Enabling Strong Neutrino Self-interaction with an Unparticle Mediator

TL;DR

This paper proposes a novel neutrino self-interaction model using unparticles as mediators, which can evade existing constraints and support strong interactions relevant to cosmology, opening new experimental possibilities.

Contribution

It introduces a continuous spectral density unparticle mediator for neutrino interactions, mitigating key constraints and expanding viable parameter space.

Findings

Unparticle mediators can pass cosmological constraints on neutrino number.

Unparticle interactions are less resonant, easing IceCube constraints.

Model supports strong neutrino self-interactions relevant for cosmology.

Abstract

Recent explorations of the cosmic microwave background and the large-scale structure of the universe have indicated a preference for sizable neutrino self-interactions, much stronger than what the Standard Model offers. When interpreted in the context of simple particle-physics models with a light, neutrinophilic scalar mediator, some of the hints are already in tension with the combination of terrestrial, astrophysical and cosmological constraints. We take a novel approach by considering neutrino self-interactions through a mediator with a smooth, continuous, spectral density function. We consider Georgi's unparticle with a mass gap as a concrete example and point out two useful effects for mitigating two leading constraints. 1) The Unparticle is ``broadband'' -- it occupies a wide range of masses which allows it to pass the early universe constraint on effective number of extra neutrinos ($\Delta N_{\rm eff.}$) even if the mass gap lies below the MeV scale. 2) Scattering involving unparticles is less resonant -- which lifts the constraint set by IceCube based on a recent measurement of ultra-high-energy cosmogenic neutrinos. Our analysis shows that an unparticle mediator can open up ample parameter space for strong neutrino self-interactions of interest to cosmology and serves a well-motivated target for upcoming experiments.