Search for solar electron anti-neutrinos due to spin-flavor precession in the Sun with Super-Kamiokande-IV

Due to a very low production rate of electron anti-neutrinos (ν̄_e) via nuclear fusion in the Sun, a flux of solar ν̄_e is unexpected. An appearance of ν̄_e in solar neutrino flux opens a new window for the new physics beyond the standard model. In particular, a spin-flavor precession process is expected to convert an electron neutrino into an electron anti-neutrino (ν_e→ν̄_e) when neutrino has a finite magnetic moment. In this work, we have searched for solar ν̄_e in the Super-Kamiokande experiment, using neutron tagging to identify their inverse beta decay signature. We identified 78 ν̄_e candidates for neutrino energies of 9.3 to 17.3 MeV in 2970.1 live days with a fiducial volume of 22.5 kiloton water (183.0 kton⋅year exposure). The energy spectrum has been consistent with background predictions and we thus derived a 90% confidence level upper limit of 4.7×10⁻⁴ on the ν_e→ν̄_e conversion probability in the Sun. We used this result to evaluate the sensitivity of future experiments, notably the Super-Kamiokande Gadolinium (SK-Gd) upgrade.