Axion in the minimal SO(10) GUT

A bstract The QCD axion is investigated within the minimal supersymmetric SO(10) grand unified theory, where the Yukawa sector involves Higgs multiplets 10 and 126̄ 126 ¯ . The relative phase between the VEVs of ( 10 , 1 , 3 ) ⊂ 126̄ 126 ¯ and ( 10̄ 10 ¯ , 1 , 3 ) ⊂ 126 under SU(4) C × SU(2) L × SU(2) R is identified with the axion. The axion is dominantly the relative phase between ∆ R and Δ ̄R ∆ ¯ R , with a small 10 -Higgs component due to the VEV hierarchy. The Peccei-Quinn and B − L symmetry breaking scales coincide through | ΔR |=| Δ̄R | Δ R = Δ ¯ R . The scalar partner of the lightest right-handed neutrino plays the role of the inflaton, realizing hybrid inflation consistent with the observed CMB density fluctuations. After inflation, both fields acquire VEVs, and the domain-wall problem is resolved through the Lazarides-Shafi mechanism, which naturally restricts the model to three generations.