A real CKM matrix and physics beyond the standard model

We study the possible existence of a real Cabibbo-Kobayashi-Maskawa (CKM) m atrix, with CP violation originating from physics beyond the standard model (SM). We show that present experimental data allow for a real CKM matrix p rovided that neu physics also contributes to Delta m(Bd) by at least 20% of the SM contribution (for rho > 0), besides generating CP violation in the kaon sector. The naturalness of a real CKM matrix is studied within the fra mework of general multi-Higgs-doublet models with spontaneous CP violation. As an example, we discuss a specific two-Higgs-doublet model and its impli cations for CP asymmetries in non-leptonic neutral B-meson decays. (C) 1999 Published by Elsevier Science B.V. All rights reserved.