Search for neutrinoless quadruple beta decay of $^{136}$Xe in XMASS-I

TL;DR

This study conducted a search for neutrinoless quadruple beta decay in $^{136}$Xe using the XMASS-I detector, setting a new lower limit on its half-life and marking the first experimental constraint on this decay mode.

Contribution

First experimental search for neutrinoless quadruple beta decay of $^{136}$Xe, establishing a new lower limit on its half-life using pulse shape discrimination techniques.

Findings

No significant signal observed.

Lower limit on half-life set at 3.7 x 10^{24} years.

First experimental constraint on this decay mode.

Abstract

A search for the neutrinoless quadruple beta decay of $^{136}$Xe was conducted with the liquid-xenon detector XMASS-I using $\rm 327\; kg \times 800.0 \; days$ of the exposure. The pulse shape discrimination based on the scintillation decay time constant which distinguishes $\gamma$-rays including the signal and $\beta$-rays was used to enhance the search sensitivity. No significant signal excess was observed from the energy spectrum fitting with precise background evaluation, and we set a lower limit of the half life of 3.7 $\times$ 10$^{24}$ years at 90$\%$ confidence level. This is the first experimental constraint of the neutrinoless quadruple beta decay of $^{136}$Xe.