Constraints on flavor-diagonal non-standard neutrino interactions from Borexino Phase-II

The Borexino detector measures solar neutrino fluxes via neutrino-electron elastic scattering. Observed spectra are determined by the solar-ν_e survival probability P_ee(E), and the chiral couplings of the neutrino and electron. Some theories of physics beyond the Standard Model postulate the existence of Non-Standard Interactions (NSI’s) which modify the chiral couplings and P_ee(E). In this paper, we search for such NSI’s, in particular, flavor-diagonal neutral current interactions that modify the ν_ee and ν_τe couplings using Borexino Phase II data. Standard Solar Model predictions of the solar neutrino fluxes for both high- and low-metallicity assumptions are considered. No indication of new physics is found at the level of sensitivity of the detector and constraints on the parameters of the NSI’s are placed. In addition, with the same dataset the value of sin²θ_W is obtained with a precision comparable to that achieved in reactor antineutrino experiments [Figure not available: see fulltext.].