Phenomenology of particle production during inflation

Abstract

This thesis is devoted to the study on particle production during the era of primordial inflation and its phenomenological impacts. The simplest models of inflation typically assume only one dynamical degree of freedom, inflaton, that is responsible for all the inflationary dynamics and predictions. Yet, it is a natural expectation that the inflaton should be coupled to some other fields, in need of successful reheating of the universe after inflation. We first consider the models in which the inflaton is coupled to a U(1) gauge field. For a pseudo-scalar inflaton, its natural coupling induces tachyonic growth of the gauge quanta, which then inverse-decay to the inflaton perturbations. This imprints non-Gaussianity in the cosmic microwave background (CMB) anisotropies. This non-Gaussianity has a nearly equilateral shape, and the fact that we have not observed it with Planck provides a bound on the axion decay constant, which is in the range naturally obtained in UV completed theories. The produced gauge quanta also source gravitational waves (GWs). Future GW interferometer experiments can improve over the CMB non-Gaussianity limits. We then study a different model characterized by a scalar inflaton coupled to gauge fields via a dilation-like interaction. This coupling can result in a nearly scale-invariant spectrum for the gauge field. Also in this case, the produced gauge quanta source inflaton perturbations, but the resulting non-Gaussianity now has a shape peaked for squeezed triangles, and which exhibits a peculiar angular dependence, that, if detected, would be a smoking gun of the higher-spin fields involved.

In the above two models, the GW signals are always subdominant at the CMB scales, due to the non-Gaussianity bounds from the scalar perturbations (namely, from the perturbations generated by the inflaton quanta produced by the gauge fields). We study the radically different situation in which some field other than the inflaton produces the gauge quanta, and these quanta have no direct coupling (apart from the unavoidable gravitational interaction) to the inflaton. We study whether this production can result in a detectable GW signal at CMB scales, without conflicting with the bounds from non-Gaussianity of the scalar perturbations. We study two possibilities: (i) gauge quanta production due to a sudden variation of their mass, and (ii) gauge quanta production from a rolling pseudo scalar. In case (i), we find that GW signals are unlikely to be detectable, due to the suppressed quadrupole moment of non-relativistic quanta. In case (ii), we instead find that GWs from particle production can actually exceed the usual inflationary vacuum fluctuations. The observable B-mode polarization can be obtained for any choice of inflaton potential, and the amplitude of the signal is not necessarily correlated with the scale of inflation.