Considerations for future neutrinoless double beta decay experiments AFCI Neutrino Mass Workshop J. F. Wilkerson December 14, 2015
Outline • Brief overview of 0 νββ and sensitivity to neutrino mass. • Is there a preferred 0 νββ isotope in terms of sensitivity? • What levels of backgrounds and exposure are required for future 0 νββ experiments to cover the inverted ordering region? • What are prospects and considerations for future ton scale 0 νββ experiments? • Relationship to other measurements? • Summary Considerations for Future AFCI Neutrino Mass Workshop 2 0 νββ Experiments. 14 December 2015
Outline • Brief overview of 0 νββ and sensitivity to neutrino mass. • Is there a preferred 0 νββ isotope in terms of sensitivity? • What levels of backgrounds and exposure are required for future 0 νββ experiments to cover the inverted ordering region? • What are prospects and considerations for future ton scale 0 νββ experiments? • Relationship to other measurements? • Summary Considerations for Future AFCI Neutrino Mass Workshop 3 0 νββ Experiments. 14 December 2015
0 νββ decay Requires: • neutrino to have non-zero mass • “wrong-handed” helicity admixture ~ m i /E ν i Any process that allows 0 νββ to occur requires e - e - Majorana neutrinos with non-zero mass. ν i (R) ν i (L) Schechter and Valle, 1982 U ei U ei W - W - • Lepton number violation > > • No experimental evidence that Lepton Nuclear Process (A, Z) (A, Z+2) number must be conserved (i.e. allowed based on general SM principles, such as electroweak-isospin conservation and renormalizability) If 0 νββ decay is observed ⇒ neutrinos are Majorana particles lepton number is violated Considerations for Future AFCI Neutrino Mass Workshop 4 0 νββ Experiments. 14 December 2015
0 νββ and ν mass Observable (decay rate) depends on nuclear processes & nature of lepton number violating interactions ( η ). e - e - − 1 2 η 2 0 ν ⎡ ⎤ T = G 0 ν M 0 ν ( η ) ⎢ ⎥ 1/2 ν i (R) ν i (L) ⎣ ⎦ U ei U ei W - W - ⇓ 2 − 1 > > 2 m ββ Nuclear Process 0 ν ⎡ ⎤ T = G 0 ν M 0 ν ⎢ ⎥ (A, Z) (A, Z+2) 1/2 m e ⎣ ⎦ • Phase space, G 0 ν is calculable. • Nuclear matrix elements (NME) via theory. • Effective neutrino mass, <m ββ >, depends directly on the assumed form of lepton number violating (LNV) interactions. Considerations for Future AFCI Neutrino Mass Workshop 5 0 νββ Experiments. 14 December 2015
Extracting ν mass from observed 0 ν ββ rates • Requires a lepton number violating (LNV) mechanism (model) 1 - In the “usual” model – light Majorana e - e - e - e - e - e - e - e - [eV] neutrino and SM interactions – <m ββ > -1 ν heavy 10 β χ β ν i (R) ν i (L) mass, m ν depends on mass hierarchy, lepton matrix U ei U ei Inverted _ _ mixing values, & Majorana phases. W - W - -2 10 W R- W L- W R- e W R- e β β ν • The combination of certain θ ij , m i , and Effective 0 Normal > > > > > > > > -3 10 Nuclear Process Nuclear Process Nuclear Process Nuclear Process phases φ k values cancel out and could (A, Z) (A, Z) (A, Z+2) (A, Z+2) (A, Z) (A, Z) (A, Z+2) (A, Z+2) yield no observable decay. -4 10 -4 -3 -2 -1 10 10 10 10 1 Lightest neutrino mass [eV] • Requires calculation of reliable theoretical nuclear matrix elements. • Advantage of multiple isotopes but one “true” value of <m ββ >: 48 Ca, 76 Ge, 82 Se, 96 Zr 100 Mo, 116 Cd 130 Te, 136 Xe, 150 Nd • Potential measurements of excited state decays. • Knowledge of effective weak-axial coupling constant. Considerations for Future AFCI Neutrino Mass Workshop 6 0 νββ Experiments. 14 December 2015
Nuclear matrix elements - M 0 ν 2 − 1 2 m ββ 0 ν " % T = G 0 ν M 0 ν $ ' 1/2 m e # & • Available model results P . Vogel 2014 differ by factors of 2-3 • Improvement is highly desirable: the matrix elements are essential for interpretation — Recently funded theory initiative in the U.S. with goal of quantifying uncertainties. • Discovery goals set by Matrix elements for “standard mechanism” taking “pessimistic” matrix elements Considerations for Future AFCI Neutrino Mass Workshop 7 0 νββ Experiments. 14 December 2015
0 νββ Decay and <m ββ > Assuming LNV mechanism is light Majorana neutrino exchange and SM interactions (W) 2 − 1 2 m ββ 0 ν " % T = G 0 ν M 0 ν $ ' 1/2 m e # & 10 t-yr 100 t-yr 2015 NSAC Long Range Plan Considerations for Future for Nuclear Science AFCI Neutrino Mass Workshop 8 0 νββ Experiments. 14 December 2015
Outline • Brief overview of 0 νββ and sensitivity to neutrino mass. • Is there a preferred 0 νββ isotope in terms of sensitivity? • What levels of backgrounds and exposure are required for future 0 νββ experiments to cover the inverted ordering region? • What are prospects and considerations for future ton scale 0 νββ experiments? • Relationship to other measurements? • Summary Considerations for Future AFCI Neutrino Mass Workshop 9 0 νββ Experiments. 14 December 2015
Sensitivity to <m ββ > per atom (<m ββ >=17.5meV) T ½ 10 28 y 10 27 y 10 26 y Figure source: A. Dueck, W. Rodejohann, and K. Zuber, Phys. Rev. D83 (2011) 113010. Considerations for Future AFCI Neutrino Mass Workshop 10 0 νββ Experiments. 14 December 2015
R.G.H. Robertson Rates (sensitivity) per unit mass MPL A 28 (2013) 1350021 (arXiv 1301.1323) Typically phase space is expressed in activity per atom, not per unit mass. − 1 2 ⎡ ⎤ 2 0 ν m ββ 4 M 0 ν = G 0 ν g A T The phase space G 0 ν is in activty per atom ⎢ ⎥ 1/2 ⎣ ⎦ m e λ 0 ν N 2 2 M = ln(2) N A 4 M 0 ν 2 G 0 ν g A m ββ Am e The specific phase space H 0 ν is in 2 2 4 M 0 ν ≡ H 0 ν g A m ββ activity per unit mass Considerations for Future AFCI Neutrino Mass Workshop 0 νββ Experiments. 14 December 2015 11
R.G.H. Robertson, Sensitivity to <m ββ > MPL A 28 (2013) 1350021 (arXiv 1301.1323) For Ge, Te, Xe, Nd uncertainty on NME 2 uncertainty on value of g A4 Signal of 1 cnt/t-y for corresponding values of NME and g A Considerations for Future AFCI Neutrino Mass Workshop 0 νββ Experiments. 14 December 2015 12
Sensitivity per unit mass of isotope ➡ Isotopes have comparable sensitivities in terms of rate per unit mass R.G.H. Robertson, MPL A 28 (2013) 1350021 (arXiv 1301.1323) Inverse correlation observed between phase space and the square of the nuclear matrix element . The points in order of increasing abscissa value are: 48 Ca, 150 Nd, 136 Xe, 96 Zr, 116 Cd, 124 Sn, 130 Te, 82 Se, 76 Ge, 100 Mo and 110 Pd. geometric mean of the squared matrix element range limits & the phase-space factor evaluated at g A =1 Considerations for Future AFCI Neutrino Mass Workshop 0 νββ Experiments. 14 December 2015 13
0 νββ Isotopes : Natural Abundances 48Ca 76Ge ββ Isotope Natural Abundance 82Se 48 Ca 0.187 7.8 76 Ge 100Mo 9.2 82 Se 116Cd 9.6 100 Mo 7.6 116 Cd 130Te 34.5 130 Te 8.9 136 Xe 136Xe 5.6 150 Nd 150Nd 0 10 20 30 40 Natural Abundance (%) Clearly 130 Te has an advantage. For the others, Isotopic enrichment ($s) is needed Considerations for Future AFCI Neutrino Mass Workshop 14 0 νββ Experiments. 14 December 2015
0 νββ Isotopes : Q-Values 208 Tl 2614 line 48Ca 76Ge ββ Isotope Q-Value 4273.7 48 Ca 82Se 2039.1 76 Ge 2995.5 82 Se 100Mo 3035 100 Mo 116Cd 2809.1 116 Cd 2530.3 130 Te 130Te 2457.8 136 Xe 3367.3 150 Nd 136Xe 150Nd 0 1250 2500 3750 5000 Q-Value (keV) • Higher Q-value will result in the ββ -decay signal being above potential backgrounds. Considerations for Future AFCI Neutrino Mass Workshop 15 0 νββ Experiments. 14 December 2015
0 νββ Isotope : 2 νββ T 1/2 2 νββ 48Ca 0 νββ 76Ge 82Se 100Mo 116Cd 130Te ββ Isotope 2 νββ T 1/2 10 20 years 136Xe 0.44 48 Ca 150Nd 15 76 Ge 0.92 82 Se 0 5 10 15 20 25 0.07 100 Mo 10 20 years 0.29 116 Cd 9.1 130 Te 21 136 Xe Longer 2 νββ T 1/2 (better) ⇒ lower rate 0.08 150 Nd Irreducible background ⇒ minimize with good resolution Considerations for Future AFCI Neutrino Mass Workshop 16 0 νββ Experiments. 14 December 2015
A preferred 0 νββ isotope in terms of sensitivity? • No preferred isotope in terms of per unit mass - within current uncertainties on NME and g A . • Need to enrich - 130 Te has an advantage • Backgrounds - higher Q value (especially above 208 Tl line helps) • 2 νββ rate (irreducible background) - 76 Ge 130 Te, 136 Xe are the best. - good resolution important No clear winner. Need to evaluate on case-by-case basis. Backgrounds and resolution are critically important. Considerations for Future AFCI Neutrino Mass Workshop 17 0 νββ Experiments. 14 December 2015
Outline • Brief overview of 0 νββ and sensitivity to neutrino mass. • Is there a preferred 0 νββ isotope in terms of sensitivity? • What levels of backgrounds and exposure are required for future 0 νββ experiments to cover the inverted ordering region? • What are prospects and considerations for future ton scale 0 νββ experiments? • Relationship to other measurements? • Summary Considerations for Future AFCI Neutrino Mass Workshop 18 0 νββ Experiments. 14 December 2015
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