Gauge field production in supergravity inflation: Local non-Gaussianity and primordial black holes

Abstract

When inflation is driven by a pseudoscalar field $\ensuremath{\chi}$ coupled to vectors as $\frac{\ensuremath{\alpha}}{4}\ensuremath{\chi}F\stackrel{\texttildelow{}}{F}$, this coupling may lead to a copious production of gauge quanta, which in turns induces non-Gaussian and non-scale-invariant corrections to curvature perturbations. We point out that this mechanism is generically at work in a broad class of inflationary models in supergravity, hence providing them with a rich set of observational predictions. When the gauge fields are massless, significant effects on cosmic microwave background scales emerge only for relatively large $\ensuremath{\alpha}$. We show that in this regime, the curvature perturbations produced at the last stages of inflation have a relatively large amplitude that is of the order of the upper bound set by the possible production of primordial black holes by non-Gaussian perturbations. On the other hand, within the supergravity framework described in our paper, the gauge fields can often acquire a mass through a coupling to additional light scalar fields. Perturbations of these fields modulate the duration of inflation, which serves as a source for non-Gaussian perturbations of the metric. In this regime, the bounds from primordial black holes are parametrically satisfied and non-Gaussianity of the local type can be generated at the observationally interesting level ${f}_{\mathrm{NL}}\ensuremath{\sim}\mathcal{O}(10)$.