Measurement of two-neutrino double electron capture half-life of ${}^{124}$Xe with PandaX-4T

Abstract

Detailed studies of two-neutrino double electron capture (2$\hat{I}½$DEC) is a crucial step towards searching for the neutrino-less mode to explore the Majorana nature of neutrinos. We have measured precisely the half-life of the 2$\hat{I}½$DEC process in ${}^{124}$Xe, utilizing a total exposure of 1.73 tonne$\cdot$year from the commissioning run and the first science run of the PandaX-4T experiment. A time-dependent background model in the $\mathcal{O}$(10 keV) energy is constructed for the first time in PandaX-4T data. With an unbinned maximum likelihood fit, we determine the half-life of the 2$\hat{I}½$DEC process to be $(1.03\pm 0.15_{\mathrm{stat}}\pm 0.08_{\mathrm{sys}})\times 10^{22}$yr. Furthermore, we have evaluated the branching ratio for both electrons captured from the $K$ shell ($KK$) to be $(65\pm 5)\%$, which aligns with the ${}^{124}$Xe nuclear model calculations within 1.8$\ddot{I}ƒ$.

19 pages, 5 figures, 4 tables; version3 accepted by JHEP