Towards Axion Monodromy Inflation with Warped KK-Modes

TL;DR

This paper proposes a simple string theory model of axion monodromy inflation using warped KK modes from the Klebanov-Strassler throat, highlighting its potential for controlled large field inflation with ultra-light modes.

Contribution

It introduces a novel axion monodromy model based on KK modes in warped throats, avoiding common issues like back-reaction and supersymmetry breaking, and explores its inflationary viability.

Findings

The model features an ultra-light KK mode with exponentially suppressed mass.

Back-reaction effects are significant near Planck-scale field excursions.

Numerical analysis needed to confirm the model's suitability for realistic inflation.

Abstract

We present a particularly simple model of axion monodromy: Our axion is the lowest-lying KK-mode of the RR-2-form-potential $C_2$ in the standard Klebanov-Strassler throat. One can think of this inflaton candidate as being defined by the integral of $C_2$ over the $S^2$ cycle of the throat. It obtains an exponentially small mass from the IR-region in which the $S^2$ shrinks to zero size both with respect to the Planck scale and the mass scale of local modes of the throat. Crucially, the $S^2$ cycle has to be shared between two throats, such that the second locus where the $S^2$ shrinks is also in a warped region. Well-known problems like the potentially dangerous back-reaction of brane/antibrane pairs and explicit supersymmetry breaking are not present in our scenario. However, the inflaton back-reaction starts to deform the geometry strongly once the field excursion approaches the Planck scale. We derive the system of differential equations required to treat this effect quantitatively. Numerical work is required to decide whether back-reaction makes the model suitable for realistic inflation. While we have to leave this crucial issue to future studies, we find it interesting that such a simple and explicit stringy monodromy model allows an originally sub-Planckian axion to go through many periods with full quantitative control before back-reaction becomes strong. Also, the mere existence of our ultra-light throat mode (with double exponentially suppressed mass) is noteworthy.