data, 0νββ corresponding to T1/2=6.6x1027 yr 106 105 104 103 102 10 1 0.1 4780kg fiducial 3000kg fiducial 1000kg fiducial 500kg fiducial SS Counts/20keV 106 105 104 103 102 10 1 0.1 MS Counts/20keV 1 2 3 Energy (MeV) 1 2 3 Energy (MeV) 1 2 3 Energy (MeV) Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Note that all the observables have a role in separating signal from background. For instance, a very large, homogeneous detector has great advantages but only if its energy resolution is sufficient to sufficiently suppress the 2ν mode. 136Xe The 2ν background is worse for other isotopes, as the 2ν half-life is shorter. Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta nEXO sensitivity as a function of time for the best-case NME (GCM) Normal hierarchy Inverted hierarchy GCM: Rodriguez, Martinez-Pinedo, Phys. Rev. Lett. 105 (2010) 252503 Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Early ββ decay experiments were based on the identification of trace amounts of element B in a sample of element A (after a geological or anyway long time). Can we imagine doing this in nEXO, but real time and for individual atoms so that the “chemical tag” can be used in conjunction with other parameters of the decay. The final state atom in the ββ decay of 136Xe is 136Ba. A substantial R&D program to develop spectroscopic techniques to achieve this is in progress. Jlab user group, Jun 2016 Double-beta decay - Gratta

Images of few Ba atoms in solid xenon in a laser beam PRELIMINARY Images of the blue points are shown.. PRELIMINARY P~0.18mW, 3 counts/photon, 205 counts/ion, 70 photons/ion PRELIMINARY PRELIMINARY PRELIMINARY Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta So one could imagine a second phase of nEXO, when sub 10 meV sensitivities could be reached. Baseline detector Ba- tagging enhanced Normal hierarchy Inverted hierarchy GCM: Rodriguez, Martinez-Pinedo, Phys. Rev. Lett. 105 (2010) 252503 Jlab user group, Jun 2016 Double-beta decay - Gratta

Double-beta decay - Gratta Conclusions 0νββ searches are discovery physics, with connections to many areas of modern physics Results from 100 kg yr searches are here! No discovery yet, with sensitivities >1025 yr Looking at more than one isotope is important We are ready to build ton-scale experiments with negligible background (in the US) the (NP) community has selected this effort as the top priority for the next large project The 10meV region is within reach! Jlab user group, Jun 2016 Double-beta decay - Gratta