Search for WIMP-${}^{129}$Xe inelastic scattering with particle identification in XMASS-I

A search for Weakly Interacting Massive Particles (WIMPs) was conducted with the single-phase liquid-xenon detector XMASS through inelastic scattering in which ${}^{129}$Xe nuclei were excited, using an exposure ($\rm 327; kg \times 800.0 ; days$) 48 times larger than that of our previous study. The inelastic excitation sensitivity was improved by detailed evaluation of background, event classification based on scintillation timing that distinguished $\gamma$-rays and $\beta$-rays, and simultaneous fitting of the energy spectra of $\gamma$-like and $\beta$-like samples. No evidence of a WIMP signal was found. Thus, we set the upper limits of the inelastic channel cross section at 90% confidence level, for example, $4.1\times {10}^{-39}\mathrm{c}{\mathrm{m}}^2$ for a $200; {\rm GeV}/c^2$ WIMP. This result provides the most stringent limits on the SD WIMP-neutron interaction and is better by a factor of 7.7 at $200;{\rm GeV}/c^2$ than the existing experimental limit.