Cosmological Relaxation through the Dark Axion Portal
  The dark axion portal is a coupling of an axion-like particle to a dark
photon kinetically mixed with the visible photon. We show how this portal, when
applied to the relaxion, can lead to cosmological relaxation of the weak scale
using dark photon production. The key backreaction mechanism involves the
Schwinger effect: As long as electroweak symmetry is unbroken, Schwinger
production of massless Standard Model fermions, which carry dark millicharges,
suppresses the dark photon production. Once the electroweak symmetry is broken,
the fermions acquire mass and the suppression is lifted. An enhanced dark
photon dissipation then traps the relaxion at a naturally small weak scale. Our
model thus provides a novel link between the phenomenological dark axion
portal, dark photons, and the hierarchy problem of the Higgs mass.
