UC Berkeley

This dissertation describes an experimental search for neutrinoless double beta (0νββ) decay of ¹³⁰Te. An observation of 0νββ decay would establish that neutrinos 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 experiments 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 temperature. 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 combined 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 Γ^0νββ(¹³⁰Te) = (-0.6 ± 1.4 (stat.) ± 0.4 (syst.)) × 10^-25 y^-1, which corresponds to a lower limit on the partial half-life for 0νββ decay of ¹³⁰Te of T_1/2^0νββ(¹³⁰Te) > 3.0 × 10²⁴ y (90% C.L.). Converting the half-life limit to an upper limit on the effective Majorana neutrino mass, m_ββ, using a set of recent nuclear matrix element calculations results in m_ββ < 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.