Precise ${}^{136}$Xe Double Beta Decay Measurement in PandaX-4T with Implications on the Nuclear Matrix Elements and Majorons

Abstract

The continuous spectrum of double beta decay (${\hat{I}}^2{\hat{I}}^2$) provides a sensitive probe to test the predictions of the standard model and to search for signatures of new physics beyond it. We present a comprehensive analysis of the ${}^{136}$Xe ${\hat{I}}^2{\hat{I}}^2$ spectrum utilizing $39.1\pm 0.7\text{kg}\cdot \text{yr}$ of ${}^{136}$Xe exposure from the PandaX-4T experiment. The analysis yields the most precise measurement to date of the ${}^{136}$Xe two-neutrino double beta decay ($2\hat{I}½{\hat{I}}^2{\hat{I}}^2$) half-life, $(2.14\pm 0.05)\times 10^{21}$ years, the uncertainty of which is reduced by a factor of 2 compared to our previous result. We measure the parameter $\hat{I}{¾}_{31}^{2\hat{I}½}$, defined as the ratio between the subleading and leading components of the ${}^{136}$Xe $2\hat{I}½{\hat{I}}^2{\hat{I}}^2$ nuclear matrix element, to be $0.59_{-0.38}^{+0.41}$, which is consistent with theoretical predictions. We also search for Majoron-emitting modes of ${}^{136}$Xe ${\hat{I}}^2{\hat{I}}^2$, establishing the most stringent limit for the spectral index $n=7$.

8 pages, 4 figures