Abstract
A search for dark matter (DM) with mass in the sub-GeV region (0.32–1 GeV) was conducted by looking for an annual modulation signal in XMASS, a single-phase liquid xenon detector. Inelastic nuclear scattering accompanied by bremsstrahlung emission was used to search down to an electron equivalent energy of 1 keV. The data used had a live time of 2.8 years (3.5 years in calendar time), resulting in a total exposure of 2.38 ton-years. No significant modulation signal was observed and 90% confidence level upper limits of 1.6×10−33 cm2 at 0.5 GeV was set for the DM-nucleon cross section. This is the first experimental result of a search for DM mediated by the bremsstrahlung effect. In addition, a search for DM with mass in the multi-GeV region (4–20 GeV) was conducted with a lower energy threshold than previous analysis of XMASS. Elastic nuclear scattering was used to search down to a nuclear recoil equivalent energy of 2.3 keV, and upper limits of 2.9 ×10−42 cm2 at 8 GeV was obtained.
Highlights
The nature of dark matter (DM) is a key mystery in cosmology, and detecting it via any force other than gravity is essential for advancing particle physics beyond the standard model
The energy threshold for multi-GeV DM analysis via nuclear recoil is set to 2.3 keVnr such that we could suppress an impact of the systematic error caused by the flasher events explained in Section 6, to be smaller than other errors
Considering that we found no significant signal, the 90% confidence level (CL) upper limit on the DM-nucleon cross section σup was calculated by the Bayesian approach [19]: σup
Summary
The nature of dark matter (DM) is a key mystery in cosmology, and detecting it via any force other than gravity is essential for advancing particle physics beyond the standard model. A search via DM-electron scattering by existing detectors have been performed [12, 13] In addition to these detectors, conventional xenon detectors should be sensitive to DM with sub-GeV mass [14, 15], due to the irreducible contribution of the bremsstrahlung effect accompanying nuclear recoils [14]. December 27, 2018 for Migdal, ∼10−8 for Bremsstrahlung at 1 GeV), because these inelastic effects lead to larger energy deposition than elastic nuclear recoil, it should be possible to detect sub-GeV DM through these effects. For multi-GeV DM search, data with lower energy threshold than in previous studies [17, 18] were used to improve sensitivity in the low mass range. These searches were conducted by looking for the annual modulation of the event rate in the XMASS data
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