Search for single top-quark production via flavour changing neutral currents at 8 TeV with the ATLAS detector

Abstract

A search for single top-quark production via flavour-changing neutral current processes from gluon plus up- or charm-quark initial states in proton-proton collisions at the LHC is presented. Data collected with the ATLAS detector in 2012 at a centre-of-mass energy of 8 TeV and corresponding to an integrated luminosity of 20.3 fb${}^{-1}$ are used. Candidate events for a top quark decaying into a lepton, a neutrino and a jet are selected and classified into signal- and background-like candidates using a neural network. No signal is observed and an upper limit on the production cross-section multiplied by the $t\to \ell \nu b$ branching fraction is set. The observed 95% CL limit is ${\sigma }_{qg\to t}\times \mathcal{B}(t\to Wb)\times \mathcal{B}(W\to \ell \nu )<3.4$ pb and the expected 95% CL limit is ${\sigma }_{qg\to t}\times \mathcal{B}(t\to Wb)\times \mathcal{B}(W\to \ell \nu )<2.9$ pb. The observed limit can be interpreted as upper limits on the coupling constants of the flavour-changing neutral current interactions divided by the scale of new physics ${\kappa }_{ugt}/\Lambda <10\times 10^{-3}$ TeV${}^{-1}$ and ${\kappa }_{cgt}/\Lambda <23\times 10^{-3}$ TeV${}^{-1}$ and on the branching fractions $\mathcal{B}(t\to ug)<1.2\times 10^{-4}$ and $\mathcal{B}(t\to cg)<6.4\times 10^{-4}$.

29 pages plus the Autors list and cover page (45 pages in total), 10 figures, 3 tables, All figures including auxiliary figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2014-13/, This version includes modifications described in the erratum that can be found at https://doi.org/10.1140/epjc/s10052-021-09929-5

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