A Search for Neutrinoless Double Beta Decay of 130Te by Adam Douglas Bryant A dissertation submitted in partial satisfaction of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate Division of the University of California, Berkeley Committee in charge: Professor Yury Kolomensky, Chair Professor Stuart Freedman Professor Eric Norman Spring 2010 A Search for Neutrinoless Double Beta Decay of 130Te Copyright 2010 by Adam Douglas Bryant 1 Abstract A Search for Neutrinoless Double Beta Decay of 130Te by Adam Douglas Bryant Doctor of Philosophy in Physics University of California, Berkeley Professor Yury Kolomensky, Chair This dissertation describes an experimental search for neutrinoless double beta (0νββ) decay of 130Te. An observation of 0νββ decay would establish that neu- trinos are Majorana fermions and would constrain the neutrino mass scale. The data analyzed were collected by two bolometric experiments: CUORICINO and an R&D experiment for CUORE known as the Three Towers Test. Both exper- iments utilized arrays of TeO2 crystals operated as bolometers at 10 mK in a dilution refrigerator. The bolometers measured the energy deposited∼ by particle interactions in the crystals by recording the induced change in crystal tempera- ture. Between the two experiments, there were 81 TeO2 bolometers used in the analysis, each of which was an independent detector of nuclear decays as well as a source of 130Te. The experiments were conducted underground at a depth of about 3300 meters water equivalent in Hall A of the Laboratori Nazionali del Gran Sasso in Assergi, Italy, in order to shield the detectors from cosmic rays. The data analyzed represent an exposure of 19:9 kg y of 130Te (18:6 kg y from CUORICINO and 1:3 kg y from the Three Towers Test).· In addition to the· com- bined analysis of the two· experiments, an analysis of CUORICINO data alone is presented in order to compare with an independent analysis being carried out by collaborators at the Univerity of Milano-Bicocca. No signal due to 0νββ decay is observed, and therefore a limit on the partial half-life for the decay is set. From a simultaneous fit to the 81 independent detectors, the rate of 0νββ decay of 130Te is measured to be Γ0νββ(130Te) = ( 0:6 25 1 − ± 1:4 (stat.) 0:4 (syst.)) 10− y− , which corresponds to a lower limit on the ± × 130 0νββ 130 24 partial half-life for 0νββ decay of Te of T1=2 ( Te) > 3:0 10 y (90% C.L.). Converting the half-life limit to an upper limit on the effective× Majorana neutrino mass, mββ, using a set of recent nuclear matrix element calculations results in mββ < 0:25{0:68 eV (90% C.L.), where the range reflects the spread of calculated nuclear matrix element values. These results disagree by at least 1:2σ, depending on the nuclear matrix element calculation, with a claim of observation of 0νββ decay of 76Ge, assuming that the dominant mechanism driving 0νββ decay is the exchange of light Majorana neutrinos. i To my family ii Contents List of Figuresv List of Tables viii 1 Introduction1 1.1 The Standard Model and neutrino masses.............2 1.2 Dirac and Majorana neutrinos....................3 1.3 Neutrino mixing............................5 2 Double beta decay 12 2.1 Nuclear matrix elements....................... 18 2.2 0νββ decay implies Majorana neutrinos............... 19 2.3 Status of experimental searches for 0νββ decay.......... 21 2.3.1 Claim of discovery...................... 24 2.3.2 Next generation experiments................. 25 3 Bolometric detectors 29 3.1 The CUORE bolometer module................... 30 3.1.1 Energy absorber: TeO2 crystal................ 30 3.1.2 Temperature sensor: NTD Ge thermistor.......... 31 3.1.3 Joule heater.......................... 33 3.1.4 Bolometer operation..................... 33 3.2 Bolometric experiments for neutrinoless double beta decay searches 36 3.2.1 CUORICINO......................... 36 3.2.2 Three Towers Test...................... 41 3.2.3 CUORE............................ 44 4 First-level data analysis 49 4.1 Raw data............................... 49 4.2 Analysis software framework..................... 50 4.3 Analysis database........................... 51 4.4 Pulse amplitude evaluation...................... 51 iii 4.5 Identification of re-triggered pulses................. 52 4.6 Offline heater flagging........................ 54 4.7 Gain stabilization........................... 60 4.8 Energy calibration.......................... 62 4.9 Pulse shape discrimination...................... 68 4.10 Thermal response transformation.................. 73 5 Data quality checks and data selection criteria 75 5.1 CUORE Online/Offline Run Check (CORC)............ 76 5.1.1 CORC pages......................... 76 5.1.2 CORC technology and operation.............. 81 5.2 Bad runs and bad intervals...................... 83 5.3 Data selection............................. 85 5.3.1 CORC check......................... 85 5.3.2 Resolution consistency requirements............. 86 5.3.3 Calibration quality and consistency requirements..... 87 5.3.4 Selection criteria for analysis................. 90 6 CUORICINO 0νββ decay fit and limit 91 6.1 Cuts.................................. 92 6.2 Signal efficiency............................ 92 6.2.1 Escape of a β ......................... 92 6.2.2 Pulse shape cut........................ 93 6.2.3 Anti-coincidence cut..................... 96 6.2.4 Signal degradation due to noise............... 96 6.2.5 Pileup............................. 98 6.3 Energy resolutions.......................... 103 6.4 Probability density function..................... 103 6.5 Fit results............................... 108 6.6 Systematic errors........................... 111 6.7 Limit technique and results..................... 112 6.8 Fit validation with toy Monte Carlo simulations.......... 117 7 Three Towers Test and CUORICINO combined analysis 122 7.1 TTT cuts............................... 122 7.2 TTT signal efficiency......................... 123 7.3 TTT energy resolutions........................ 129 7.4 Combined fit results and systematic errors............. 129 7.5 Limit results.............................. 132 iv 8 Conclusion 135 8.1 Comparison with previous CUORICINO results.......... 135 8.2 Limit on the effective neutrino mass................. 136 8.3 Comparison with the claim of discovery............... 137 Bibliography 143 A Energy spectra from CUORICINO and the Three Towers Test 149 B Tables of physical parameters, resolutions, and efficiencies 157 C Generalized pulse amplitude measurement algorithm 166 C.1 Description of the problem...................... 166 C.2 Variation in pulse onset time, t0 ................... 168 C.2.1 Determining the t0 offset................... 168 C.2.2 Truncating the pulse..................... 173 C.2.3 Residual offset......................... 173 C.3 Derivation of the amplitude algorithm............... 174 C.4 Determining the covariance matrix................. 176 C.5 Comparison with the optimal filter................. 177 C.6 Implementation in Diana modules.................. 177 C.7 Use in rejection of spurious pulses.................. 177 v List of Figures 1.1 Neutrino fluxes in SNO........................7 1.2 Neutrino oscillations observed by KamLAND............9 1.3 Neutrino mass hierarchies...................... 11 2.1 Energy levels............................. 13 2.2 Masses of A = 130 isobars...................... 14 2.3 Feynman diagrams for double beta decay.............. 15 2.4 Effective Majorana mass vs. lightest neutrino mass........ 17 2.5 Double beta decay spectra...................... 18 2.6 Nuclear matrix elements....................... 20 2.7 0νββ decay implies Majorana neutrinos............... 21 2.8 Heidelberg-Moscow spectrum.................... 25 2.9 2νββ in the Heidelberg-Moscow experiment............ 26 3.1 CUORICINO floor.......................... 31 3.2 Germanium wafer........................... 32 3.3 Thermistor biasing circuit...................... 34 3.4 NTD Ge thermistor load curve.................... 35 3.5 Optimal working point........................ 35 3.6 CUORICINO pulse.......................... 36 3.7 LNGS halls.............................. 37 3.8 CUORICINO tower.......................... 38 3.9 CUORICINO internal lead shielding................ 39 3.10 Three Towers Test detectors mounted to dilution refrigerator... 42 3.11 Three Towers Test detectors without cylindrical shields...... 43 3.12 TTT channel map.......................... 45 3.13 CUORE cryostat........................... 46 4.1 CUORICINO and TTT pulses.................... 50 4.2 Average pulse and noise power spectrum.............. 53 4.3 Re-triggered pulse........................... 54 4.4 Spectrum before and after offline heater identification....... 58 vi 4.5 Pulse from unidentified heater peak................. 59 4.6 Example of a stabilization fit..................... 61 4.7 Example of a run split for stabilization............... 63 4.8 Calibration spectrum and calibration fit.............. 65 4.9 Calibration peaks with line shape fits................ 67 4.10 Examples of spurious signals..................... 69 4.11 Rise time vs. energy......................... 70 4.12 Examples of pulse shape parameter energy dependence...... 71 4.13 Pulse shape parameter normalization................ 72 4.14 Thermal response transformed pulse................ 74 5.1 CORC History plots......................... 77 5.2 CORC History Graph plots..................... 78 5.3 CORC Summary plots........................ 79 5.4 CORC Channel plots......................... 82