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. Furthermore, 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.

We present a search for flavor changing neutral currents via quark-gluon couplings in a sample of single top quark final states corresponding to 2.3 fb−1 of integrated luminosity collected with the D0 detector at the Fermilab Tevatron Collider. We select events containing a single top quark candidates with an additional jet, and obtain separation between signal and background using Bayesian neural networks. We find consistency between background expectation and observed data, and set limits on avor changing neutral current gluon couplings of the top quark to up quarks (tgu) and charm quarks (tgc). The cross section limits at the 95% C.L. are [sigma]{sub tgu}

The top quark is the heaviest known elementary particle. With a coupling to the Higgs boson close to unity, the top quark plays a special role in electroweak symmetry breaking as well as in physics beyond the Standard Model. An accurate measurement of its properties can provide key information for understanding the physics at the TeV scale. In this dissertation, two measurements of the top quark properties are presented. The first part of this dissertation presents the first-ever ATLAS search for the associated production of a single top quark and a photon. This analysis is performed with 139 fb−1 of pp collision data collected by the ATLAS detector at the LHC at [square root symbol] s = 13 TeV. Only final states with exactly one lepton and at least one photon are considered in this analysis. In the second part, results from the combination of ATLAS searches for flavor changing neutral currents (FCNC) in t->Hq decays are presented. Four FCNC analyses, probing different Higgs decay modes, performed with 36.1 fb−1 of proton-proton collision data collected by the ATLAS detector in 2015-2016 are considered in the combination. They search for top-quark pair events in which one top quark decays into Wb and the other top quark decay into Hq