Dynamical axion misalignment with small instantons

Abstract

We present a new mechanism to relax the initial misalignment angle of the QCD axion and raise the cosmological bound on the axion decay constant. The QCD axion receives a contribution from small UV instantons during inflation, which raises its mass to the inflationary Hubble scale. This makes the axion start rolling down its potential early on. In the scenario, the standard model Yukawa couplings of quarks are dynamical, being of order one during the inflationary era and reducing to their standard model values once it ends. This means that after inflation the contribution of the small instantons is suppressed, and the axion potential reduces to the standard one from the usual IR instantons. As a result, when the axion starts to oscillate again after inflation, the initial misalignment angle is suppressed due to the dynamics during inflation. While the general idea of dynamical axion misalignment has been discussed in the literature before, we present in detail the major bottleneck on the mismatching between the minima of the axion potentials during and after inflation, and how it is circumvented in our scenario via the Froggatt-Nielsen mechanism. Taking into account of all the constraints, we find that the axion decay constant could be raised to the GUT scale, 10 15 GeV, in our scenario.

Analytic study of dark photon and gravitational wave production from axion

“…For example, avoiding the cosmological overabundance of the QCD axion puts an upper bound f a 10 12 GeV. However, bounds on the coupling can be relaxed by considering a coupling of the axion to hidden photons or monopoles [6][7][8] as such a coupling is predicted JHEP05(2021)043 in some models based on string theory [9], late time entropy production before big bang nucleosynthesis [10][11][12], and dynamical axion misalignment [13][14][15][16][17]. Having larger values of f a makes it possible to consider scenarios in which axion-like fields play significant roles in the earlier times in the expansion history of the universe which was the subject of many studies [18][19][20][21][22][23][24].…”

Axion dark matter detection by superconducting resonant frequency conversion

“…Additional dynamics or interactions in the early Universe can allow for axion-like particles to be viable DM candidates in a much larger range of gaγγ − ma, as discussed in, e.g., refs [77][78][79]…”

Feebly-interacting particles: FIPs 2020 workshop report

“…Examples of horizontal enlargement of the parameter space towards the right of the canonical axion band are heavy axion models that solve the strong CP problem at low scales (e.g. f a ∼ TeV) [460][461][462][463][464][465][466][467][468][469][470][471][472][473][474][475][476]. More recently, there has been also a proposal producing instead a left horizontal shift, that is an axion solving the strong CP problem with m a f a C aGG √ χ [477].…”