Study of capture and annihilation of a few GeV WIMPs inside the Sun by using an underground neutrino detector

Dark matter has an enigmatic invisible existence contributing most of the gravity in the universe. Weakly interacting massive particles (WIMPs) are favored as particle candidates for non-baryonic cold dark matter, since their interaction strength can explain the thermal relic abundance of dark matter. Recent event excesses or annual modulation signals reported by direct detection experiments such as DAMA/LIBRA, CoGeNT, CRESST and CDMS II Si would suggest possible evidence for WIMPs with light masses of a few to a few tens of GeV/c. However, they conflict with null results from other experiments such as XENON100, LUX and SuperCDMS. A promising way to identify a WIMP particle is to search for excess of a high-energy neutrino flux coming from the Sun, often referred to as the “solar WIMP search”. WIMPs in the dark matter halo could scatter off nuclei inside the Sun and be captured, then pair-annihilate producing neutrinos from decays of the annihilation products which propagate through the Sun and may finally be detected in neutrino detectors on the earth. Thanks to the hydrogen-rich composition and huge gravity of the Sun, tight limits on the scattering cross section of WIMPs to protons by spin-dependent interaction have been placed by neutrino telescopes such as Super-Kamiokande, IceCube, Baksan and ANTARES. For a critical interpretation of indirect searches by neutrino detectors and comparisons to direct detection experiments, astrophysical uncertainties are key issues. This is especially true for the light WIMP region, where the velocity distribution of WIMPs in the dark matter halo are known to be a large uncertainty source for direct detection searches. The effect of the uncertainties from the velocity distribution of WIMPs in the solar WIMP search are precisely evaluated. We examine various sources of uncertainties including local circular speed of the Sun, high-velocity cut of halo, halo shapes from cosmological simulations and the possible existence of the dark disc. We conclude that the impacts of the uncertainties on the velocity distribution on the solar WIMP search are not significant. The estimated size of the total uncertainty is found to be less than 24% for 20 GeV/c WIMPs for scenarios without the dark disc. For heavier WIMPs, the uncertainties are larger but still below about 50%. Super-Kamiokande (SK), a water Cherenkov neutrino observatory located in Kamioka mine in Japan, is used to search for light WIMPs. The signal acceptances for a few GeV/c WIMPs are improved by one to two orders of magnitude by using vertexcontained type of neutrino events. To discriminate signal from the overwhelming atmospheric neutrino background events, we perform a least-squares fit on event bins finely defined by using angle, energy and flavor information. By fitting 3,903-days of