Z'-induced FCNC Decays of Top, Beauty and Strange Quarks

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 → cZ ′ , based on a model of the gauged Lµ − Lτ 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 → cZ ′ via Z ′ → µ + µ − 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 → cZ ′ branching ratio for the light Z ′ case, and discuss about its observability at the LHC. We also scrutinize the possibility that the decay KL → π 0 Z ′ with Z ′ → νν may lead to apparent violation of the usual Grossman-Nir bound of B(KL → π 0 νν) < 1.4 × 10 −9 .