Abstract
The CUORE experiment is a large bolometric array searching for the lepton number violating neutrino-less double beta decay (0nu beta beta ) in the isotope mathrm {^{130}Te}. In this work we present the latest results on two searches for the double beta decay (DBD) of mathrm {^{130}Te} to the first 0^{+}_2 excited state of mathrm {^{130}Xe}: the 0nu beta beta decay and the Standard Model-allowed two-neutrinos double beta decay (2nu beta beta ). Both searches are based on a 372.5 kgtimes yr TeO_2 exposure. The de-excitation gamma rays emitted by the excited Xe nucleus in the final state yield a unique signature, which can be searched for with low background by studying coincident events in two or more bolometers. The closely packed arrangement of the CUORE crystals constitutes a significant advantage in this regard. The median limit setting sensitivities at 90% Credible Interval (C.I.) of the given searches were estimated as mathrm {S^{0nu }_{1/2} = 5.6 times 10^{24} , mathrm {yr}} for the {0nu beta beta } decay and mathrm {S^{2nu }_{1/2} = 2.1 times 10^{24} , mathrm {yr}} for the {2nu beta beta } decay. No significant evidence for either of the decay modes was observed and a Bayesian lower bound at 90% C.I. on the decay half lives is obtained as: mathrm {(T_{1/2})^{0nu }_{0^+_2} > 5.9 times 10^{24} , mathrm {yr}} for the 0nu beta beta mode and mathrm {(T_{1/2})^{2nu }_{0^+_2} > 1.3 times 10^{24} , mathrm {yr}} for the 2nu beta beta mode. These represent the most stringent limits on the DBD of ^{130}Te to excited states and improve by a factor sim 5 the previous results on this process.
Highlights
Double beta decay (DBD) is an extremely rare nuclear process where a simultaneous transmutation of a pair of neutrons into protons converts a nucleus (A, Z) into an isobar (A, Z+2), with the emission of two electrons and two antineutrinos
We evaluate for each dataset the pulse shape selection (PSA) cut efficiency term as the average of the two efficiencies obtained from such samples
We store each simulated Toy Monte Carlo simulations (ToyMC) event as a vector of ordered energy releases Eev and related bolometers chev, where the bolometers are randomly extracted from the active bolometers of each dataset according to their exposure in the data, while the energies are generated according to the selected shape of the background pdf computed with the parameters generated according to the posterior pdfs obtained with the blinded fit to the data
Summary
Double beta decay (DBD) is an extremely rare nuclear process where a simultaneous transmutation of a pair of neutrons into protons converts a nucleus (A, Z) into an isobar (A, Z+2), with the emission of two electrons and two antineutrinos. The neutrinoless mode of the decay (0νββ) is a posited Beyond Standard Model process that could shed light on many open aspects of modern particle physics and cosmology such as the existence of lepton number violation and elementary Majorana fermions, the neutrino mass scale, and the baryon asymmetry in the Universe [1,2,3,4,5] Both DBD modes can proceed through transitions to the ground state as well as to various excited states of the daughter nucleus. Due to the emission of such coincident de-excitation γ rays, both 0νββ and 2νββ decay channels allow a significant background reduction with respect to the corresponding transitions to the ground state This holds especially in an experimental setup that exploits a high detector granularity, such as the CUORE experiment
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