Neutrino Self-interaction and Weak Mixing Angle Measurements

TL;DR

This paper explores how a light scalar particle interacting with neutrinos causes energy-scale dependent modifications in the neutrino-Z boson coupling, impacting weak mixing angle measurements.

Contribution

It demonstrates a generic, UV-insensitive effect where neutrino self-interactions induce scale-dependent changes in the Z-neutrino coupling, affecting precision electroweak measurements.

Findings

Zνν coupling can vary by several percent due to the effect.

The running influences the measurement of the weak mixing angle.

Disentangling the effects requires multiple precise measurements.

Abstract

Neutrino self-interaction with a larger ``Fermi constant'' is often resorted to for understanding various puzzles of our universe. We point out that a light, neutrinophilic scalar particle $\phi$ through radiative correction leads to an energy-scale dependence in the neutrino-$Z$-boson gauge coupling. The driver behind this phenomenon is a large separation between the mass scales of $\phi$ and additional heavy particles needed for gauge invariance. This is a generic effect insensitive to details of the UV completion. We show that the running can change the $Z\nu\bar\nu$ coupling by several percent and affect the measurement of weak mixing angle through neutrino neutral-current processes. We discuss the interplay between the running of the $Z\nu\bar\nu$ coupling and $\sin^2\theta_W$ in various experimental observables. It is possible to disentangle the two effects with more than one precise measurement.