Possible roles of Peccei-Quinn symmetry in an effective low energy model

Understand

Strong $CP$ problem is known to be solved by imposing Peccei-Quinn (PQ) symmetry.

• However, domain wall problem caused by the spontaneous breaking of its remnant discrete subgroup could make models invalid in many cases.
• We propose a model in which the PQ charge is assigned quarks so as to escape this problem without introducing any extra colored fermions.
• In the low energy effective model resulting after the PQ symmetry breaking, both the quark mass hierarchy and the CKM mixing could be explained through Froggatt-Nielsen mechanism.