The Structure of the Yang-Mills Vacuum Seen by Distant Observers

Abstract

The famous solution of the long-standing U(1)-problem in the Standard Model via instanton effects was presented by ’t Hooft about two decades ago [1]. This solution demonstrated that instantons, i.e. finite-action solutions of the Euclidean Yang-Mills-equations discovered by Belavin et al. [2] should be taken seriously in gauge theories. Another problem arose in these models over flat space-times, however: if instantons really exist, they induce a P- hence CP-violating so-called θ-term in the effective Yang-Mills action. But according to accurate experimental results, such a CP-violation does not exists in QCD, for instance. The most accepted solution to this problem is the so-called Peccei-Quinn mechanism [3]. A consequence of this mechanism is the existence of a light particle, the so-called axion. This particle has not been observed yet, however.