Generalized Neutrino Interactions: constraints and parametrizations

Generalized neutrino interactions (GNI) are emerging as a convenient framework for describing effective scalar, vector, and tensor interactions. Such interactions arise naturally from extensions of the Standard Model that aim to explain neutrino properties and their mass origin. In this paper, we carefully study the two more common parametrizations for GNI and how to relate them. This allows us to compare bounds obtained from CEvNS and deep-inelastic scattering under the same footing. In addition, we present the current bounds from CEvNS measurements by COHERENT and compare them to those obtained from deep inelastic scattering on the same level. Our results focus on neutrino-quark interactions, and illustrate the complementarity between experiments working at different scales for GNI, showing that scalar interactions are better constrained by low-energy experiments like COHERENT, while tensor interactions are robustly constrained from deep inelastic scattering.

💡 Research Summary

The manuscript provides a comprehensive study of generalized neutrino interactions (GNI), focusing on scalar, vector, and tensor operators that arise in many extensions of the Standard Model (SM) aimed at explaining neutrino masses. Two widely used parametrizations are examined: the “ε‑parameter” formalism, which directly attaches left‑ and right‑handed neutrino currents to quark currents with coefficients ε_X and ˜ε_X, and the “C‑parameter” formalism, which introduces complex coefficients C_X and D_X and explicitly separates the SM vector and axial contributions. The authors derive explicit conversion relations (Eqs. 4–7) that map every ε‑parameter to a combination of C and D coefficients, thereby establishing a one‑to‑one correspondence between the two frameworks.

Armed with this mapping, the paper proceeds to evaluate how low‑energy coherent elastic neutrino‑nucleus scattering (CEvNS) experiments and high‑energy deep‑inelastic scattering (DIS) measurements constrain the same underlying GNI couplings. For CEvNS, the authors write the differential cross sections for scalar, vector, and tensor interactions in both parametrizations, incorporating nuclear form factors (Klein‑Nystrand) and nucleon‑level scalar form factors f_{Nq}. They emphasize that scalar and vector contributions receive an N² enhancement (N = neutron number), while tensor terms are not coherently enhanced and are instead suppressed by nuclear spin structure. The χ² analysis of the COHERENT CsI data includes seven nuisance parameters (signal normalization, form‑factor uncertainties, efficiency, and three background normalizations) and treats each GNI coefficient individually, yielding 90 % confidence limits. The resulting bounds show that scalar couplings are limited at the 10⁻³ level, vector/axial couplings are comparable to existing NSI limits, and tensor couplings are only weakly constrained (∼10⁻¹) by CEvNS.

In the DIS sector, the authors recast the neutral‑current cross sections measured by the CHARM and CDHS experiments in terms of the same GNI parameters. By adding the ε‑ or C‑terms to the SM partonic cross sections, they obtain modifications proportional to left‑ and right‑handed quark charges (g_{qL}, g_{qR}) and new scalar/tensor contributions that grow with the momentum transfer Q². Because DIS probes large Q² and accesses individual quark flavors, it is especially sensitive to tensor operators. The analysis shows that DIS limits tensor couplings down to ∼10⁻⁴, an order of magnitude stronger than CEvNS, while scalar couplings are constrained at a level comparable to, but slightly weaker than, the low‑energy bounds.

The key insight is the complementarity between the two experimental regimes: low‑energy CEvNS experiments provide the strongest constraints on scalar interactions due to coherent enhancement, whereas high‑energy DIS experiments dominate the limits on tensor interactions because of their Q²‑dependent sensitivity. Vector and axial couplings receive comparable constraints from both types of measurements. By establishing a unified parametrization, the authors enable direct comparison of results across experiments that previously used different conventions.

The paper concludes by highlighting the importance of a common GNI framework for future global fits, especially with upcoming facilities such as DUNE and Hyper‑K, which will span both low‑ and high‑energy neutrino interactions. The authors also note that many specific BSM scenarios (leptoquarks, light mediators, neutrino magnetic moments) map onto particular combinations of the scalar or tensor GNI coefficients, so the presented limits can be readily interpreted in model‑specific contexts. Overall, the work provides a valuable reference for both theorists and experimentalists aiming to constrain non‑standard neutrino interactions in a coherent, model‑independent manner.