Limiting the effective magnetic moment of solar neutrinos with the Borexino detector

Abstract

Borexino has provided an updated upper limit on the effective neutrino magnetic moment of solar neutrinos μ_βα < 2.8 x 10⁻¹¹ μ_औ at 90% C.L. This result represents nearly a factor of two improvement with respect to the previous result based on 192 days of the Phase-I data. The current analysis has been performed using 1291.5 days exposure of the Phase-II data, characterized by a further improved level of radio-purity of liquid scintillator. Another key ingredient of the new analysis, lowering the threshold from 260 to 186 keV, was possible thanks to a better understanding of the detector-response function at low energies. The global spectral fit was preformed up to 2970 keV energy, using constraints on the sum of the solar neutrino fluxes implied by the radiochemical gallium experiments. From the limit for the effective neutrino magnetic moment, new limits for the magnetic moments of the neutrino flavour states were derived.