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Discovery of ν flavor change - Solar neutrinos (MSW effect)

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. Discovery of ν flavor change

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Discovery of ν flavor change - Solar neutrinos (MSW effect)

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  1. Reaching for the half-life horizon: neutrino less double beta decayGiorgio Gratta StanfordWith thanks to many colleagues who provided (whether they are aware or not) material for this talk

  2. Last 20 yrs: the age of ν physics Discovery of νflavor 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 Double-beta decay - Gratta

  3. νe νe 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) Double-beta decay - Gratta

  4. 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” Double-beta decay - Gratta

  5. 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) solar ~7.6·10-5eV2 From tritium endpoint (Maintz and Troitsk) ~2.4·10-3eV2 ~2.4·10-3eV2 From Cosmology From 0νββ if ν is Majorana solar ~7.6·10-5 eV2 Double-beta decay - Gratta

  6. Fermion mass spectrum 1st 2nd 3rd TeV t b t c GeV s m d u MeV e keV eV Neutrinos meV Double-beta decay - Gratta

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

  8. Neutrinos have other peculiarities: They are the only electrically neutral fermions Generation 1st 2nd 3rd 1 e+++ u c t 2/3 d s b 1/3 Neutrinos do not carry charge What about lepton number? e Charge 0 -1/3 d s b u c t -2/3 -1 e--- Double-beta decay - Gratta

  9. Say that for neutrinos, since they have no charge… Could it be that the mass and charge peculiarities are somehow related? 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. Double-beta decay - Gratta

  10. 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 formν 0) Double-beta decay - Gratta

  11. 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)(%) Double-beta decay - Gratta

  12. 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 Standard mechanism Double-beta decay - Gratta

  13. 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 Double-beta decay - Gratta

  14. 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

  15. 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 Majoranaν mass (εi = ±1 if CP is conserved) Double-beta decay - Gratta

  16. 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 Double-beta decay - Gratta

  17. 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) ISM: J. Menendez et al., NuclPhys A 818, 139 (2009) IBM-2: J. Barea, J. Kotila, and F. Iachello, PRC 91, 034304 (2015) QRPA: F. Šimkovic et al., PRC 87 045501 (2013) SkyrmeQRPA: M.T. Mustonen and J. Engel PRC 87 064302 (2013) Double-beta decay - Gratta

  18. A useful way to relate different isotopes and compare results (assumes the standard mechanism) Double-beta decay - Gratta

  19. The state of the art for 136Xe and 76Ge (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 Double-beta decay - Gratta

  20. …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) Planck, arXiv:1502.01589 Cosmology Cosmology Double-beta decay - Gratta

  21. 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 u Peng, Ramsey-Musolf, Winslow, PRD 93, 093002 (2016) d d Note that even in this case the decay implies Majorana masses, except the relationship to the half-life is now a different one. Double-beta decay - Gratta

  22. In fact there is a theorem: “0νββ decay always implies new physics” There is no scenario in which observing 0νββdecay 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 cosmicbaryon asymmetry Double-beta decay - Gratta

  23. 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) Double-beta decay - Gratta

  24. 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-rayinduced background Several underground labs around the world, next round of experiments 1-2 km deep. Double-beta decay - Gratta

  25. 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. Double-beta decay - Gratta

  26. 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. Double-beta decay - Gratta

  27. Recent results (>1025yr half life) * Note that the range of “viable” NME is chosen by the experiments and uncertainties related to gA are not included ** All Xe. FiducialXe is more like ~150 kg yr Double-beta decay - Gratta

  28. The next step: ton-scale detectors entirely covering the inverted hierarchy Testing lepton number violation with >100x the current T1/2sensitivity Double-beta decay - Gratta

  29. “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 programand the EIC.” Double-beta decay - Gratta

  30. It is essential that action is taken in a timely fashion the US leadership position in the field should not be wasted. Double-beta decay - Gratta

  31. 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! Double-beta decay - Gratta

  32. 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. Double-beta decay - Gratta

  33. 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 Double-beta decay - Gratta

  34. 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 Double-beta decay - Gratta

  35. 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 Double-beta decay - Gratta

  36. Evolution of various technologies/experimental programs: a (very) terse summary Double-beta decay - Gratta

  37. Gamma interaction cross section Typical ββ0ν Q values Shielding a detector from MeV gammas is difficult! Example: γinteraction 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 Double-beta decay - Gratta

  38. 2.5MeV γ attenuation length 8.5cm = 5kg 150kg 5000kg This works best for a monolithic detector Double-beta decay - Gratta

  39. 0νββ searches are quite different from Dark Matter ones But there is synergy in the detector development for some of the technologies Double-beta decay - Gratta

  40. Example #1: the high resolution approachNext Generation ton scale 76Ge 0νββ • Goal: background of 0.1 c / ROI-t-y 5 yr 90% CL sensitivity: T1/2 > 6.1·1027yr 5 yr 3σ discovery: T1/2 ~ 5.9·1027yr • 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

  41. Example #2: the very large Liquid scint approach Beyond KamLAND-Zen 800: Higher energy resolution = lower2ν background: KamLAND2-ZEN expected σ(2.6MeV)= 4% → ~2％ target sensitivity 20 meV 1000+ kg xenon But eventually 2ν background becomes dominant Super-KamLAND-Zen in connection with Hyper-Kamiokande Beyond? target sensitivity 8 meV

  42. 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 Double-beta decay - Gratta

  43. Preliminary artist view of nEXO in the SNOlabCryopit 13m 14m 14 m Ø13 m Double-beta decay - Gratta

  44. Combining Ionization and Scintillation Anticorrelation between scintillation and ionization in LXe known since early EXO R&D E.Conti et al. PhysRev 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%) Double-beta decay - Gratta

  45. Using event multiplicity to recognize backgrounds (from EXO-200 data) single cluster multiple cluster Low background data 2νββ 228Th calibration source γ γ Double-beta decay - Gratta

  46. 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=6.6x1027yr 3000kg fiducial 500kg fiducial 4780kg fiducial 1000kg fiducial 106 105 104 103 102 10 1 0.1 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) Double-beta decay - Gratta

  47. 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. Double-beta decay - Gratta

  48. 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 Double-beta decay - Gratta

  49. Early ββdecay 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. Double-beta decay - Gratta

  50. Images of few Ba atoms in solid xenon in a laser beam Images of the blue points are shown.. PRELIMINARY PRELIMINARY PRELIMINARY PRELIMINARY PRELIMINARY Double-beta decay - Gratta

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