$$B^0 - \bar B^0 $$ mixing within and beyond the standard model

Abstract

The standard model with three fermion families is found to be compatible with the Argus observation of a relatively large amount of\(B_{^d }^0 - \bar B_d^0 \) mixing provided that the top quark is rather heavy. From an analysis of the existing information on mixing angles, and of the constraints imposed by the kaon system, in particular by the e parameter, we conclude that almost certainlym t 90 GeV. In view of the Argus result the standard model unambiguously leads to the prediction of a nearly maximal amount of\(B_{^s }^0 - \bar B_s^0 \) mixing. Apart from the rather obvious case of a fourth family of quarks, most “minimal” extensions of the standard model preserve the prediction of a large\(B_{^s }^0 - \bar B_s^0 \) mixing. We discuss\(B^0 - \bar B^0 \) mixing in minimal supersymmetric models compatible with the UA1 lower bounds on gluino and squark masses. Contributions from charged Higgs exchange in the box diagrams are also discussed. While supersymmetry (although marginally) and charged Higgses can lead to an appreciable enhancement of\(B^0 - \bar B^0 \) mixing, minimal left-right symmetric models actually predict a modest suppression of the effect with respect to the standard model.