Search for FCNC in top-quark events in ATLAS

TL;DR

This paper reports on ATLAS searches for flavor-changing neutral currents in top-quark events, setting upper limits on FCNC processes and branching ratios using 7 TeV collision data from 2011.

Contribution

First analysis to set experimental upper limits on FCNC in top-quark production and decay at the LHC using neural networks and Bayesian methods.

Findings

No FCNC signal observed in top-quark production.

Upper limits on FCNC couplings and branching fractions established.

Constraints improve previous bounds on top-quark FCNC processes.

Abstract

Searches for flavor changing neutral current (FCNC) processes in top-quark production and decays by the ATLAS Collaboration are presented. Data collected from $pp$ collisions at the LHC at a centre-of-mass energy of 7 TeV during 2011, corresponding to an integrated luminosity of 2.05/fb, are used. In a first analysis single top-quarks produced via FCNC are searched for. Candidate events with a semileptonic top-quark decay signature are classified as signal or background-like events by using several kinematic variables as input to a neural network. No signal is observed in the neural network output distribution and a Bayesian upper limit is placed on the production cross-section. The observed upper limit is converted using a model-independent approach into upper limits on the coupling strengths k_{ugt}/Lambda < 6.9x10^{-3}/TeV and k_{cgt}/Lambda < 1.6x10^{-2}/TeV, where Lambda is the new physics scale, and on the branching fractions BF(t-->ug) < 5.7x10^{-5} and BF(t-->cg) < 2.7x10^{-4}. A second search is performed for top-quark pair-production events, with one top quark decaying through the t-->Zq FCNC (q=u,c) channel, and the other through the Standard Model dominant mode t-->Wb. Only the decays of the Z boson to charged leptons and leptonic W-boson decays are considered as signal. No evidence for an FCNC signal is found and thus an upper limit on the t-->Zq branching ratio of BF(t-->Zq)<0.73% is set at the 95% confidence level.