The Strong CP Problem and Axions

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 $\bar{\theta }$. To agree with experimental bounds, however, this parameter must be very small $(\bar{\theta }\le 10^{-9}$). 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.

14 pages, to appear in the Lecture Notes in Physics volume on Axions, (Springer Verlag)

References in corpus (2)

• Wilkinson Microwave Anisotropy Probe (WMAP) Three Year Results: Implications for Cosmology
• An Improved Experimental Limit on the Electric Dipole Moment of the Neutron

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