Searches for Double Beta Decay The Allure of Ultra-Sensitive Experiments Carter Hall, Univ. of Maryland - February 18, 2014! Outline • The Majorana/Dirac question • Is neutrino mass an example of beyond-standard-model physics? • Double beta decay as a Majorana/Dirac discriminant • Experimental techniques and challenges • Current status of experimental efforts Spin 1/2 mass spectrum 1st 2nd 3rd TeV Higgs! t b GeV c τ s 5 µ ~10 d MeV u e Fermion Mass Fermion keV ~105 eV at least one neutrino meV Charged fermions masses are due to interactions with the Higgs top quark: H H H H tL tL tL tL tL t t tR R R tR tR H H H H H mass term H electron - - e L - e R e R H Dirac mass term: conserves charge (e- →e-) Spin 1/2 mass spectrum 1st 2nd 3rd TeV t b GeV c τ s 5 µ ~10 d MeV u e Fermion Mass Fermion keV ~105 eV at least one neutrino meV Higgs? Types of Neutrino mass terms Dirac Neutrino Mass Standard model X ν Higgs creates only νR L Conserves Dirac masses! lepton number Majorana Mass 1 X ν νR L violates conservation of lepton number Majorana Mass 2 Neutrinos could, X νR νL in principle, have more than one mass term. What if neutrinos have two mass terms? Dirac Mass Dirac Mass (Higgs?) (Higgs?) X X νL ν νR R X νL Majorana Mass (New physics) Majorana ν νL Neutrino R Mass If neutrinos have more than one mass term, then they are Majorana particles. OR A Dirac neutrino is a special case where both Majorana mass terms are exactly zero. See-saw mechanism generated by the Higgs Suppose m(ν)Dirac ~ 100 GeV, like the top quark 15 and m(ν)Majorana ~ 10 GeV generated by some type of new physics at the Grand Unification energy scale Then we would observe two Majorana neutrinos, m2 D −2 like atmospheric oscillations with m1 ≈ ≈10 eV ν M GUT m2 ≈ M GUT too large to be seen directly M.Gellman, R.Slanksy, P.Ramond, R.Mohapatra, G.Senjanovic Spin 1/2 mass spectrum 1st 2nd 3rd TeV t b c τ GeV s µ d Higgs MeV u e Are neutrino masses Fermion Mass Fermion suppressed by grand keV unification physics? eV at least one neutrino meV Are neutrino masses Dirac or Majorana? Is the neutrino its own anti-particle? Question: Is neutrino mass a clear case of new physics? Or can it be accommodated in the standard model? What does a ‘standard model neutrino mass’ look like? 1. It should be due to the Higgs mechanism, which implies a Dirac neutrino: X νL νR mD X ν νL R mD • Note that we require the existence of νR and νL. These states are unusual: unlike all other SM particles, they have no gauge interactions…. they are “sterile”. • We do not yet have definitive evidence for their existence. Question: Is neutrino mass a clear case of new physics? Or can it be accommodated in the standard model? 2. Having introduced νR and νL, what prevents the appearance of Majorana mass terms? For example, what prevents this? X ν νR L MR – In QFT, ‘that which is not forbidden is mandatory’. – If we wish to maintain a Dirac neutrino, we should impose a global symmetry to enforce lepton number conservation and prevent the appearance of Majorana neutrino masses. – Lepton number conservation is fundamentally different than the conservation of color and electric charge. Those conservation laws are a consequence of the standard model’s gauge symmetries. Lepton number conservation has no comparable theoretical or experimental foundation. Question: Is neutrino mass a clear case of new physics? Or can it be accommodated in the standard model? 3. Summary: To accommodate neutrino mass in the standard model, we 1. introduced two new states (νR and νL), and 2. altered the symmetry structure of the theory. – Many people would say that these two features alone are sufficient to qualify as non-standard model physics. – Secondly, we would still like to have an explanation for why the Higgs couplings of the neutrinos are so small. – Conversely, if we allow for a new-physics mechanism at very high energies to generate Majorana mass terms, then the light neutrino masses are a generic consequence, which is appealing. – To distinguish these scenarios, we must determine if neutrinos are Dirac or Majorana. Use nuclear physics to test lepton number conservation in neutrinos: beta decay e - acts a neutrino source L Majorana neutrino? W-W- n ν p Inside a nucleus Use nuclear physics to test lepton number conservation in neutrinos: eL- Majorana neutrino? W-W- 2n eL- ν W- Inside a nucleus 2p second vertex acts as the detector Neutrinoless Double Beta Decay (ββ0ν) Forbidden if neutrino mass is Dirac only eL- N(Z,A)→N(Z+2,A)e-e- Majorana neutrino W-W- 2n e - ν L X m(ν) W- Forbidden Inside a nucleus by lepton number conservation if neutrinos are Dirac. “Neutrino mass 2p mechanism” for double beta decay The equivalent HEP process is e-e-→W-W- Imagine a 1 TeV e-e- collider must not carry Need both νL and νR, lepton number so νe must be massive If TeV scale Majorana neutrinos exist, this process could be used to discover them. For the light neutrinos that we know and love, the rate is far too small. 16 The ββ0ν half-life depends directly on the absolute neutrino masses Decay rate −1 2 2 i T 0νββ G0νββ NME m , m U 2 m e αi ( 1/ 2 ) = ⋅ ⋅ ν ν = ∑ ei i i effective mass Majorana mixing matrix phases phase space Nuclear Matrix (with phases) Element mass eigenvalues We can use the ββ0ν decay rate to measure the absolute neutrino mass if: 1) neutrinos are Majorana 2) we have a reliable calculation of the nuclear matrix element 3) the neutrino mass mechanism dominates the decay Many types of new physics lead to ββ0ν Right handed weak currents, leptoquarks, supersymmetry, ect., d u W- e- h-- W- e- d u “Doubly charged Higgs mechanism” for double beta decay Does this spoil neutrino-physics interpretations of ββ0ν ? 18 Black box theorem guarantees that ββ0ν always implies Majorana ν’s Anything that converts ν into ν implies that neutrinos are Majorana ν ν If we see ββ0ν, we know that Majorana neutrinos exist. Neutrino mass spectrum 48.5 ± 1.2 meV 8.71 ± 0.15 meV Comparison of ν mass techniques ββ0ν vs cosmology 1 EXO, KamLAND, ) ν 0 Gerda ββ 10-1 inverted hierarchy CMB ) – eV – ( ) – eV ββ (Planck + -2 m( 10 WMAP+ normal hierarchy high L) Σm - eV – (cosmology) 21 Adapted from G.L Fogli, et al, PRD 78 033010 (2008) Two-Neutrino Double Beta Decay: eL- W-W- 2n eL- νR Two neutrons convert to two protons and four leptons. W- νR First direct observation by Moe, Elliott, and Hahn 2p in 100Se (1988) No direct implications for neutrino mass, but useful for constraining and testing the nuclear matrix element calculations ββ0ν signature: a peak in the ββ energy spectrum ββ2ν spectrum (normalized to 1) 0νββ peak (5% FWHM) (normalized to 10-6) ββ0ν signal (5% FWHM) (normalized to 10-2) Summed electron energy in units of the kinematic endpoint (Q) Choice of ββ source isotope atomic mass atomic decay energy number of protons Choice of ββ source isotope Isotope Q value Abundance centrifuge (MeV) (%) enrichment? 48Ca 4.271 0.187 No 76Ge 2.039 7.8 Yes 82Se 2.995 9.2 Yes 96Zr 3.350 2.8 No 100Mo 3.034 9.6 Yes 110Pd 2.013 11.8 116Cd 2.802 7.5 Yes 124Sn 2.228 5.64 130Te 2.533 34.5 Yes 136Xe 2.457 8.9 Yes 150Nd 3.367 5.6 No Q values and natural abundance 5 4.5 48Ca Better Better 4 3.5 150Nd 96Zr 100Mo 3 82Se 116Cd Q value (MeV) 130Te 2.5 136Xe 124Sn 110Pd 2 76Ge 1.5 0 10 20 30 40 50 natural abundance (%) Better 26 27 Neutrino mass sensitivity for T1/2 = 10 years −1 2 T 0νββ G0νββ NME 2 m ( 1/2 ) = ⋅ ⋅ ν From P. Vogel, J. Phys. G: Nucl. Part. Phys. 39 (2012) Radioactive backgrounds: Nuclide half lives “Stable”: ~1015 years “Peninsula ββ0ν halflife is 1010 times longer of stability” half-life comparable to age of the earth half-life short compared to age of the earth Peninsula of Stability Uranium-235 Curium-247 Plutonium-244 Thorium-232 Uranium-238 Uranium-238 decay chain start Thorium-232 decay chain start gamma sources end end Radioactive decay here on earth 76 136Xe Ge 130Te 116Cd 82Se ββ0ν Q values: 100Mo 150Nd 48Ca Source: http://npgroup.pd.infn.it/luna/images/background.jpg Double beta decay frustration theorem Gamma interaction cross section typical ββ Q values gammas travel about 2 cm before scattering in lead Lead shielding Historical progress T1/2 limit - 0ν decay 1026 1025 1024 1023 Years 1022 1021 1020 1960 1970 1980 1990 2000 2010 2020 2030 Publication Date Historical progress m(ν) limit 102 76Ge 136Xe 130 10 Te 100Mo eV 1 -1 10 NME: RQRPA, Phys.Rev.C 79, 055501 (2009) 1960 1970 1980 1990 2000 2010 2020 2030 Publication Date Germanium Diode Experiments Heidelberg-Moscow and IGEX 2-3 kg Ge diodes, 80% 76Ge Lead shielding Cu cryostats • Precise energy resolution (4 keV FWHM). • Pulse shape analysis rejects multiple site events within a single crystal. • Modest Q value (2039 keV) 76Ge: Heidelberg-Moscow Full spectrum ROI closeup 25 T1/2(ββ0ν) > 1.9 x 10 yrs (90%C.L.) mββ < 350 meV Exposure: 709 mol*years Eur.