## Abstract

It is pointed out that there exist a few problems to be overcome toward an observable sub-eV QCD axion in superstring compactification. We give a general expression for the axion decay constant. For a large domain wall number Now, the axion decay constant can be substantially lowered from a generic value of a scalar singlet VEV. The Yukawa coupling structure in the recent Z_{12-1} model is studied completely, including the needed nonrenormalizable terms toward realistic quark and lepton masses. In this model we find an approximate global symmetry and vacuum so that a QCD axion results but its decay constant is at the GUT scale. The axion-photon-photon coupling is calculated with a reasonable assumption on a realistic vacuum satisfying the quark and lepton mass matrix conditions. It is the first time calculation of c _{αγγ} in realistic string compactifications: C _{αγγ}= 5/3- 1.93 ≃ -0.26.

Original language | English |
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Journal | Journal of High Energy Physics |

Volume | 2007 |

Issue number | 3 |

DOIs | |

State | Published - 1 Mar 2007 |

## Keywords

- GUT
- Global symmetries compactification and string models
- Quark masses and SM parameters