Neutrino non-standard interactions meet precision measurements of $N_{\rm eff}$

TL;DR

This paper investigates how non-standard neutrino interactions influence the effective number of relativistic species, $N_{\rm eff}$, providing constraints on new physics scales using cosmological measurements.

Contribution

It develops a comprehensive analytical framework for neutrino collision terms with non-standard interactions and derives new bounds on interaction scales from $N_{\rm eff}$ measurements.

Findings

The most stringent constraint is on the dimension-6 vector neutrino-electron operator.

Planck data constrains the operator scale above ~195 GeV.

Future CMB-S4 measurements could improve these constraints significantly.

Abstract

The number of relativistic species, $N_{\rm eff}$, has been precisely calculated in the standard model, and would be measured to the percent level by CMB-S4 in future. Neutral-current non-standard interactions would affect neutrino decoupling in the early Universe, thus modifying $N_{\rm eff}$. We parameterize those operators up to dimension-7 in the effective field theory framework, and then provide a complete, generic and analytical dictionary for the collision term integrals. From precision measurements of $N_{\rm eff}$, the most stringent constraint is obtained for the dimension-6 vector-type neutrino-electron operator, whose scale is constrained to be above about 195 (331) GeV from Planck (CMB-S4). We find our results complementary to other experiments like neutrino coherent scattering, neutrino oscillation, collider, and neutrino deep inelastic scattering experiments.