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; 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.
Highlights
The electroweak (EW) hierarchy problem of the standard model (SM) remains a major challenge after the null search at the Large Hadron Collider (LHC), inspiring many new ideas in addition to the traditional approaches from supersymmetry [1], extra dimensions [2,3], and strong dynamics [4,5]
One generally expects a superPlanckian field excursion of the axion with a gigantic efolding number during ultra low-scale inflation. These requirements come from slowly scanning enough Higgs mass range so that it is independent from initial conditions during the slow-roll era dominated by classical evolutions other than quantum fluctuations
We propose a new relaxion mechanism with a stochastic stopping mechanism that is different from the dynamical or thermal stopping mechanisms we mention above
Summary
The electroweak (EW) hierarchy problem of the standard model (SM) remains a major challenge after the null search at the Large Hadron Collider (LHC), inspiring many new ideas in addition to the traditional approaches from supersymmetry [1], extra dimensions [2,3], and strong dynamics [4,5] Among these new ideas, the so-called relaxion mechanism [6] has received much attention recently. One generally expects a superPlanckian field excursion of the axion with a gigantic efolding number during ultra low-scale inflation. These requirements come from slowly scanning enough Higgs mass range so that it is independent from initial conditions during the slow-roll era dominated by classical evolutions other than quantum fluctuations. A consistent choice of parameters can be identified to render a QCD relaxion as all of the cold dark matter (CDM)
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