Opening up the QCD axion window

Journal of High Energy Physics, Mar 2018

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

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Opening up the QCD axion window

HJE Opening up the QCD axion window Prateek Agrawal 0 3 Gustavo Marques-Tavares 0 1 Wei Xue 0 2 Oxford Street 0 Cambridge 0 U.S.A. 0 Via Pueblo Mall 0 Stanford 0 U.S.A. 0 0 77 Massachusetts Avenue , Cambridge , U.S.A 1 Stanford Institute for Theoretical Physics, Stanford University 2 Center for Theoretical Physics, Massachusetts Institute of Technology , USA 3 Department of Physics, Harvard University 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 eld 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 in ation. The depletion also ampli es the power spectrum at scales that enter the horizon before particle production begins, potentially leading to axion miniclusters. Cosmology of Theories beyond the SM; CP violation - 1 Introduction 2 3 4 2.1 2.2 4.1 4.2 4.3 4.4 Axion dark matter review The axion Lagrangian and potential Axion cosmology A simple model for particle production Particle production and DM The equations of motion Conditions for e cient particle production Dark photon contribution to Ne Numerical results 5 Future directions and conclusions A UV completions of the alignment model A.1 A 5D model A.2 Con ning dynamics A.3 Clockwork equation of state of matter, making it a dark matter candidate [7{9]. Further, the axion makes the CP-violating QCD angle dynamical, with a minimum at = 0 [10]. Thus, the axion can naturally explain the severe constraints on CP violation in the strong sector, . 10 10, from null measurements of the neutron/Hg electric dipole moments [11{13]. The mass of the QCD axion is set by its decay constant fa. Its couplings to other SM elds are model-dependent, but are expected to be given by higher-dimension operators suppressed by fa. Astrophysical observations impose a lower bound on the decay constant, fa & 109 GeV for generic couplings [14{18] (for detailed discussion of the constraints { 1 { see [6]). On the other hand, axion decay constants of fa > 1012 GeV predict a dark matter abundance in excess of observations for generic initial conditions. These two considerations determine the axion window, 109 GeV < fa < 1012 GeV, within which axions can naturally account for the observed dark matter density. There are a number of ongoing and proposed experiments for probing the QCD axion window [19{23]. We propose a new mechanism for depleting the axion abundance in the early Universe, such that the region fa > 1012 GeV becomes viable. This region is well-motivated theoretically. Generic expectations from string theory predict fa values higher than the axion window [24{26]. Beyond theoretical arguments, a practical reason for exploring viable cosmological models outside the axion window is that there are a number of exciting proposed experiments that should have sensitivity to very light (and thus large-fa) axions [27{32]. Our mechanism uses particle production in the time-dependent axion background. We assume that the axion couples to a massless dark photon, a U( 1 ) gauge boson that is decoupled from the SM. As the axion starts oscillating in the early universe, certain modes of the gauge eld become tachyonic, leading to an exponential growth of such modes. A similar mechanism has been previously used for generating primordial magnetic elds [33{39], for in ation and (p)reheating after in ation [40{47], for moduli decay [48], and for the relaxion [49]. Our mechanism di ers from previous cases because we focus on smaller couplings of the axion to the gauge elds, which requires multiple oscillations of the axion until the gauge eld grow su ciently to in uence the axion dynamics. In this regime the relevant modes of the gauge eld go through two separate phases, a tachyonic growth period and a subsequent parametric resonance period. Thus, our mechanism shares features both with tachyonic preheating and with more standard preheating scenarios based on parametric resonant decays. Even though the coupling responsible for this mechanism is quite generic for axion-like particles, the presence of light charged degrees of freedom can make the production of gauge elds very ine cient due to plasma screening e ects. For this reason we focus on the coupling of the axion to a dark photon instead of making use of its couplings to the SM photon. We show that this mechanism can be very e cient in transferring energy from the axion into the dark photons. As long as backscattering e ect (...truncated)


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Prateek Agrawal, Gustavo Marques-Tavares, Wei Xue. Opening up the QCD axion window, Journal of High Energy Physics, 2018, pp. 49, Volume 2018, Issue 3, DOI: 10.1007/JHEP03(2018)049