A harbour for strong CP problem

Missing points in previous literature about strong CP are remarked. Emphasis is put on vacuum alignment taking heavy quark into account. After obtaining the master equations of vacuum alignment, the authors point out that mCd = 0 is the sufficient and necessary condition for = 0, where m is the mass of a quark and Cd the dynamical condensate of the same quark. The other authors have discussed the scenario with a vanishing u quark mass. A different scenario with Chd = 0 where Chd is the dynamical condensate of a heavy quark is proposed