CP violation in the cubic coupling of neutral gauge bosons
  We investigate the CP violating form factor of the ZZZ$ and $ZZ\gamma$ vertices in the pair production of $Z^0$ bosons. Useful observables in azimuthal distributions are constructed to probe $CP$ nonconservation which may originate from these vertices. A simple Two Higgs Model of $CP$ violation is used as an illustration. 
Darwin Chang, Wai-Yee Keung and Palash Pal