The Spectral Index and its Running in Axionic Curvaton

TL;DR

This paper proposes that the axionic curvaton model with a modulated potential can naturally explain a sizable running spectral index observed in recent data, without requiring large-field inflation.

Contribution

It introduces a novel axionic curvaton model with a two-sinusoid potential to account for the running spectral index and its relation to string axiverse scenarios.

Findings

The running spectral index is proportional to (n_s - 1) and inversely proportional to the number of e-folds.

The model can produce both sizable running and red-tilted spectra.

It does not depend on large-field inflation to explain the spectral features.

Abstract

We show that a sizable running spectral index suggested by the recent SPT data can be explained in the axionic curvaton model with a potential that consists of two sinusoidal contributions of different height and period. We find that the running spectral index is generically given by d ns/dlnk ~ (2pi/dN)(n_s - 1), where dN is the e-folds during one period of modulations. In the string axiverse, axions naturally acquire a mass from multiple contributions, and one of the axions may be responsible for the density perturbations with a sizable running spectral index via the curvaton mechanism. We note that the axionic curvaton model with modulations can also accommodate the red-tilted spectrum with a negligible running, without relying on large-field inflation.