Double- -beta decay: beta decay: Double and new results and new - PowerPoint PPT Presentation

Double- -beta decay: beta decay: Double and new results and new results from EXO- -200 200 from EXO G.Gratta G.Gratta Physics Dept Physics Dept Stanford University Stanford University SPP 2012, Groningen, Jun 2012 SPP 2012, Groningen, Jun 2012

Double- -beta decay beta decay : Double : Candidate nuclei with Q>2 MeV MeV Candidate nuclei with Q>2 a second- -order process order process a second only detectable if first only detectable if first Candidate Q Q Abund. . Candidate Abund order beta decay is order beta decay is (MeV MeV) ) (%) ( (%) energetically forbidden energetically forbidden 48 Ca → 48 Ti 4.271 0.187 76 Ge → 76 Se 2.040 7.8 82 Se → 82 Kr 2.995 9.2 96 Zr → 96 Mo 3.350 2.8 100 Mo → 100 Ru 3.034 9.6 110 Pd → 110 Cd 2.013 11.8 116 Cd → 116 Sn 2.802 7.5 124 Sn → 124 Te 2.228 5.64 130 Te → 130 Xe 2.533 34.5 136 Xe → 136 Ba 2.458 8.9 150 Nd → 150 Sm 3.367 5.6 SPP 2012, Groningen Jun 2011 DoubleBeta decay 2

There are two varieties of ββ decay 0 ν mode: a hypothetical process can happen 2 ν mode: only if: M ν ≠ 0 a conventional 2 nd order process ν = ν in nuclear physics | ∆ L|=2 | ∆ (B-L)|=2 SPP 2012, Groningen Jun 2011 DoubleBeta decay 3

ν ⎛ ⎞ ⎜ ⎟ L “Dirac” neutrinos ν ⎜ ⎟ ν = L D (some “redundant” information but the ⎜ ⎟ ν “good feeling” of things we know…) ⎜ ⎟ R ⎜ ⎟ ν ⎝ ⎠ “Majorana” neutrinos R ν ⎛ ⎞ (more efficient description, no lepton ⎜ ⎟ ν = L M ⎜ ⎟ ν number conservation, new paradigm…) ⎝ ⎠ R Which way Nature chose to proceed is an experimental question � But the alternative is only meaningful/testable for massive particles… which we now know neutrinos are! SPP 2012, Groningen Jun 2011 DoubleBeta decay 4

Our knowledge of the ν ν mass pattern mass pattern Our knowledge of the ~20 eV Time of flight from SN1987A ~2 eV ~1 eV From tritium endpoint (Maintz and Troitsk) (PDG 2002) ~0.3 eV solar~ 7.6·10 -5 eV 2 From Cosmology From 0 νββ if ν is Majorana ~ 2.4·10 -3 eV 2 ~ 2.4·10 -3 eV 2 solar~ 7.6·10 -5 eV 2 The connection of ν masses with cosmological measurements is particularly interesting because it ties together very different fields. We need both, the connection between the two is the interesting part! SPP 2012, Groningen Jun 2011 DoubleBeta decay 5

In the last 10 years there has been a transition 1) From a few kg detectors to 100s or 1000s kg detectors � Think big: qualitative transition from cottage industry to large experiments 2) From “random shooting” to the knowledge that at least the inverted hierarchy will be tested Discovering 0 νββ νββ decay: decay: Discovering 0 � Discovery of the neutrino mass scale Discovery of the neutrino mass scale � � Discovery of Discovery of Majorana Majorana particles particles � � Discovery of Discovery of Majorana Majorana masses masses � � Discovery of lepton number violation Discovery of lepton number violation � SPP 2012, Groningen Jun 2011 DoubleBeta decay 6

If 0 νββ is due to light ν Majorana masses − 1 ⎛ ⎞ 2 2 ⎜ ( ) ⎟ g 2 νββ νββ νββ νββ = − 0 0 0 0 , V m T G E Z M M ⎜ ⎟ ν 1 / 2 0 2 GT F g ⎝ ⎠ A can be calculated within can be calculated within νββ νββ 0 0 and and M M particular nuclear models particular nuclear models F GT νββ 0 a known phasespace phasespace factor factor a known G is the quantity to is the quantity to νββ 0 T be measured be measured 1 / 2 3 effective Majorana ν mass ∑ 2 = ε m U m ( ε i = ±1 if CP is conserved) ν , e i i i = 1 i SPP 2012, Groningen Jun 2011 DoubleBeta decay 7

Calculations differ by about a factor of two (but care is necessary in treating some of them generally regarded as obsolete) S.M. Bilenky and C.Giunti arXiv:1203.5250v2 SPP 2012, Groningen Jun 2011 DoubleBeta decay 8

Note, however, that to discover Majorana neutrinos and lepton number violation the value of the nuclear matrix element is inessential! � 0 νββ decay always implies new physics This is comforting for the ones of us spending their time building experiments! SPP 2012, Groningen Jun 2011 DoubleBeta decay 9

Simplified List of Limits for ββ ββ 0 0 ν ν decay decay Simplified List of Limits for Candidate Detector Present <m> (eV) 0 νββ (yr) nucleus type (kg yr) T 1/2 48 Ca >5.8*10 22 (90%CL) 76 Ge Ge diode 47.7 >1.9*10 25 (90%CL) <0.35 82 Se >2.1*10 23 (90%CL) 96 Zr >9.2*10 21 (90%CL) 100 Mo Foil.Geiger tubes >5.8*10 23 (90%CL) 116 Cd >1.7*10 23 (90%CL) 128 Te >1.1*10 23 (90%CL) ~12 >3*10 24 (90%CL) <0.19–0.68 130 Te TeO 2 cryo ~4.5 >1.2*10 24 (90%CL) <1.1-2.9 136 Xe Xe scint Xe TPC 32.3 >1.6*10 25 (90%CL) <0.14-0.38 150 Nd >1.8*10 22 (90%CL) 160 Gd >1.3*10 21 (90%CL) SPP 2012, Groningen Jun 2011 DoubleBeta decay 10

ββ 0 ν discovery claim Fit model: 214 Bi Q value 6 gaussians + linear bknd. 214 Bi Fitted excess @ Q ββ 28.75 ± 6.86. ??? Claimed significance: 4.2 σ + = ⋅ 0 . 44 24 2 . 23 10 T yr − 1 / 2 0 . 31 = ± 0 . 32 0 . 03 m eV ν [H.V.Klapdor-Kleingrothaus and I.Krivosheina, Mod.Phys.Lett. A21 (2006) 1547] However, this is a very controversial matter See e.g. Strumia+Vissani Nucl Phys B726 (2005) 294 SPP 2012, Groningen Jun 2011 DoubleBeta decay 11

Need very large fiducial Candidate Q Q Abund. . Candidate Abund (MeV MeV) ) (%) ( (%) mass (tons) of isotopically separated material 48 Ca → 48 Ti 4.271 0.187 (except for 130 Te) 76 Ge → 76 Se 2.040 7.8 [using natural material typically 82 Se → 82 Kr 2.995 9.2 means that 90% of the source 96 Zr → 96 Mo 3.350 2.8 produced background but not signal] 100 Mo → 100 Ru 3.034 9.6 This is expensive and provides 110 Pd → 110 Cd 2.013 11.8 encouragement to use the 116 Cd → 116 Sn 2.802 7.5 material in the best 124 Sn → 124 Te 2.228 5.64 possible way: 130 Te → 130 Xe 2.533 34.5 136 Xe → 136 Ba 2.458 8.9 νββ ∝ ∝ For no bkgnd bkgnd 0 For no 1 / 1 / 150 Nd → 150 Sm 3.367 5.6 m T Nt ν 1 / 2 ( ) νββ 1 / 4 ∝ 0 ∝ For statistical bkgnd bkgnd subtraction subtraction For statistical 1 / 1 / m T Nt ν 1 / 2 SPP 2012, Groningen Jun 2011 DoubleBeta decay 12

How to “ “organize organize” ” an experiment: the source an experiment: the source How to Better C.Hall SLAC Summer Institure 2010 Better - High Q value reduces backgrounds and increases the phase space & decay rate, - Large abundance makes the experiment cheaper - A number of isotopes have similar matrix element performance SPP 2012, Groningen Jun 2011 DoubleBeta decay 13

How to “ “organize organize” ” an experiment: the technique an experiment: the technique How to • Final state ID: Final state ID: 1) “Geochemical”: search • 1) “Geochemical”: search for an abnormal abundance for an abnormal abundance of (A,Z+2) in a material containing (A,Z) of (A,Z+2) in a material contai ning (A,Z) 2) “Radiochemical”: store in a mine some material (A,Z) 2) “Radiochemical”: store in a mine some m aterial (A,Z) and after some time try to find and after some time try to find (A,Z+2) in it (A,Z+2) in it + Very specific signature + Very specific signature + Large live times (particularly for 1) + Large live times (particularly for 1) + Large masses + Large masses - Possible only for a few isotopes (in the case of 1) - Possible only for a few isotopes (in the case of 1) - - No distinction between 0 No distinction between 0 ν , 2 2 ν ν or other modes or other modes ν , • “Real time”: “Real time”: ionization or scintillation is detected in the decay • ionization or scintillation is detected in the decay a) “Homogeneous”: source=detector a) “Homogeneous”: source=detector b) “Heterogeneous”: source b) “Heterogeneous”: source ≠ ≠ detector detector + Energy/some tracking available (can distinguish modes) + Energy/some tracking available (can distinguish modes) + In principle universal (b) + In principle universal (b) - - Many Many γ γ backgrounds can fake signature backgrounds can fake signature - Exposure is limited by human patience - Exposure is limited by human patience SPP 2012, Groningen Jun 2011 DoubleBeta decay 14

Shielding a detector from gammas is difficult because the absorption cross section is small. Gamma interaction cross section Typical ββ 0 ν Example: Q values 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 int lengths SPP 2012, Groningen Jun 2011 DoubleBeta decay 15

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