Novel cosmological bounds on thermally-produced axion-like particles

Abstract

We constrain the coupling of thermally-produced axion-like particles (here axions) with photons and gluons, using data from the cosmic microwave background (CMB) spectra and baryon acoustic oscillations. The axion possesses an explicit soft breaking mass term and it is produced thermally in the early Universe from either axion-photon or axion-gluon processes, accounting for the recent progresses in the field. We derive the most stringent bounds on the axion-gluon coupling to date on the mass range considered 10-4 ≲ ma / eV ≲ 100, superseding the current bounds from SN1987A. The bounds on the axion-photon coupling are competitive with the results from the CAST collaboration for the axion mass ma ≳ 3eV. We comment on the forecast reaches that will be available given the sensitivity of future CMB-S4 missions.

Constraints on ultralight axions, vector gauge bosons, and unparticles from geodetic and frame-dragging measurements

“…For m a 1.45 × 10 −22 eV the effects add coherently whereas for m a 1.45 × 10 −22 eV the precessional velocity oscillates with time. For relatively heavier axions, one can constrain the axion EDM parameter from spin precession measurements, CMB spectra, and baryon acoustic oscillations as discussed in [74,75,127].…”

Probing the Blue Axion with Cosmic Optical Background Anisotropies

“…a model dependent number and α the fine structure constant. A similar bound, limiting the QCD axion mass to m a 10 −1 eV, arises from the constraints on the neutron electric dipole moment [55,56], the only model-independent bound as other couplings can be smaller at the cost of some fine tuning [57]. Notice however that suppressing the coupling to neutrons (that limits the QCD axion mass to be m a 10 −2 eV [58]) would require even further nontrivial assumptions [59].…”

“…Additional interactions (e.g. to nucleons) can keep the axion in thermal equilibrium at smaller temperatures [56,75,77].…”

Dark radiation from the primordial thermal bath in momentum space

“…Furthermore, we could observe the cosmological effects of a non-vanishing QCD axion mass [62][63][64][65][66][67][68]. With the phase space distribution in hands, it is possible to reconsider existing analyses [69][70][71][72][73] and update the axion mass bound for production channels different than pion scatterings. We leave these developments of our general formalism to future studies.…”