# Thraxions: Ultralight Throat Axions

## TL;DR

This paper introduces a new class of ultralight axions, called thraxions, arising in warped regions of string theory, with potential implications for cosmology and the Weak Gravity Conjecture.

## Contribution

It identifies and analyzes the properties of thraxions, a novel ultralight axion type in string theory, including their mass suppression mechanism and potential for super-Planckian field ranges.

## Key findings

- Thraxions have masses suppressed by the cube of the warp factor.
- They exhibit finite monodromy and flattened potentials due to backreaction.
- Potential oscillates on sub-Planckian scales, limiting inflation applications.

## Abstract

We argue that a new type of extremely light axion is generically present in the type IIB part of the string theory landscape. Its mass is suppressed by the third power of the warp factor of a strongly warped region (Klebanov-Strassler throat), suggesting the name thraxion. Our observation is based on the generic presence of several throats sharing the same 2-cycle. This cycle shrinks to zero volume at the end of each throat. It is hence trivial in homology and the corresponding $C_2$ axion is massive. However, the mass is warping-suppressed since, if one were to cut off the strongly warped regions, a proper 2-cycle would re-emerge. Since the kinetic term of the axion is dominated in the UV, an even stronger, quadratic mass suppression results. Moreover, if the axion is excited, the angular modes of the throats backreact. This gives our effective $C_2$ axion a finite monodromy and flattens its potential even further. Eventually, the mass turns out to scale as the third power of the warp factor. We briefly discuss possible implications for phenomenology and potential violations of the Weak Gravity Conjecture for axions. Moreover we identify a mechanism for generating super-Planckian axionic field ranges which we call drifting monodromies. However, in the examples we consider, the potential oscillates on sub-Planckian distances in field space, preventing us from building a natural inflation model on the basis of this idea.