Study of solar and other unknown anti-neutrino fluxes with Borexino at LNGS

Abstract We report on the search for anti-neutrinos of yet unknown origin with the Borexino detector at the Laboratori Nazionali del Gran Sasso. In particular, a hypothetical anti-neutrino flux from the Sun is investigated. Anti-neutrinos are detected through the neutron inverse β decay reaction in a large liquid organic scintillator target. We set a new upper limit for a hypothetical solar ν ¯ e flux of 760 cm − 2 s − 1 , obtained assuming an undistorted solar 8 B energy spectrum. This corresponds to a limit on the transition probability of solar neutrinos to anti-neutrinos of 1.3 × 10 − 4 (90% C.L.) for E ν ¯ > 1.8 MeV , covering the entire 8 B spectrum. Best differential limits on anti-neutrino fluxes from unknown sources are also obtained between the detection energy threshold of 1.8 MeV and 17.8 MeV with more than 2 years of data.