Runaway Relaxion from Finite Density

TL;DR

This paper investigates how finite density effects, such as those in stars and neutron stars, can trigger relaxion field instabilities, leading to phase transitions that constrain relaxion model parameters.

Contribution

It introduces a novel mechanism where stellar environments induce relaxion runaway bubbles, providing new constraints on relaxion models from astrophysical phenomena.

Findings

Finite density effects can destabilize relaxion vacua in stars.

Stars and neutron stars can trigger relaxion phase transitions.

New astrophysical constraints on relaxion parameter space.

Abstract

Finite density effects can destabilize the metastable vacua in relaxion models. Focusing on stars as nucleation seeds, we derive the conditions that lead to the formation and runaway of a relaxion bubble of a lower energy minimum than in vacuum. The resulting late-time phase transition in the universe allows us to set new constraints on the parameter space of relaxion models. We also find that similar instabilities can be triggered by the large electromagnetic fields around rotating neutron stars.