# $\sin^2(\theta)w$ estimate and bounds on nonstandard interactions at source and detector in the solar neutrino low-energy regime

## TL;DR

This paper uses Borexino's low-energy solar neutrino data to set bounds on nonstandard neutrino interactions and to measure the weak mixing angle at an unprecedented low-energy scale, with implications for future experiments.

## Contribution

It provides the first bounds on NSI parameters at low energies and measures the weak mixing angle using solar neutrino data, extending the energy frontier for these parameters.

## Key findings

- Bounded NSI parameters at energies below reactor experiments.
- Measured ( heta)w=0.224b10.016 at low energy.
- Projected future experiments could improve sensitivity fivefold.

## Abstract

We explore the implications of the Borexino experiment's real time measurements of the lowest energy part of the neutrino spectrum from the primary pp fusion process up to 0.420 MeV through the 7^Be decay at 0.862 MeV to the pep reaction at 1.44 MeV. We exploit the fact that at such low energies, the large mixing angle solution to the Mikheyev-Smirnov-Wolfenstein matter effects in the sun are small for 7^Be and pep and negligible for pp. Consequently, the neutrinos produced in the sun change their flavor almost entirely through vacuum oscillations during propagation from the sun's surface and through possible nonstandard interactions acting at the solar source and Borexino detector. We combine the different NSI effects at source and detector in a single framework and use the current Borexino data to bound NSI non-universal and flavor- changing parameters at energies below the reach of reactor neutrino experiments. We also study the implication of the current data for the weak- mixing angle at this "low-energy frontier" data from the Borexino experiment, where it is expected to be slightly larger than its value at the Z mass. We find $\sin^2(\theta)w=0.224+-0.016$, the lowest energy-scale estimate to date. Looking to the future, we use projected sensitivities to solar neutrinos in next generation dedicated solar experiments and direct dark matter detection experiments and find a potential factor five improvement in determination of the weak-mixing angle and up to an order of magnitude improvement in probing the NSI parameters space.