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

Understand

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 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 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 form 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.