Opening up the QCD axion window

Abstract

We present a new mechanism to deplete the energy density of the QCD axion, making decay constants as high as f a 10 17 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.

On the wondrous stability of ALP dark matter

“…Requiring that the timescale of the energy growth is smaller than the time that the cosine preserves its sign, in addition to the tachyonic condition, gives [44]…”

Adiabatic suppression of the axion abundance and isocurvature due to coupling to hidden monopoles

“…refs. [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49] for other scenarios to suppress the axion isocurvature.…”

General continuum clockwork

“…To get the continuum limit of the lagrangian densities (2.1) and (2.3), one can make the following substitutions: 19) together with the field and parameter redefinitions: 20) JHEP07 (2018)113 and finally take the limit ∆r → 0. One then finds…”

“…Among the many possible implementations of the mechanism [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23], the CW axions [2,3] and U(1) gauge bosons [4,5] are particularly interesting as the key features of the mechanism can be understood in terms of a specific pattern of symmetry breaking of the underlying N + 1 (global or local) U(1) symmetries [U (1)] N +1 = N i=0 U(1) i , which is (explicitly or spontaneously) broken down to a U(1) CW subgroup. Furthermore, the key model parameters such as the CW parameter q and the involved axion-instanton couplings and U(1) gauge charges, are required to be integervalued (in appropriate units) by the compact [U (1)] N +1 , so the model has a built-in criterion for natural size of these model parameters.…”