Searching for Two-Neutrino and Neutrinoless Double Beta Decay of ${}^{134}\mathrm{Xe}$ with the PandaX-4T Experiment

${}^{134}\mathrm{Xe}$ is a candidate isotope for neutrinoless double beta decay ($0\nu \beta \beta$) search. In addition, the two-neutrino case ($2\nu \beta \beta$) allowed by the standard model of particle physics has not yet been observed. With the 656-kg natural xenon in the fiducial volume of the PandaX-4T detector, which contains 10.4% of ${}^{134}\mathrm{Xe}$, and its initial 94.9-day exposure, we have established the most stringent constraints on $2\nu \beta \beta$ and $0\nu \beta \beta$ of ${}^{134}\mathrm{Xe}$ half-lives, with limits of $2.8\times {10}^{22}\mathrm{yr}$ and $3.0\times {10}^{23}\mathrm{yr}$ at 90% confidence level, respectively. The $2\nu \beta \beta$ ($0\nu \beta \beta$) limit surpasses the previously reported best result by a factor of 32 (2.7), highlighting the potential of large monolithic natural xenon detectors for double beta decay searches.

Figure 1

Relative energy resolution $\sigma /\mathrm{E}$ as a function of energy. The black line represents a fit of the experimental data with the function $0.41/\sqrt{\mathrm{E}[\mathrm{keV}]}+2.6\times {10}^{-6}\mathrm{E}[\mathrm{keV}]+0.008$.

Figure 2

Event distributions of the physics data in the energy range of [200, 1000] keV in $z$ vs ${\mathrm{R}}^2$ coordinates, with the color bar indicating counts in each bin. The orange rectangle represents the cylindrical FV.

Figure 3

Energy spectra of ${}^{232}\mathrm{Th}$ calibration data and MC simulation for $\mathrm{MS}+\mathrm{SS}$ and SS events.

Figure 4

The energy spectrum for the ${}^{134}\mathrm{Xe}$ $2\nu \beta \beta$ and $0\nu \beta \beta$ search with fitted background components and the signals at the 90% CL upper limits. The background contributions of xenon isotopes except ${}^{136}\mathrm{Xe}$ are grouped as one curve. The bottom panel shows the residuals.