Suppressed flavor changing neutral current in new physics with shared flavor symmetry

Abstract

Many extensions of the standard model (SM) generate contributions to flavor changing neutral current (FCNC) processes that must have sufficient flavor suppression to be consistent with experiments, if the new physics (NP) is associated with a scale of a TeV. Here we present a mechanism for suppressing the NP effects to FCNC processes while allowing for new non-SM $CP$ violating phases. We consider the possibility that the source of NP contributions to FCNC processes share the same flavor symmetry underlying the SM source of FCNC processes which are the quark and lepton mass matrices. We call this the principle of shared flavor symmetry. In the flavor symmetric limit, the quark and lepton mixing matrices have fixed forms and there are no NP FCNC processes. In the flavor symmetric limit, we take the quark mixing matrix to be the identity matrix and the lepton mixing matrix to be given by tri-bimaximal mixing. Realistic mixing matrices are obtained by the small breaking of the flavor symmetry. New contributions to FCNC processes arise because of nonuniversal breaking of the flavor symmetry in the quark and lepton mass matrices and the NP sources of FCNC processes. In particular, we will focus on new FCNC effects that arise due to the breaking of flavor symmetry only in the quark and charged lepton mass matrices but not in the NP sector. In this scenario, NP contributions to FCNC processes are linked to the source of flavor symmetry breaking in the quark and charged lepton mass matrices. The breaking of flavor symmetry in the NP sector is assumed to produce FCNC effects that are at most the size of NP FCNC effects due to the breaking of flavor symmetry in the quark and charged lepton mass matrices. To demonstrate the mechanism we use a two Higgs doublet model as an example of beyond the SM physics though one should be able to adapt this mechanism to other models of new physics.