Search for astrophysical electron antineutrinos in Super-Kamiokande with 0.01wt% gadolinium-loaded water

TL;DR

This study presents the first search for astrophysical electron antineutrinos in the 10 MeV range using gadolinium-loaded Super-Kamiokande, achieving improved background rejection and setting flux upper limits.

Contribution

It introduces gadolinium loading in Super-Kamiokande for enhanced neutron tagging, enabling more efficient detection of electron antineutrinos at lower energies.

Findings

No significant excess of antineutrinos observed.

Flux upper limits established at 90% confidence level.

Enhanced sensitivity comparable to previous pure-water results.

Abstract

We report the first search result for the flux of astrophysical electron antineutrinos for energies O(10) MeV in the gadolinium-loaded Super-Kamiokande (SK) detector. In June 2020, gadolinium was introduced to the ultra-pure water of the SK detector in order to detect neutrons more efficiently. In this new experimental phase, SK-Gd, we can search for electron antineutrinos via inverse beta decay with efficient background rejection and higher signal efficiency thanks to the high efficiency of the neutron tagging technique. In this paper, we report the result for the initial stage of SK-Gd with a $22.5\times552$ $\rm kton\cdot day$ exposure at 0.01% Gd mass concentration. No significant excess over the expected background in the observed events is found for the neutrino energies below 31.3 MeV. Thus, the flux upper limits are placed at the 90% confidence level. The limits and sensitivities are already comparable with the previous SK result with pure-water ($22.5 \times 2970 \rm kton\cdot day$) owing to the enhanced neutron tagging.