Couplings between QCD axion and photon from string compactification

The QCD axion couplings of various invisible axion models are presented. In particular, the exact global symmetry U(1)PQU(1)PQ in the superpotential is possible for the anomalous U(1) from string compactification, broken only by the gauge anomalies at one loop level, and is shown to have the resultant invisible axion coupling to photon, View the MathML sourcecaγγ≥83−caγγchbr where View the MathML sourcecaγγchbr≃2. However, this bound is not applicable in approximate U(1)PQU(1)PQ models with sufficiently suppressed U(1)PQU(1)PQ-breaking superpotential terms. We also present a simple method to obtain View the MathML sourcecaγγ0 which is the value obtained above the electroweak scale. ©2016 The Authors.1441sciescopu