Axions and the Strong CP Problem in M-theory

TL;DR

This paper explores how string-theoretic axions in M-theory could solve the strong CP problem, emphasizing the importance of large compactification radii and specific model conditions for viability.

Contribution

It demonstrates that under certain conditions, large-radius M-theory compactifications can produce viable QCD axions capable of solving the strong CP problem.

Findings

Large compactification radius can suppress unwanted symmetry breakings.

Viability depends on quantized gauge kinetic function coefficients.

Cosmologically viable axions are possible with late-time entropy production.

Abstract

We examine the possibility that the strong CP problem is solved by string-theoretic axions in strong-coupling limit of the E_8 x E_8 heterotic string theory (M-theory). We first discuss some generic features of gauge kinetic functions in compactified M-theory, and examine in detail the axion potential induced by the explicit breakings other than the QCD anomaly of the non-linear U(1)_{PQ} symmetries of string-theoretic axions. It is argued based on supersymmetry and discrete gauge symmetries that if the compactification radius is large enough, there can be a U(1)_{PQ}-symmetry whose breaking other than the QCD anomaly, whatever its microscopic origin is, is suppressed enough for the axion mechanism to work. Phenomenological viability of such a large radius crucially depends upon the quantized coefficients in gauge kinetic functions. We note that the large radius required for the axion mechanism is viable only in a limited class of models. For instance, for compactifications on a smooth Calabi-Yau manifold with a vanishing second E_8 field strength, it is viable only when the quantized flux of the antisymmetric tensor field in M-theory has a minimal nonzero value. It is also stressed that this large compactification radius allows the QCD axion in M-theory to be cosmologically viable in the presence of a late time entropy production.