Features of heavy physics in the CMB power spectrum

TL;DR

This paper investigates how rapid turns in multi-field inflation trajectories can leave observable imprints, such as oscillations and modified sound speeds, in the primordial power spectrum, revealing potential signatures of heavy physics.

Contribution

It provides a general computation of the power spectrum in multi-field inflation, highlighting the effects of trajectory turns and deriving an effective low-energy theory for the adiabatic mode.

Findings

Turning trajectories induce damped oscillations in the power spectrum.

Effective low-energy theory shows a modified speed of sound for the adiabatic mode.

Heavy fields can leave observable imprints even when their masses approach the cutoff.

Abstract

The computation of the primordial power spectrum in multi-field inflation models requires us to correctly account for all relevant interactions between adiabatic and non-adiabatic modes around and after horizon crossing. One specific complication arises from derivative interactions induced by the curvilinear trajectory of the inflaton in a multi-dimensional field space. In this work we compute the power spectrum in general multi-field models and show that certain inflaton trajectories may lead to observationally significant imprints of `heavy' physics in the primordial power spectrum if the inflaton trajectory turns, that is, traverses a bend, sufficiently fast (without interrupting slow roll), even in cases where the normal modes have masses approaching the cutoff of our theory. We emphasise that turning is defined with respect to the geodesics of the sigma model metric, irrespective of whether this is canonical or non-trivial. The imprints generically take the form of damped superimposed oscillations on the power spectrum. In the particular case of two-field models, if one of the fields is sufficiently massive compared to the scale of inflation, we are able to compute an effective low energy theory for the adiabatic mode encapsulating certain relevant operators of the full multi-field dynamics. As expected, a particular characteristic of this effective theory is a modified speed of sound for the adiabatic mode which is a functional of the background inflaton trajectory and the turns traversed during inflation. Hence in addition, we expect non-Gaussian signatures directly related to the features imprinted in the power spectrum.