Small and an invisible axion as automatic consequences of grand unification

This chapter explores the possibility of solving the strong CP problem in a grand unified theory in which a global PecceiQuinn (PQ) symmetry is an automatic consequence of the gauge structure. Points out that the low energy effective theory differs from that of the standard model only by the presence of an extremely light, weakly interacting axion. Describes 4 models which incorporate the PQ mechanism. The simplest model has two SU(2) doublets of scalar particles, but predicts a light pseudo-Goldstone boson (PGB), the axion, which has yet to be experimentally confirmed. The second model removes such an axion, at the expense of introducing a new fine tuning problem. This is resolved by embedding the model in a grand unified theory (GUT). Gives a GUT which seeks to explain the origin of U(1)PQ. Discusses two aspects of gauge theories which are considered by many to be undesirable; namely, global symmetries and the fine tuning of parameters of the theory