Y. Onishi

Search for solar axions in XMASS, a large liquid-xenon detector

Abstract XMASS, a low-background, large liquid-xenon detector, was used to search for solar axions that would be produced by bremsstrahlung and Compton effects in the Sun. With an exposure of 5.6 ton days of liquid xenon, the model-independent limit on the coupling for mass ≪1 keV is | g a e e | 5.4 × 10 − 11 (90% C.L.), which is a factor of two stronger than the existing experimental limit. The bounds on the axion masses for the DFSZ and KSVZ axion models are 1.9 and 250 eV, respectively. In the mass range of 10–40 keV, this study produced the most stringent limit, which is better than that previously derived from astrophysical arguments regarding the Sun to date.

Light WIMP search in XMASS

Abstract A search for light dark matter using low-threshold data from the single phase liquid xenon scintillation detector XMASS, has been conducted. Using the entire 835 kg inner volume as target, the analysis threshold can be lowered to 0.3 keVee (electron-equivalent) to search for light dark matter. With low-threshold data corresponding to a 5591.4 kgu2009day exposure of the detector and without discriminating between nuclear-recoil and electronic events, XMASS excludes part of the parameter space favored by other experiments.

Search for bosonic superweakly interacting massive dark matter particles with the XMASS-I detector

Bosonic superweakly interacting massive particles (super-WIMPs) are a candidate for warm dark matter. With the absorption of such a boson by a xenon atom, these dark matter candidates would deposit an energy equivalent to their rest mass in the detector. This is the first direct detection experiment exploring the vector super-WIMPs in the mass range between 40 and 120 keV. With the use of 165.9 day of data, no significant excess above background was observed in the fiducial mass of 41 kg. The present limit for the vector super-WIMPs excludes the possibility that such particles constitute all of dark matter. The absence of a signal also provides the most stringent direct constraint on the coupling constant of pseudoscalar super-WIMPs to electrons. The unprecedented sensitivity was achieved exploiting the low background at a level 10(-4)u2009u2009kg-1u2009keVee-1u2009day-1 in the detector.

Search for inelastic WIMP nucleus scattering on 129Xe in data from the XMASS-I experiment

A search for inelastic scattering of Weakly Interacting Massive Particles (WIMPs) on the isotope

Search for two-neutrino double electron capture on 124 Xe with the XMASS-I detector

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A measurement of the time profile of scintillation induced by low energy gamma-rays in liquid xenon with the XMASS-I detector

Xe was done in data taken with the single phase liquid xenon detector XMASS at the Kamioka Observatory. Using a restricted volume containing 41 kg of LXe at the very center of our detector we observed no significant excess of events in 165.9 days of data. Our background reduction allowed us to derive our limits without explicitly subtracting the remaining events which are compatible with background expectations and derive for e.g. a 50 GeV WIMP an upper limit for its inelastic cross section on

Detectability of galactic supernova neutrinos coherently scattered on xenon nuclei in XMASS

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Development of a high sensitivity radon detector for purified gases

Xe nuclei of 3.2 pb at the 90% confidence level.

Direct dark matter search by annual modulation in XMASS-I

Abstract Double electron capture is a rare nuclear decay process in which two orbital electrons are captured simultaneously in the same nucleus. Measurement of its two-neutrino mode would provide a new reference for the calculation of nuclear matrix elements whereas observation of its neutrinoless mode would demonstrate lepton number violation. A search for two-neutrino double electron capture on 124 Xe is performed using 165.9 days of data collected with the XMASS-I liquid xenon detector. No significant excess above background was observed and we set a lower limit on the half-life as 4.7 × 10 21 years at 90% confidence level. The obtained limit has ruled out parts of some theoretical expectations. We obtain a lower limit on the 126 Xe two-neutrino double electron capture half-life of 4.3 × 10 21 years at 90% confidence level as well.

Micro-source development for XMASS experiment

Abstract We report the measurement of the emission time profile of scintillation from gamma-ray induced events in the XMASS-I 832xa0kg liquid xenon scintillation detector. Decay time constant was derived from a comparison of scintillation photon timing distributions between the observed data and simulated samples in order to take into account optical processes such as absorption and scattering in liquid xenon. Calibration data of radioactive sources, 55 Fe, 241 Am, and 57 Co were used to obtain the decay time constant. Assuming two decay components, τ 1 and τ 2 , the decay time constant τ 2 increased from 27.9xa0ns to 37.0xa0ns as the gamma-ray energy increased from 5.9xa0keV to 122xa0keV. The accuracy of the measurement was better than 1.5xa0ns at all energy levels. A fast decay component with τ 1 ∼ 2 ns was necessary to reproduce data. Energy dependencies of τ 2 and the fraction of the fast decay component were studied as a function of the kinetic energy of electrons induced by gamma-rays. The obtained data almost reproduced previously reported results and extended them to the lower energy region relevant to direct dark matter searches.