Outline of the lecture:
In this lecture I will explain the origin of the Strong CP Problem, which is connected to the non- trivial structure of the vacuum state of the Standard Model. The most natural solution to this problem involves introducing an additional global chiral U(1) symmetry in the theory. Because this symmetry is both spontaneously broken and anomalous, this solution necessitates the presence of a very light, very weakly coupled boson, known as the axion.
Physical properties of axions, including their mixing with pions and etas and their coupling to two-photons are described. Using these results, it is possible to rule out weak-scale axions. However, invisible axions models are still viable. Some of the properties of these latter models will be also discussed.