Search for Flavour Changing Neutral Currents in Single Top-Quark Production at √<em>s</em> = 7 TeV with the ATLAS Detector

This thesis represents the search for single top-quark production through flavour changing neutral currents using data collected by the ATLAS detector in 2011, at a centre-of-mass energy of √s = 7 TeV, corresponding to an integrated luminosity of 2.05 fb-1. Candidate events are selected with one isolated lepton, missing transverse momentum associated to the undetected neutrino and a jet originated from the hadronisation of a b quark. Given the large expected number of background events and the small number of expected signal events, a neural network classifier is developed to combine many kinematic variables to create a powerful separator in order classify the events as a signal- or a background-like events. As no sign of new physics is seen in the neural network output distribution, a Bayesian statistical method is used to set an upper limit at 95% confidence level (C.L.) on the single top-quark production cross section through FCNC processes. The observed upper limit at 95% C.L. on the cross-section multiplied by the t → Wb branching fraction is measured to be σqg→t × B(t → Wb) ugt/Λ -3 TeV-1 and Κcgt/Λ -2 TeV-1, where Λ is the new physics scale, and on the branching fractions B(t→ug) -5 and B(t→cg) -4. The limits on the branching fractions are the world's best limits to date and significantly improving the previous limits obtained by the DO collaboration by a factor of 15

Reconstruction of primary vertices at the ATLAS experiment in Run 1 proton–proton collisions at the LHC

This paper presents the method and performance of primary vertex reconstruction in proton–proton collision data recorded by the ATLAS experiment during Run 1 of the LHC. The studies presented focus on data taken during 2012 at a centre-of-mass energy of √s=8 TeV. The performance has been measured as a function of the number of interactions per bunch crossing over a wide range, from one to seventy. The measurement of the position and size of the luminous region and its use as a constraint to improve the primary vertex resolution are discussed. A longitudinal vertex position resolution of about 30μm is achieved for events with high multiplicity of reconstructed tracks. The transverse position resolution is better than 20μm and is dominated by the precision on the size of the luminous region. An analytical model is proposed to describe the primary vertex reconstruction efficiency as a function of the number of interactions per bunch crossing and of the longitudinal size of the luminous region. Agreement between the data and the predictions of this model is better than 3% up to seventy interactions per bunch crossing

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements

A new method to distinguish hadronically decaying boosted Z bosons from W bosons using the ATLAS detector

The distribution of particles inside hadronic jets produced in the decay of boosted W and Zbosons can be used to discriminate such jets from the continuum background. Given that a jet has been identified as likely resulting from the hadronic decay of a boosted W or Z boson, this paper presents a technique for further differentiating Z bosons from W bosons. The variables used are jet mass, jet charge, and a b-tagging discriminant. A likelihood tagger is constructed from these variables and tested in the simulation of W′→WZW′→WZ for bosons in the transverse momentum range 200 GeV &lt;pT&lt;&lt;pT&lt; 400 GeV in √s = 8 TeV pp collisions with the ATLAS detector at the LHC. For Z-boson tagging efficiencies of ϵZ=90ϵZ=90, 50, and 10%10%, one can achieve W+W+-boson tagging rejection factors (1/ϵW+1/ϵW+) of 1.7, 8.3 and 1000, respectively. It is not possible to measure these efficiencies in the data due to the lack of a pure sample of high pTpT, hadronically decaying Z bosons. However, the modelling of the tagger inputs for boosted Wbosons is studied in data using a tt¯tt¯-enriched sample of events in 20.3 fb−1−1 of data at √s = 8TeV. The inputs are well modelled within uncertainties, which builds confidence in the expected tagger performance

ATLAS Run 1 searches for direct pair production of third-generation squarks at the Large Hadron Collider

This paper reviews and extends searches for the direct pair production of the scalar supersymmetric partners of the top and bottom quarks in proton-proton collisions collected by the ATLAS collaboration during the LHC Run 1. Most of the analyses use 20 fb$^{-1}$ of collisions at a centre-of-mass energy of $\sqrt{s}$ = 8 TeV, although in some case an additional 4.7 fb$^{-1}$ of collision data at $\sqrt{s}$ = 7 TeV are used. New analyses are introduced to improve the sensitivity to specific regions of the model parameter space. Since no evidence of third-generation squarks is found, exclusion limits are derived by combining several analyses and are presented in both a simplified model framework, assuming simple decay chains, as well as within the context of more elaborate phenomenological supersymmetric models