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

TL;DR

This study precisely measures the half-life of two-neutrino double electron capture in $^{124}$Xe using PandaX-4T data, providing important insights for neutrino physics and Majorana neutrino searches.

Contribution

The paper introduces a novel time-dependent background model and applies unbinned maximum likelihood fitting to accurately determine the half-life of 2$ u$DEC in $^{124}$Xe.

Findings

Half-life of 2$ u$DEC in $^{124}$Xe is $(1.03 o0.15_{ m stat} o0.08_{ m sys}) imes 10^{22}$ years.

Branching ratio for electrons captured from the $K$ shell is $(65 o5)\%$, consistent with nuclear model predictions.

First application of a time-dependent background model in PandaX-4T data for this measurement.

Abstract

Detailed studies of two-neutrino double electron capture (2$\nu$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$\nu$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$\nu$DEC process to be $(1.03\pm0.15_{\rm stat}\pm0.08_{\rm sys})\times 10^{22}$$\,$yr. Furthermore, we have evaluated the branching ratio for both electrons captured from the $K$ shell ($KK$) to be $(65\pm5)\%$, which aligns with the $^{124}$Xe nuclear model calculations within 1.8$\,$$\sigma$.