# Constraining the Self-Interacting Neutrino Interpretation of the Hubble Tension

## TL;DR

This paper investigates the feasibility of self-interacting neutrinos as a solution to the Hubble tension, showing most models are excluded by existing bounds unless specific conditions are met, such as coupling to tau neutrinos and Majorana nature.

## Contribution

It constrains the parameter space of neutrino self-interactions for resolving the Hubble tension, highlighting the necessity of specific mediator properties and neutrino characteristics.

## Key findings

- Most self-interacting neutrino models are excluded by current bounds.
- Only models with a spin-0 mediator coupling to tau neutrinos remain viable.
- Neutrinos must be Majorana for these models to work.

## Abstract

Large, non-standard neutrino self-interactions have been shown to resolve the $\sim 4\sigma$ tension in Hubble constant measurements and a milder tension in the amplitude of matter fluctuations. We demonstrate that interactions of the necessary size imply the existence of a force-carrier with a large neutrino coupling ($> 10^{-4}$) and mass in the keV -- 100 MeV range. This mediator is subject to stringent cosmological and laboratory bounds, and we find that nearly all realizations of such a particle are excluded by existing data unless it carries spin 0 and couples almost exclusively to $\tau$-flavored neutrinos. Furthermore, we find that the light neutrinos must be Majorana, and that a UV-complete model requires a non-minimal mechanism to simultaneously generate neutrino masses and appreciable self-interactions.