End-cap calorimeter performance and identification of the t-channel single top quark process with the ATLAS detector

The LHC collider will provide proton-proton collisions with 14 TeV centre of mass energy and an expected peak luminosity of 10 34 cm -2 s -1 . ATLAS is one of the multipurpose detectors that will be used for particles detection and measurement of properties. The first part of this thesis focuses on the study of the response of the ATLAS electromagnetic and hadronic end-cap calorimeters (EMEC and HEC, respectively) in a beam test performed in the summer of 2002. For the EMEC, the dependence of the measured signal versus the beam energy was found to be linear and an electromagnetic conversion constant [Special characters omitted.] = (0.446 ± 0.009) MeV/nA was calculated. The energy resolution for the EMEC was [Special characters omitted.] = [Special characters omitted.] ⊕ (0.4 ± 0.1)%, while for the HEC it was [Special characters omitted.] = [Special characters omitted.] ⊕ (3.0 ± 0.2)%, where the reconstructed energy, E reco , is in GeV. These results feed back into the tuning of the calorimeter Monte Carlo simulations. The second part of this thesis is on the analysis of the electroweak production of the top quark at the LHC, particularly the so-called single top t-channel, by investigating the possible isolation of signal events from the diverse background present at LHC energies. Kinematic characteristics of relevant physics objects were studied for their use as discriminant variables. After identifying these discriminants, a cut flow analysis was performed on both signal and backgrounds. It was found that the selection efficiency, ε (%), for the t-channel was ε (%) = 2.86 ± 2.5% Stat ± 2% TopMass ± 5% JES ± 5% B-tagging ± 8% ISR/FSR ± 2% PDF , for an integrated luminosity of 1 fb -1 . For the same luminosity, a signal to background ratio (S/B) of 0.28 and a signal significance (S/[Special characters omitted.] ) of 21 were obtained. The single top production channels provide an unique opportunity to study directly properties of the top quark. These channels are also sensitive to new physics beyond the Standard Model of particle physics.