Constraints on non-standard neutrino interactions from Borexino extended data-set

TL;DR

This work extends the Borexino NSI program by incorporating Phase III solar neutrino data and generalizing to all off-diagonal NSI terms, enabling a full 12-parameter NSI analysis. It employs a novel MC-based PDF reweighting method to propagate NSI effects through both solar neutrino propagation and ν–e scattering, within a robust likelihood framework that includes independent penalties from backgrounds and solar fluxes. The analysis reports improved constraints on vector and axial NSI parameters across diagonal and off-diagonal flavors, finds no significant deviation from the Standard Model, and demonstrates consistency with, and refinement of, previous Borexino results and other experiments. Overall, Borexino’s extended data set reinforces its role in probing beyond-Standard-Model neutrino interactions and narrows the viable NSI parameter space relevant to solar neutrino phenomenology.

Abstract

Neutrino non-standard interactions (NSI) constitute an active research field, as they are closely related to potential new physics associated with dark matter searches and exotic interactions arising from fundamental symmetry violations. The Borexino's unprecedented sensitivity to solar neutrinos, derived from its low background and precision spectral measurements, enables stringent constraints on potential deviations from the standard three-flavor neutrino oscillation paradigm. This work presents an update on the analysis of flavor-diagonal NSI using the full Borexino Phase-III data set, extending the study previously reported in Constraints on flavor-diagonal non-standard neutrino interactions from Borexino Phase-II (JHEP 2020, 38). The updated analysis incorporates the extended temporal and statistical coverage of Phase-III. The results indicate improved sensitivity to the diagonal NSI parameters, with constraints exceeding those obtained in Phase-II. Furthermore, a more general analysis that includes all possible off-diagonal NSI terms is presented for the first time, providing a comprehensive exploration of the NSI parameter space associated with the flavors of the incoming and outgoing neutrinos. This work once again underlines Borexino's critical role in probing new physics scenarios and reinforces its legacy in neutrino research. Detailed comparisons with Phase-II results are discussed, along with implications for theoretical models of NSI.

Figures (16)

• Figure 1: Electron neutrino survival probability $P_{ee}(E)$ as a function of neutrino energy.bib:bxnsi
• Figure 2: Distortion of the electron recoil spectrum for $^7\rm{Be}$ solar neutrino due to non-zero values of $\varepsilon_{e}^{L/R}$ without detector resolution.
• Figure 3: Flow chart of the reweighting process. Starting from the PDFs of electron recoil spectra simulated with G4Bx, the detector response is deconvoluted comparing the final energy deposition to the Monte Carlo truth. The obtained response function is later convoluted to the NSI-distorted energy spectra. The final spectra are then used in the NSI spectral fit.
• Figure 4: Example of the fitting result of Borexino energy spectrum.
• Figure 5: Comparison of 1-dimensional profiling $\varepsilon_{ee}^{eeL/R}$, the solid lines represent our result and the dashed lines show the published constrain bib:bxnsi.