Search for flavour-changing neutral current top-quark decays t -> qZ in proton-proton collisions at root s=13 TeV with the ATLAS detector

M. Aaboud, G. Aad, B. Abbott, O. Abdinov, B Abeloos, S.H. Abidi, Ossama Sherif Alexander Abouzeid, NL Abraham, H. Abramowicz, H. Abreu, Y. Abulaiti, B.S. Acharya, Sosuke Adachi, L. Adamczyk, J. Adelman, M. Adersberger, T. Adye, A. A. Affolder, Y. Afik, C. AgheorghieseiJ. A. Aguilar-Saavedra, F. Ahmadov, Alejandro Alonso Diaz, Geert-Jan Besjes, Milena Bajic, Flavia de Almeida Dias, Mogens Dam, Gorm Aske Gram Krohn Galster, Jørn Dines Hansen, Jørgen Beck Hansen, Peter Henrik Hansen, James William Monk, Troels Christian Petersen, Simon Holm Stark, Fabian Alexander Jürgen Thiele, Graig Wiglesworth, Stefania Xella

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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.[Figure not available: see fulltext.].

OriginalsprogEngelsk
Artikelnummer176
TidsskriftThe Journal of High Energy Physics
Vol/bind2018
Udgave nummer7
Antal sider41
ISSN1126-6708
DOI
StatusUdgivet - 1 jul. 2018

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