Constraints on anomalous $tcZ$ coupling from $\bar B \to \bar K^* \mu^+ \mu^-$ and $B_s \to \mu^+ \mu^-$ decays

TL;DR

This paper constrains anomalous $tcZ$ couplings using $B$ meson decay data, deriving bounds on the coupling and related top quark decay branching ratios, and discusses implications for future collider searches.

Contribution

It provides the first combined experimental constraints on the anomalous $tcZ$ coupling from $B$ decay data and explores their impact on top quark decay predictions.

Findings

Bound on $|X_{ct}^L|$ dominated by $B$ decay branching ratios.

One sign-flipped solution excluded by $K^*$ longitudinal fraction.

Predicted upper bound on $t o cZ$ branching ratio is $1.0 imes 10^{-4}$.

Abstract

In this paper, we analyze the possible anomalous $tcZ$ coupling effects in the $b \to s $ mediated decays $\bar B \to \bar K^* \mu^+ \mu^-$ and $B_s \to \mu^+ \mu^-$. After exploiting the available experimental data, the combined constraints on the anomalous coupling $X_{ct}^L$ are derived. It is found that, the bound on the magnitude $|X_{ct}^L|$ is dominated by the branching ratios of these two decays. Furthermore, one sign-flipped solution is excluded by the longitudinal fraction of $\bar B \to \bar K^* \mu^+ \mu^-$ at the low dilepton mass region. After considering the combined constraints, for general complex coupling $X_{ct}^L$, the predicted upper bound on $\mathcal B (t \to cZ)$ are compatible with that from the recent CMS direct search. In particular, for the case of real coupling $X_{ct}^L$, the upper bound reads $\mathcal B (t \to cZ)<1.0\times 10^{-4}$, which is much lower than the current CMS bound but still accessible at the LHC. With improved measurements at the LHC, the colser correlations between the $t \to cZ$ and $b \to s$ mediated (semi-) leptonic decays are expected in the near future.