CP Violation in Higgs Diphoton Decay in Models with Vectorlike Heavy   Fermions
  The amplitude of the diphoton decay of the Higgs boson is considered in an extension of the Standard Model involving heavy vectorlike fermions, which was recently suggested in the literature in order to explain a possible enhancement of the h -> gamma gamma decay rate. It is pointed out that generally in such models the decay amplitude acquires a sizable CP-odd contribution from the complex phases in the mass matrix of the new heavy fermions. The resulting CP violation in the diphoton decay can be studied experimentally when both photons are converted into e^+ e^- pairs by measuring the distribution over the angle between the planes of the pairs. Such measurement, if feasible, would be of a great interest on its own, regardless specific models. 
M.B. Voloshin
hep-ph hep-ex