Calculation of axion–photon–photon coupling in string theory

The axion search experiments invite a plausible estimation of the axion–photon–photon coupling constant c¯aγγ in string models with phenomenologically acceptable visible sectors. We present the calculation of c¯aγγ with an exact Peccei–Quinn symmetry. In the Huh–Kim–Kyae Z12−IZ12−I orbifold compactification, we obtain c¯aγγ=1123388, and the low-temperature axion search experiments will probe the QCD corrected coupling, caγγ≃c¯aγγ−1.98≃0.91.