Abstract
Due to a very low production rate of electron anti-neutrinos (ν̄e) via nuclear fusion in the Sun, a flux of solar ν̄e is unexpected. An appearance of ν̄e 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 (νe→ν̄e) when neutrino has a finite magnetic moment. In this work, we have searched for solar ν̄e in the Super-Kamiokande experiment, using neutron tagging to identify their inverse beta decay signature. We identified 78 ν̄e 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 νe→ν̄e conversion probability in the Sun. We used this result to evaluate the sensitivity of future experiments, notably the Super-Kamiokande Gadolinium (SK-Gd) upgrade.
Highlights
While the Sun is known to produce neutrinos through nuclear fusion processes abundantly, small amounts of antineutrinos can be emitted through multiple channels
The selected sample is consistent with background predictions and no significant signal was found
We expect the sensitivity of the solar νe search to significantly improve in Super-Kamiokande Gadolinium (SK-Gd), since this upgrade considerably improves the neutron identification efficiency
Summary
While the Sun is known to produce neutrinos through nuclear fusion processes abundantly, small amounts of antineutrinos can be emitted through multiple channels. The Borexino experiment reported a solar νe flux limit of 384 cm−2s−1 at 90% C.L. in the neutrino energy region of 1.8–16.8 MeV after 2485 live days, which corresponds to Pν →ν < 7.2 × 10−5 at e. Cherenkov detector, where νe can be detected via the charged-current reaction on deuterium, νe+d → e++n+n For this channel, SNO reported an upper limit on the νe flux from the Sun of φν < 3.4×104 cm−2s−1 (90% C.L.) in e the 4–14.8 MeV energy range after 305.9 live days, which corresponds to Pν →ν < 8.1 × 10−3 (90% C.L.) [11]. The first phase of Super-Kamiokande (SK-I) found no significant excess for solar νe in selecting events whose directions were not aligned with the direction from the Sun (cos θsun < 0.5) 5 It set an upper limit on the conversion probability of 8 × 10−3 (90% C.L.).
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