The Strong CP Problem and Axions

TL;DR

The paper analyzes the strong CP problem in QCD, detailing how the U(1)_A anomaly and the theta vacuum introduce CP violation and are constrained by neutron EDM measurements. It advocates the Peccei–Quinn mechanism as a dynamical solution, yielding the axion, a light pseudoscalar whose mass and couplings arise from QCD anomalies and meson mixing. It discusses two main invisible axion realizations, KSVZ and DFSZ, and derives their mass scales and interactions, alongside stringent astrophysical and cosmological bounds that limit the PQ scale and the axion parameter space. The work highlights the potential for axions to be dark matter and notes ongoing experimental efforts to detect them.

Abstract

I describe how the QCD vacuum structure, necessary to resolve the $U(1)_A$ problem, predicts the presence of a P, T and CP violating term proportional to the vacuum angle $\barθ$. To agree with experimental bounds, however, this parameter must be very small $(\barθ \leq 10^{-9}$). After briefly discussing some possible other solutions to this, so-called, strong CP problem, I concentrate on the chiral solution proposed by Peccei and Quinn which has associated with it a light pseudoscalar particle, the axion. I discuss in detail the properties and dynamics of axions, focusing particularly on invisible axion models where axions are very light, very weakly coupled and very long-lived. Astrophysical and cosmological bounds on invisible axions are also briefly touched upon.