Relaxion Dark Matter from Stochastic Misalignment

TL;DR

This paper explores how stochastic fluctuations of the relaxion field during inflation can lead to its misalignment, enabling it to account for dark matter across a broader parameter space, including larger couplings and masses.

Contribution

It introduces the concept of stochastic misalignment driven by inflationary fluctuations as a new mechanism for relaxion dark matter production, expanding viable parameter space.

Findings

Relaxation of the electroweak scale via the relaxion mechanism.

Stochastic fluctuations can generate sufficient relaxion abundance.

New parameter regions allow relaxion to explain dark matter with larger couplings and masses.

Abstract

Cosmological relaxation of the electroweak scale via Higgs-axion interplay, named as relaxion mechanism, provides a dynamical solution to the Higgs mass hierarchy. In the original proposal by Graham, Kaplan and Rajendran, the relaxion abundance today is too small to explain the dark matter of the universe because of the high suppression of the misalignment angle after inflation. It was then realised by Banerjee, Kim and Perez that reheating effects can displace the relaxion, thus enabling it to account for the dark matter abundance from the misalignment mechanism. However, this scenario is realised in a limited region of parameter space to avoid runaway. We show that in the regime where inflationary fluctuations dominate over the classical slow-roll, the "stochastic misalignment" of the field due to fluctuations can be large. We study the evolution of the relaxion after inflation, including the high-temperature scenario, in which the barriers of the potential shrink and destabilise temporarily the local minimum. We open new regions of parameter space where the relaxion can naturally explain the observed dark matter density in the universe, towards larger coupling, larger mass, larger mixing angle, smaller decay constant, as well as larger scale of inflation.