| Abstract: |
In the model of QCD axion where the axion is identified as the fifth component of the bulk $U(1)_{X}$ gauge field in 5D, the non-derivative couplings of the axion is governed by 5D $U(1)_{X}$ gauge symmetry and 5D locality. In the absence of matter field charged under $U(1)_{X}$, the Chern-Simons coupling of axion becomes the only source for violation of $U(1)_{\rm PQ}$, and thus the axion quality is protected from the quantum gravity effects. On the other hand, when the theory is supported on a slice of AdS$_{5}$, the small instantons of the bulk $SU(3)_{c}$ may make significant contribution to the axion potential due to the non-standard RGE of the bulk $SU(3)_{c}$, reviving the concern for the axion quality again. In this work, we study how large this effect is by computing the enhanced axion mass and the misalignment of axion potential induced by small instantons. Our analysis provides a unavoidable large QCD axion mass in the model and constraints on CP-violating operators for maintaining the success of the PQ mechanism to solve the strong CP problem. |
