Eur. Phys. J. C (2024) 84: 999
https://doi.org/10.1140/epjc/s10052-024-13330-3

Regular Article

Generalising axion-like particle as the curvaton: sourcing primordial density perturbation and non-Gaussianities

¹ Faculty of Physics, Institute of Theoretical Physics, University of Warsaw, ul. Pasteura 5, 02-093, Warsaw, Poland
² Department of Physics, Indian Institute of Technology Bombay, 400076, Mumbai, India
³ School of Physical Science and Technology, ShanghaiTech University, 393 Middle Huaxia Road, 201210, Shanghai, China

^a anish.ghoshal@fuw.edu.pl

Received: 24 December 2023
Accepted: 4 September 2024
Published online: 5 October 2024

Abstract

We investigate the non-perturbatively generated axion-like particle (ALP) potential, involving fermions in the dark sector that couple to the ALP, in an early universe cosmological inflationary stage with the ALP being a spectator field. The potential here deviates from the standard cosine nature due to the presence of the two dark sector fermion masses $m_u$ and $m_d$ which couple to the ALP. The ALP is a spectator field during inflation but it starts to oscillate and dominates the energy density of the universe after inflation ends, thereby sourcing isocurvature perturbations, while standard curvature fluctuations from the inflaton are assumed to be sub-dominant. Subsequently the ALP decays converting the isocurvature perturbations to adiabatic perturbations thereby acting as the origin of the primordial density perturbations. We identify the parameter space involving the axion decay constant $f_{\varphi }$, scale of confinement $\mathrm{\Lambda }$, ALP mass m and the masses of the fermions, $m_u$ and $m_d$ where it can satisfactorily behave as the curvaton and source the observed primordial density perturbation. We also predict local non-Gaussianity signals for bi-spectrum and tri-spectrum $f_{\mathit{NL}}$ and $g_{\mathit{NL}}$, as a function of the ratio $m_u/m_d$, which are within the allowed range in the latest Planck observations and are detectable with future observations. Particularly we find that the value of $f_{\mathit{NL}}$ and $g_{\mathit{NL}}$ are dependent on the ratio of $m_u$ and $m_d$: $f_{\mathit{NL}}$ is more or less positive for all scenarios except $m_u=m_d$ and $g_{\mathit{NL}}$ is always positive irrespective of the ratio between $m_u$ and $m_d$. The results of our analysis in the limit $m_u=m_d$ resembles vanilla curvaton scenario while in the limit $m_u\gg m_d$ resembles pure axion cosine potential. This is because the model has the unique feature where the extra matter in the dark sector determines the nature and shape of the potential.