QCD Axion Dark Matter with a Small Decay Constant

Abstract

The QCD axion is a good dark matter candidate. The observed dark matter abundance can arise from misalignment or defect mechanisms, which generically require an axion decay constant f a ∼ Oð10 11 Þ GeV (or higher). We introduce a new cosmological origin for axion dark matter, parametric resonance from oscillations of the Peccei-Quinn symmetry breaking field, that requires f a ∼ ð10 8 -10 11 Þ GeV. The axions may be warm enough to give deviations from cold dark matter in large scale structure.

Misalignment & Co.: (pseudo-)scalar and vector dark matter with curvature couplings

“…The initial (homogeneous) field value in our observable Universe can simply correspond to the initial misalignment of the field away from the vacuum, or it can arise "stochastically" [36][37][38][39][40][41] from the accumulated effect of fluctuations during a long phase of inflation. Additional contributions may arise from production via inflationary fluctuations [42][43][44][45][46][47] as well as from decays of precursor particles [48][49][50][51][52][53]. Modifications to the usual misalignment mechanism have been for example explored in [54,55].…”

Weighing the solar axion

“…We have shown that over a limited range IAXO will have sensitivity to the masses of KSVZ axions. Some new theoretical predictions propose that this range of axion masses could be of importance in describing both inflation and dark matter [29][30][31][32][33]77]. Therefore IAXO, as well as being simply a search for solar axions, may additionally prove valuable in the search for dark matter, and a companion to future haloscopes searching in the same range [28].…”

“…In the range of masses accessible to a helioscope, the QCD axion is generally associated with being a subdominant DM component in the simplest cosmologies. Although given the uncertainties, it could in fact easily account for all of it in both the postinflationary [29][30][31][32], and preinflationary scenarios [33,34]. Furthermore there are long-standing astrophysical anomalies for which an axion in this range has been shown to be a viable explanation [35][36][37][38][39][40][41][42][43][44][45][46].…”

Real-time dynamics of axion particle production due to spontaneous decay of a coherent axion field

“…We stress that quantum tunneling plays an essential role here. Indeed, the fluctuation cannot grow if there exists only one minimum, which is in contrast to the axion particle production via parametric resonance due to oscillating coherent axion field [23][24][25][26].…”