Radiative Corrections from Heavy Fast-Roll Fields during Inflation

TL;DR

This paper explores how heavy fields during inflation can cause temporary or sustained running of the spectral index, potentially explaining observed anomalies and challenging the usual small field inflation expectations.

Contribution

It introduces a model where radiative corrections from heavy fast-roll fields induce spectral index running, linking it to observable inflationary signatures and tensor modes.

Findings

A logarithmic one-loop correction can cause strong or sustained running.

Large running correlates with small field models and low tensor-to-scalar ratio.

Running can be induced by a curvaton, allowing observable tensor modes in small field inflation.

Abstract

We investigate radiative corrections to the inflaton potential from heavy fields undergoing a fast-roll phase transition. We find that a logarithmic one-loop correction to the inflaton potential involving this field can induce a temporary running of the spectral index. The induced running can be a short burst of strong running, which may be related to the observed anomalies on large scales in the cosmic microwave spectrum, or extend over many e-folds, sustaining an effectively constant running to be searched for in the future. We implement this in a general class of models, where effects are mediated through a heavy messenger field sitting in its minimum. Interestingly, within the present framework it is a generic outcome that a large running implies a small field model with a vanishing tensor-to-scalar ratio, circumventing the normal expectation that small field models typically lead to an unobservable small running of the spectral index. An observable level of tensor modes can also be accommodated, but, surprisingly, this requires running to be induced by a curvaton. If upcoming observations are consistent with a small tensor-to-scalar ratio as predicted by small field models of inflation, then the present study serves as an explicit example contrary to the general expectation that the running will be unobservable.