Search for new physics in single-top-quark production with the ATLAS detector at the LHC

This thesis presents two analyses devoted to the search of new physics in single-top-quark events with data collected by the ATLAS detector at the LHC. In spite of its great success, it is believed that there must be a more general theory underlying the Standard Model of particle physics which sheds light to unanswered questions such as the matter-antimatter asymmetry of the Universe or the existence of Dark Matter. This thesis covers two different approaches to address the two later topics. The aim of the first analysis is to probe the structure of the Wtb vertex through the measurement of angular asymmetries related to W-boson polarization observables using single-top-quark events produced in the t-channel. The dataset corresponds to proton-proton collision events produced at a center-of-mass energy of 8 TeV with a total integrated luminosity of 20.2/fb. Selected events contain one isolated lepton (electron or muon), large missing transverse momentum and exactly two jets, with one of them identified as likely to contain a b-hadron. Further selection requirements are applied to further separate signal t-channel single-top-quark events from background. The W boson polarization observables are extracted from asymmetries in angular distributions of the charged lepton measured with respect to a defined spin quantization axes. The asymmetry measurements are performed at parton level by correcting the observed angular distributions for detector effects and hadronization after subtracting the background contributions. The measured W-boson polarization values are in agreement with the Standard Model predictions. The results are used to set limits on the imaginary part of the anomalous coupling gR in a model-independent way. The second analysis presents a search for events with one top quark and large missing transverse energy at the final state. Such final states are not possible within the Standard Model of particle physics. The dataset corresponds to proton-proton collision data at a center-of-mass energy of 13 TeV collected by the ATLAS detector during 2015 and 2016, corresponding to an integrated luminosity of 36.1/fb. The channel with leptonic decay of the W boson from the top quark is covered in this thesis. Selected events contain exactly one isolated lepton (electron or muon), large missing transverse momentum and exactly one jet, identified as likely to contain a b-hadron. In addition, further selection requirements are applied to discriminate signal events from background. The shape of the missing transverse energy distribution is used to fit the expected backgrounds to the observed data. The obtained results are combined with the analysis of the hadronic W boson decay and interpreted in the context of generic models for Dark Matter production. In the absence of evidence for any of these signals, 95% confidence level upper limits on the corresponding production cross-sections are obtained and these limits are translated into constraints on the parameter space of the considered models. In addition to the two mentioned analyses, some contributions to the alignment of the ATLAS inner detector are presented in this manuscript.