Fundamental CP-violating quantities in a SU(2)$ \otimes $U(1) model with   many Higgs doublets
  We consider a SU(2)$ \otimes $U(1) gauge theory with many scalar doublets, but without fermions. We devise a systematic method of constructing quantities which are invariant under changes of basis of the scalar fields, but acquire a minus sign under a general CP transformation. Those quantities provide a basis-independent way of checking for the existence of CP violation. 
L. Lavoura and Joao P. Silva