String theory and the strong CP problem

Abstract

The axion solution to the strong CP problem is reexamined. It is noted that in order for the axion to solve the problem, it is necessary that high-energy contributions to the axion potential be sufficiently small. Examples are constructed where this is not the case. It is noted that this problem arises in many interesting compactifications of string theory and in popular supergravity models, and that this provides a significant phenomenological constraint on model building. In string theory, these constraints are investigated both for compactifications with unification in E 6 and in O(10) and SU(5). We observe that the latter case is especially exciting, since one can have extra light Higgs doublets while satisfying renormalization group constraints.