# Constraints on Flavor-Diagonal Non-Standard Neutrino Interactions from Borexino Phase-II

## TL;DR

This study uses Borexino Phase-II data to search for flavor-diagonal non-standard neutrino interactions, setting constraints on new physics beyond the Standard Model without finding evidence for such interactions.

## Contribution

It provides new limits on flavor-diagonal NSI parameters using solar neutrino data and measures the weak mixing angle with competitive precision.

## Key findings

- No evidence of non-standard interactions was observed.
- Constraints on NSI parameters were established.
- The weak mixing angle was measured with high precision.

## Abstract

The Borexino detector measures solar neutrino fluxes via neutrino-electron elastic scattering. Observed spectra are determined by the solar-$\nu_{e}$ 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 $\nu_e e$ and $\nu_\tau e$ 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 $\sin^2\theta_W$ is obtained with a precision comparable to that achieved in reactor antineutrino experiments.