Z'-induced FCNC decays of top, beauty and strange quarks

TL;DR

This paper investigates flavor-changing neutral current decays of the top quark induced by a Z' boson within a gauged L_mu - L_tau symmetry model, exploring experimental constraints and potential observability at the LHC, including implications for rare meson decays.

Contribution

It introduces a model linking Z' boson-induced FCNC top decays to LHCb anomalies and explores experimental bounds and signatures for different Z' mass ranges.

Findings

LHC Run 1 data can probe unexplored Z' parameter space for masses around 10 GeV.

Upper limits on t --> c Z' branching ratios are established for light Z' scenarios.

Possible violation of the Grossman-Nir bound due to K_L --> pi^0 Z' decays is discussed.

Abstract

Anomalous b --> s transitions from LHCb data may suggest a new massive gauge boson Z' that couples to the left-handed b --> s current, which in turn implies a coupling to the t --> c current. In this paper, we study flavor-changing neutral current (FCNC) decays of the top quark induced by a Z' boson, namely t --> c Z', based on a model of the gauged L_mu - L_tau symmetry (the difference between the muon and tauon numbers) with vector-like quarks, which was introduced to explain the anomalous LHCb data. We illustrate that searching for t --> c Z' via Z' --> mu^+ mu^- with LHC Run 1 data can already probe a parameter region which is unexplored by B physics for the Z' mass around O(10) GeV or more. We further extend the model to very light Z' with mass below 400 MeV, which is motivated by the muon g-2 anomaly. Taking rare B and K meson decay data into account, we give upper limits on the t --> c Z' branching ratio for the light Z' case, and discuss about its observability at the LHC. We also scrutinize the possibility that the decay K_L --> pi^0 Z' with Z' --> nu nubar may lead to apparent violation of the usual Grossman-Nir bound of B(K_L --> pi^0 nu nubar) < 1.4 x 10^-9.