Paper-boat relaxion

Abstract

A new relaxion mechanism is proposed where a small electroweak scale is preferably selected earlier than the larger one due to a potential instability, which is different from previously proposed stopping mechanisms by either Hubble friction from an increasing periodic barrier or thermal friction from gauge boson production. The sub-Planckian field excursion of an axion can be achieved without violating the bound on the $e$-folding number from a quantum gravity perspective; and our relaxion can be identified as a QCD axion, preserving the Peccei-Quinn solution to the strong $CP$ problem as well as making up all the cold dark matter in our current Universe.

Comments: v1, 6 pages, 1 figure; v2, references added, minor improvement, a misused constraint corrected; v3, accepted by PRD, discussion added, title changed from "Mexican Relaxion" to meet the journal requirement; v4, final version to match the published version