Abstract
We present a new mechanism to deplete the energy density of the QCD axion, making decay constants as high as fa ≃ 1017 GeV viable for generic initial conditions. In our setup, the axion couples to a massless dark photon with a coupling that is moderately stronger than the axion coupling to gluons. Dark photons are produced copiously through a tachyonic instability when the axion field starts oscillating, and an exponential suppression of the axion density can be achieved. For a large part of the parameter space this dark radiation component of the universe can be observable in upcoming CMB experiments. Such dynamical depletion of the axion density ameliorates the isocurvature bound on the scale of inflation. The depletion also amplifies the power spectrum at scales that enter the horizon before particle production begins, potentially leading to axion miniclusters.
Highlights
We present a new mechanism to deplete the energy density of the QCD axion, making decay constants as high as fa 1017 GeV viable for generic initial conditions
We present one such extension where the axion has a coupling to a dark photon
This leads to an efficient depletion of the axion density into dark photons, such that large-f QCD axions can be misaligned by a generic amount from their minimum
Summary
The cosmology of the axion depends sensitively on whether the spontaneous breaking of PQ symmetry occurs before/during or after inflation. Due to the astrophysical constraints requiring fa 109 GeV, and the bound on the inflationary scale from non-detection of gravitational B-modes in the CMB [60, 61], HI 1014 GeV,. This scenario represents a large fraction of the allowed axion parameter space. In this case, due to the exponential expansion during inflation, the axion field takes on a constant value throughout our Hubble volume, which is generically away from the late time minimum of the potential. The dark matter abundance arises out of this misalignment, and depends on the axion mass and on the size of the misalignment
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