# Flavor signatures of complex anomalous $tcZ$ couplings

## TL;DR

This paper investigates how anomalous $tcZ$ couplings influence rare meson decays and top quark transitions, establishing bounds and potential observable effects in $K$, $B$, and top decays, with implications for CP violation.

## Contribution

It provides new constraints on anomalous $tcZ$ couplings from meson decay data and explores their potential to produce observable effects in top quark and $B$ meson decays.

## Key findings

- Upper bound on ${
m BR}(t o c Z)$ is $1.47 imes 10^{-5}$ for real couplings.
- Complex couplings could lead to ${
m BR}(t o c Z)$ up to $1.91 imes 10^{-4}$.
- Enhancement of order 10 in ${
m BR}(K_L o ext{pi}^0
u ar{
u})$ possible.

## Abstract

In this work we study the effects of anomalous $tcZ$ couplings. Such couplings would potentially affect several neutral current decays of $K$ and $B$ mesons via $Z$-penguin diagrams. Using constraints from relevant observables in $K$ and $B$ sectors, we find that the 2$\sigma$ upper bound on the branching ratio of ${\cal B}(t \to c Z)$ is $1.47 \times 10^{-5}$ for real coupling and $1.91 \times 10^{-4}$ for complex coupling. The current experimental upper bound from ATLAS and CMS collaborations on the branching ratio of $t \to c Z$ are $2.4 \times 10^{-4}$ and $4.9\times 10^{-4}$ at $95\%$ C.L., respectively. Hence the possibility of observation of $t \to c Z$ decay at the level of $10^{-4}$ would imply the anomalous couplings to be complex. Such complex couplings should also show up its presence in other related decays. We find that an order of magnitude enhancement is possible in the branching ratio of $K_L \to \pi^0 \nu \bar{\nu}$. Further, the complex $tcZ$ coupling can also provide large enhancements in many $CP$ violating angular observables in $B \to K^* \mu^+ \mu^-$ decay.