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

Abstract

A search for the neutrinoless quadruple beta decay of ${}^{136}$Xe was conducted with the liquid-xenon detector XMASS-I using $327\mathrm{kg}\times 800.0\mathrm{days}$ of the exposure. The pulse shape discrimination based on the scintillation decay time constant which distinguishes ${\hat{I}}^3$-rays including the signal and ${\hat{I}}^2$-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.