Bounds on the Solar Antineutrino total Flux and Energy spectrum from the SK experiment

TL;DR

This paper sets upper limits on the flux and energy spectrum of solar antineutrinos using Super-Kamiokande data, constraining their possible contribution and conversion probabilities based on the absence of a significant signal.

Contribution

It provides the first stringent bounds on solar antineutrino flux and spectrum from SK data, improving understanding of neutrino-antineutrino conversions.

Findings

Upper bound on solar antineutrino flux: < 1.8×10^5 cm^-2 s^-1

Conversion probability less than 3.5% for ${}^8 B$ neutrinos

Conversion probability below 8% for energies >6.5 MeV

Abstract

A search for inverse beta decay electron antineutrinos has been carried out using the 825 days sample of solar data obtained at SK. The absence of a significant signal, that is, contributions to the total SK background and their angular variations has set upper bounds on a) the absolute flux of solar antineutrinos originated from ${}^8 B$ neutrinos $\Phi_{\bar{\nu}}({}^8 B)=< 1.8\times 10^5 cm^{-2} s^{-1}$ which is equivalent to an averaged conversion probability bound of P<3.5% (SSM-BP98 model) and b) their differential energy spectrum, the conversion probability is smaller than 8% for all $E_{e,vis}>6.5$ MeV going down the 5% level above $E_{e,vis}\approx 10$ MeV. It is shown that an antineutrino flux would have the net effect of enhancing the SK signal at {\em hep} neutrino energies. The magnitude of this enhancement would highly depend on the, otherwise rather uncertain, steepness of the solar neutrino spectrum at these energies.