$B_s^0-\bar B_s^0$ mixing in a family non-universal $Z^{\prime}$ model revisited

TL;DR

This paper investigates whether a family non-universal $Z'$ model can simultaneously explain recent $B_s^0-ar B_s^0$ mixing measurements, including CP asymmetries, and finds it cannot under current constraints.

Contribution

It revisits the $B_s^0-ar B_s^0$ mixing in a specific $Z'$ model, analyzing its ability to explain recent experimental anomalies and constraints.

Findings

The $Z'$ model cannot explain the measured $a_{fs}^s$ within 1$\sigma$ constraints.

Adding $Z'$ contributions to $\Gamma_{12}^s$ does not reconcile all data.

Future measurements may clarify the viability of such models.

Abstract

Motivated by the very recent measurements performed at the LHCb and the Tevatron of the $B_s^0-\bar B_s^0$ mixing, in this paper we revisit it in a family non-universal $Z^{\prime}$ model, to check if a simultaneous explanation for all the mixing observables, especially for the like-sign dimuon charge asymmetry observed by the D0 collaboration, could be made in such a specific model. In the first scenario where the $Z^\prime$ boson contributes only to the off-diagonal element $M_{12}^s$, it is found that, once the combined constraints from $\Delta M_s$, $\phi_s$ and $\Delta \Gamma_s$ are imposed, the model could not explain the measured flavour-specific CP asymmetry $a_{fs}^s$, at least within its $1\sigma$ ranges. In the second scenario where the NP contributes also to the absorptive part $\Gamma_{12}^s$ via tree-level $Z^\prime$-induced $b\to c\bar{c}s$ operators, we find that, with the constraints from $\Delta M_s$, $\phi_s$ and the indirect CP asymmetry in $\bar{B}_d\to J/\psi K_S$ taken into account, the present measured $1\sigma$ experimental ranges for $a_{fs}^s$ could not be reproduced too. Thus, such a specific $Z^\prime$ model with our specific assumptions could not simultaneously reconcile all the present data on $B_s^0-\bar B_s^0$ mixing. Future improved measurements from the LHCb and the proposed superB experiments, especially of the flavour-specific CP asymmetries, are expected to shed light on the issue.