Calculation of axion-photon-photon coupling in string theory

Abstract

The axion search experiments invite a plausible estimation of the axion--photon--photon coupling constant ${\overline{c}}_{a\gamma \gamma }$ in string models with phenomenologically acceptable visible sectors. We present the calculation of ${\overline{c}}_{a\gamma \gamma }$ with an exact Peccei-Quinn symmetry. In the Huh-Kim-Kyae ${\mathbb{Z}}_{12-I}$ orbifold compactification, we obtain ${\overline{c}}_{a\gamma \gamma }=\frac{8}{3}$ even with a lot of singlets at the GUT scale, and the low-temperature axion search experiments will probe the QCD corrected coupling, $c_{a\gamma \gamma }\simeq {\overline{c}}_{a\gamma \gamma }-1.98\simeq 0.69$.

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