Abstract

A search for dark matter using an underground single-phase liquid xenon detector was conducted at the Kamioka Observatory in Japan, particularly for Weakly Interacting Massive Particles (WIMPs). We have used 705.9 live days of data in a fiducial volume containing 97kg of liquid xenon at the center of the detector. The event rate in the fiducial volume after the data reduction was (4.2±0.2)×10−3day−1kg−1keVee−1 at 5keVee, with a signal efficiency of 20%. All the remaining events are consistent with our background evaluation, mostly of the “mis-reconstructed events” originated from 210Pb in the copper plates lining the detector's inner surface. The obtained upper limit on a spin-independent WIMP-nucleon cross section was 2.2×10−44cm2 for a WIMP mass of 60GeV/c2 at the 90% confidence level, which was the most stringent limit among results from single-phase liquid xenon detectors.

Highlights

  • The existence of dark matter (DM) in the universe is inferred from many cosmological and astrophysical observations [1, 2]

  • The attribution of mis-reconstructed events to dead PMTs is confirmed by analytically masking normal PMTs and watching the effect on the event distribution. We found that these mis-reconstructed events tend to move in the direction of the line connecting that PMT and the detector center, and that the probability of entering the fiducial volume is determined by the distance to the line and energy

  • The systematic errors coming from the LXe optical parameters and the scintillation decay time were evaluated by comparing Weakly Interacting Massive Particles (WIMPs) Monte Carlo (MC) simulations generated with different absorption and scattering lengths and scintillation decay times of 26.9+−10..82 ns

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Summary

Introduction

The existence of dark matter (DM) in the universe is inferred from many cosmological and astrophysical observations [1, 2]. A single-phase detector has a simple geometry; a minimum requirement is the target and surrounding PMTs. The detector design of XMASS pursues this simplicity as a potential for a scaling-up, low BG with a minimum detector component, and a low energy threshold with a large photo-coverage. The detector design of XMASS pursues this simplicity as a potential for a scaling-up, low BG with a minimum detector component, and a low energy threshold with a large photo-coverage Another key idea to achieve low BG with the single-phase LXe detector which does not have a decent particle identification is shielding of γ-rays from outside material with a largeZ material xenon itself (self-shielding). The abundances of radioactive isotopes (RIs) assumed in the BG prediction were independently measured with dedicated equipment or estimated from the XMASS-I data itself

The XMASS-I detector and the simulations
Event selection
Radioactive BG in XMASS-I
BG events in the fiducial volume and their systematic errors
DM search in the fiducial volume
Findings
Conclusions

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