$Z^\prime$-Tandem Mechanism for the Suppression of New Physics in Quark Mixing with Implications for K, D and B Decays

Abstract

Z models belong to the ones that can most easily explain the anomalies in b → sµ + µ − transitions. However, such an explanation by a single Z gauge boson, as done in the literature, is severly constrained by the B 0 s − B0 s mixing. Also the recent finding, that the mass differences ∆M s , ∆M d , the CP-violating parameter ε K , and the mixing induced CP-asymmetries S ψK S and S ψφ can be simultaneously well described within the SM without new physics (NP) contributions, is a challenge for Z models with a single Z contributing at tree-level to quark mixing. We point out that including a second Z in the model allows to eliminate simultaneously tree-level contributions to the five ∆F = 2 observables used in the determination of the CKM parameters while leaving the room for NP in ∆M K and ∆M D . The latter one can be removed at the price of infecting ∆M s or ∆M d by NP which is presently disfavoured. This pattern is transparently seen using the new mixing matrix for Z interactions with quarks. This strategy allows significant tree-level contributions to K, B s and B d decays thereby allowing to explain the existing anomalies in b → sµ + µ − transitions and the anticipated anomaly in the ratio ε /ε much easier than in Z -Single scenarios. The proposed Z -Tandem mechanism bears some similarities to the GIM mechanism for the suppression of the FCNCs in the SM with the role of the charm quark played here by the second Z . However, it differs from the latter profoundly in that only NP contributions to quark mixing are eliminated at tree-level. We discuss briefly the implied flavour patterns in K and B decay observables in this NP scenario.

Natural Suppression of FCNCs at the One-Loop Level in $Z^\prime$ Models with Implications for K, D and B Decays

“…In this Z -Tandem scenario the new parameters in (15) are determined exclusively from ∆F = 1 processes with the CKM parameters determined already through ∆F = 2 processes and given in (47). Z -Tandem scenario is proposed and discussed in [40]. The parametrization in (17) presented here plays the crucial role in this new framework.…”