Bounds on the solar antineutrino total flux and energy spectrum from the SK experiment

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 Bur of solar antincutrinos originated from B-8 neutrinos Phi (-)(1')(B-8) less than or equal to 1.8 x 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,E-vis > 6.5 MeV going down the 5% level above E-e,E-vis approximate to 10 MeV. It is shown that an antineutrino flux would have the net effect of enhancing the SK signal at hep neutrino energies. The magnitude of this enhancement would highly depend on the, otherwise rather uncertain at this moment, steepness of the solar neutrino spectrum at these energies. (C) 2000 Elsevier Science B.V. All rights reserved.