The abundance of relativistic axions in a flaton model of Peccei-Quinn symmetry

TL;DR

The paper investigates a canonical flaton model of Peccei–Quinn symmetry in which flaton fields drive thermal inflation and generate a calculable cosmology. It constructs the full flaton/flatino/axion sector, deriving the two CP-even flatons $F_1,F_2$, the CP-odd flaton $\\psi'$, and two flatinos, with the axion $a$ arising from the same PQ sector; all relevant derivative and cubic interactions are computed. By evaluating decay rates among flatons, flatinos, and axions and comparing them to hadronic channels in KSVZ and DFSZ models, the work translates these rates into a bound on the relativistic-axion population at nucleosynthesis, quantified as $\delta N_ u$. The analysis finds that the KSVZ case is likely ruled out by current $\delta N_ u$ constraints, while the DFSZ case remains viable in substantial parameter regions, offering a potential explanation for any future detection of nonzero $\delta N_ u$. Overall, the study links microphysical flaton/axion couplings to cosmological observables, providing a testable framework for PQ symmetry breaking, axion dark matter, and post-thermal-inflation baryogenesis scenarios.

Abstract

Flaton models of Peccei-Quinn symmetry have good particle physics motivation, and are likely to cause thermal inflation leading to a well-defined cosmology. They can solve the $μ$ problem, and generate viable neutrino masses. Canonical flaton models predict an axion decay constant F_a of order 10^{10} GeV and generic flaton models give F_a of order 10^9 GeV as required by observation. The axion is a good candidate for cold dark matter in all cases, because its density is diluted by flaton decay if F_a is bigger than 10^{12} GeV. In addition to the dark matter axions, a population of relativistic axions is produced by flaton decay, which at nucleosynthesis is equivalent to some number δN_νof extra neutrino species. Focussing on the canonical model, containing three flaton particles and two flatinos, we evaluate all of the flaton-flatino-axion interactions and the corresponding axionic decay rates. They are compared with the dominant hadronic decay rates, for both DFSZ and KSVZ models. These formulas provide the basis for a precise calculation of the equivalent δN_νin terms of the parameters (masses and couplings). The KSVZ case is probably already ruled out by the existing bound δN_ν\lsim 1. The DFSZ case is allowed in a significant region of parameter space, and will provide a possible explanation for any future detection of nonzero $δN_ν$.