Is the Universal String Axion the QCD Axion?

TL;DR

This paper examines whether the universal string axion can serve as the QCD axion within certain supergravity models derived from heterotic string theory, highlighting conditions under which it can solve the strong CP problem.

Contribution

It demonstrates how axion properties are modified in heterotic string models and identifies conditions where the universal axion can effectively act as the QCD axion.

Findings

The axion remains massless above the QCD scale in simple hidden sector models.

A modified axion-gluon coupling affects the axion mass and its relation to the Planck scale.

Most heterotic string compactifications can naturally avoid the strong CP problem.

Abstract

We consider the class of effective supergravity theories from the weakly coupled heterotic string in which local supersymmetry is broken by gaugino condensation in a hidden sector, with dilaton stabilization achieved through corrections to the classical dilaton K\"ahler potential. If there is a single hidden condensing (simple) gauge group, the axion is massless (up to contributions from higher dimension operators) above the QCD condensation scale. We show how the standard relation between the axion mass and its Planck scale coupling constant is modified in this class of models due to a contribution to the axion-gluon coupling that appears below the scale of supersymmetry breaking when gluinos are integrated out. In particular there is a point of enhanced symmetry in parameter space where the axion mass is suppressed. We revisit the question of the universal axion as the Peccei-Quinn axion in the light of these results, and find that the strong CP problem is avoided in most compactifications of the weakly coupled heterotic string.