Pure chromo-natural inflation: signatures of particle production from weak to strong backreaction

Abstract

We consider, in the context of axion-inflation, the Pure Natural Inflation (PNI) model coupled with an SU(2) gauge sector via a Chern-Simons term. As the axion rolls down its potential, it dissipates energy in the gauge sector thus sourcing fluctuations of scalar and tensor degrees of freedom therein. Gauge field fluctuations will, in turn, feed primordial gravitational waves as well as curvature perturbations. Remarkably, we can use upcoming cosmological probes to test this mechanism across a vast range of scales, from the CMB to laser interferometers. Due to their flat plateau at large field values, we find that PNI potentials fare better vis-á-vis CMB observations than the conventional sinusoidal potential of chromo-natural inflation (CNI). We show that, largely independently from the details of the potential, even when the dynamics begin in the weak backreaction regime, the rolling of the axion leads to a build-up of gauge-quanta production, invariably triggering the strong backreaction of the gauge tensors on the background dynamics. This transition results in the copious production of both scalar and tensor perturbations, which we study in detail. The gravitational wave signatures include a rich peak structure with a characteristic scale-dependent chirality, a compelling target for future gravitational wave detectors. Additionally, the peak in scalar perturbations may lead to the formation of primordial black holes, potentially accounting for a significant fraction of the observed dark matter abundance.