Abstract
Abstract:
A search for flavour-changing neutral-current processes in top-quark decays is presented.
Data collected with the ATLAS detector from proton–proton collisions at the Large Hadron Collider
at a centre-of-mass energy of √s = 13 TeV, corresponding to an integrated luminosity
of 36.1 fb−1, are analysed. The search is performed using top-quark pair events, with one top quark
decaying through the t → qZ (q = u, c) flavour-changing neutral-current channel, and the other
through the dominant Standard Model mode t → bW . Only Z boson decays into charged leptons and
leptonic W boson decays are considered as signal. Consequently, the final-state topology is
characterized by the presence of three isolated charged leptons (electrons or muons), at least two
jets, one of the jets originating from a b-quark, and missing transverse momentum from the
undetected neutrino. The data are consistent with Standard Model background contributions, and at
95% confidence level the search sets observed (expected) upper limits of 1.7 × 10−4 (2.4 × 10−4)
on the t → uZ branching ratio and
2.4 × 10−4 (3.2 × 10−4) on the t → cZ branching ratio, constituting the most stringent limits
to date.