Resonant CP violation induced by particle mixing in transition amplitudes

Abstract

We analyze CP violation in resonant transitions involving scalar as well as fermionic intermediate states, using a gauge-invariant resummation approach implemented by the pinch technique. We derive the necessary conditions for resonantly enhanced CP violation induced by particle mixing, by paying special attention to CPT invariance, and apply the results of our analysis to two representative new-physics scenarios: (i) the indirect mixing of a CP-even Higgs scalar with a CP-odd Higgs particle in two-Higgs doublet models and (ii) the CP-asymmetric mixing between the top quark and a new heavy up-type quark, $t'$, in mirror fermion models. Furthermore, we explicitly demonstrate the equivalence of our scattering-amplitude formalism with that based on the effective Hamiltonian and discuss the possibility of maximal CP violation in the limiting case of two degenerate particles.