Search for Double Beta Decay of $^{136}$Xe to the $0^+_1$ Excited State of $^{136}$Ba with PandaX-4T

TL;DR

This study searches for a rare double beta decay of xenon-136 to an excited state of barium-136 using PandaX-4T data, setting a new lower limit on the decay's half-life and improving background modeling.

Contribution

First experimental search for $^{136}$Xe double beta decay to the $0^+_1$ excited state using a large xenon detector with improved background analysis.

Findings

No significant decay signal observed.

Established a lower half-life limit of $7.5 imes 10^{22}$ years.

Enhanced background modeling with multi-site event reconstruction.

Abstract

We perform a search of double beta decay of $^{136}$Xe to the excited state, $0^+_1$, of $^{136}$Ba (2$\nu \beta \beta$-0$_1^+$), using the dual-phase xenon detector of PandaX-4T with the first 94.9-day commissioning data. The multi-site events are reconstructed up to the MeV energy scale, which helps to improve the background model significantly. The background contribution from the stainless steel platform outside PandaX-4T cryostat is evaluated for the first time. No significant evidence for 2$\nu \beta \beta$-$0_1^+$ is observed, resulting in a lower limit on half-life of $7.5 \times 10^{22}$ yr at the 90% confidence level. This is the first experimental limit on such a rare decay in a natural xenon-based detector.