Relaxion Fluctuations (Self-stopping Relaxion) and Overview of Relaxion Stopping Mechanisms

TL;DR

This paper investigates how quantum fluctuations of the relaxion field can produce particles that serve as an effective stopping mechanism, potentially reducing the need for inflation in the relaxion cosmological model.

Contribution

It introduces the role of relaxion fluctuations and particle production as a novel stopping mechanism, expanding the understanding of relaxion dynamics and constraints.

Findings

Relaxion fluctuations can grow and produce particles that halt its evolution.

Certain parameter regions allow stopping without inflation.

Electroweak gauge boson production imposes strong constraints.

Abstract

In implementations of the electroweak scale cosmological relaxation mechanism proposed so far, the effect of the quantum fluctuations of the homogeneous relaxion field has been ignored. We show that they can grow during the classical cosmological evolution of the relaxion field passing through its many potential barriers. The resulting production of relaxion particles can act as an efficient stopping mechanism for the relaxion. We revisit the original relaxion proposal and determine under which conditions inflation may no longer be needed as a source of friction. We review alternative stopping mechanisms and determine in detail the allowed parameter space for each of them (whether happening before, during and after inflation), also considering and severely constraining the case of friction from electroweak gauge boson production in models with large and Higgs-independent barriers.