Scintillation-only based pulse shape discrimination for nuclear and electron recoils in liquid xenon

In a dedicated test setup at the Kamioka Observatory we studied pulse shape discrimination (PSD) in liquid xenon (LXe) for dark matter searches in the absence of an externally applied electric field. PSD in LXe was based on the observation that scintillation light from electron events was emitted over a longer period of time than that of nuclear recoil events, and our method used a simple ratio of early to total scintillation light emission in a single scintillation event. Requiring an efficiency of 50% for nuclear recoil retention we reduced the electron background by a factor of 7.7±1.1(stat) ±_0.6^1.2(sys)×10^-2 at energies between 4.8 and 7.2 keV_ee and 7.7±2.8(stat)± _2.8^2.5(sys)×10^-3 at energies between 9.6 and 12 keV_ee for a scintillation light yield of 20.9 photoelectrons/keV_ee. Further study was done by masking some of that light to reduce this yield to 4.6 photoelectrons/keV_ee. Under these conditions the same method results in an electron event reduction by a factor of 2.4±0.2(stat)±_0.2^0.3(sys)×10 ^-1 for the lower of the energy regions above. We also observe that in contrast to nuclear recoils the fluctuations in our early to total ratio for electron events are larger than expected from statistical fluctuations.