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 ${\hat{I}}^2$ decay reaction in a large liquid organic scintillator target. We set a new upper limit for a hypothetical solar $\bar{\hat{I}}{½}_e$ flux of 760 ${\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}$, 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 $1.3\times 10^{-4}$ (90\% C.L.) for $E_{\bar{\hat{I}}½}>1.8$ 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.

8 pages, 2 figures, 1 table