Search for Neutrinoless $β^+EC$ Decay of ${}^{120}$Te with CUORE

Abstract

CUORE is a large scale cryogenic experiment searching for neutrinoless double beta decay ($0\hat{I}½{\hat{I}}^2{\hat{I}}^2$) in ${}^{130}$Te. The CUORE detector is made of natural tellurium, providing the possibility of rare event searches on isotopes other than ${}^{130}$Te. In this work we describe a search for neutrinoless positron emitting electron capture ($0νβ^+EC$) decay in ${}^{120}$Te with a total TeO${}_2$ exposure of 355.7 kg $\cdot$ yr, corresponding to 0.2405 kg $\cdot$ yr of ${}^{120}$Te. Albeit $0\hat{I}½{\hat{I}}^2{\hat{I}}^2$ with two final state electrons represents the most promising channel, the emission of a positron and two 511-keV ${\hat{I}}^3$s make $0νβ^+EC$ decay signature extremely clear. To fully exploit the potential offered by the detector modularity we include events with different topology and perform a simultaneous fit of five selected signal signatures. Using blinded data we extract a median exclusion sensitivity of $3.4\cdot 10^{22}$ yr at 90% Credibility Interval (C.I.). After unblinding we find no evidence of $0νβ^+EC$ signal and set a 90% C.I. Bayesian lower limit of $2.9\cdot 10^{22}$ yr on ${}^{120}$Te half-life. This result improves by an order of magnitude the existing limit from the combined analysis of CUORE-0 and Cuoricino.