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

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.

💡 Research Summary

This paper presents an updated and comprehensive search for non‑standard neutrino interactions (NSI) using the full Borexino Phase‑III data set, collected between July 2016 and October 2021. The authors exploit roughly 1500 days of exposure with a fiducial mass of 70 t, corresponding to an increase of about 80 % in statistics compared with the previous Phase‑II analysis. The unprecedented radiopurity achieved during Phase‑III—thanks to a thermal‑stabilisation programme and a final water‑extraction purification—drastically reduced the dominant backgrounds (210Po, 210Bi, 85Kr), allowing a clean measurement of the solar neutrino spectrum from pp up to 8B energies.

The theoretical framework adopts the standard effective‑operator description of NSI, introducing neutral‑current (NC) and charged‑current (CC) coefficients εαβ that modify the ν‑e elastic‑scattering cross‑section and the matter potential in the Mikheyev–Smirnov–Wolfenstein (MSW) effect. While the earlier Borexino study (Phase‑II) considered only the three diagonal parameters (εee, εμμ, εττ), the present work expands the analysis to include all off‑diagonal terms (εeμ, εeτ, εμτ) and both their real and imaginary parts, yielding a total of nine complex parameters. This broader parameter space enables the first simultaneous probe of flavor‑changing NSI with solar neutrinos.

Methodologically, the analysis relies on a high‑statistics Monte‑Carlo simulation of the Borexino detector response. A baseline Standard Model (SM) spectrum is generated, and the impact of each NSI coefficient is evaluated by linearising the spectrum with respect to small variations of εαβ. A multi‑dimensional χ² function is constructed, incorporating systematic uncertainties as nuisance parameters: energy scale and resolution, fiducial‑volume definition, residual 210Po/210Bi rates, 85Kr limits, and the solar flux normalisations. The χ² is profiled over all nuisance parameters to obtain one‑dimensional confidence intervals for each NSI coefficient. A complementary Bayesian Markov‑Chain Monte‑Carlo (MCMC) cross‑check confirms the frequentist results.

The results show a marked improvement over Phase‑II. The diagonal NSI parameters are constrained to |εαα| < 0.04 (90 % C.L.), roughly 30 % tighter than the previous limits (|εαα| < 0.06). For the off‑diagonal coefficients, the analysis yields |εeμ|, |εeτ| < 0.02 and |εμτ| < 0.03 (90 % C.L.). These bounds are competitive with, and in some cases surpass, those obtained from high‑energy accelerator experiments such as CHARM and COHERENT, which traditionally provide the strongest constraints on NSI involving muon neutrinos. The study also quantifies how NSI would shift the extracted solar neutrino fluxes: a positive εee enhances the ν‑e scattering cross‑section, leading to a lower inferred 7Be and pp flux, whereas a negative εeμ reduces flavor conversion in the Sun, increasing the electron‑neutrino survival probability. The Borexino data, however, are consistent with the Standard Model predictions within the reduced uncertainties, thereby limiting any NSI‑induced distortion to a few percent.

The authors discuss the broader implications of their findings. When combined with global oscillation fits, the Borexino limits further shrink the allowed NSI parameter space, especially for flavor‑changing interactions that could otherwise mimic CP‑violating effects in long‑baseline experiments. They highlight the synergy between low‑energy solar neutrino measurements and upcoming high‑precision detectors (JUNO, Hyper‑Kamiokande, DUNE), suggesting that joint analyses could push NSI sensitivities down to the 10⁻³ level. The paper also emphasizes that the inclusion of off‑diagonal terms opens a new avenue for testing models that predict light mediators or lepton‑flavour‑violating couplings, which are often unconstrained by collider data.

In conclusion, this work demonstrates that Borexino, even in its decommissioning phase, remains a leading laboratory for probing physics beyond the Standard Model. By leveraging its unrivaled low‑background environment, extended exposure, and refined analysis techniques, the collaboration has set the most stringent solar‑neutrino‑based limits on both diagonal and off‑diagonal NSI to date. The results reinforce the consistency of the three‑flavour oscillation paradigm and provide essential input for future theoretical and experimental efforts aimed at uncovering subtle non‑standard interactions in the neutrino sector.