Top quark pair production cross-section in proton-proton collisions at √s = 7 TeV

With proton beams colliding at an energy of 7 TeV in the center-of-mass (8 TeV in 2012), the Large Hadron Collider allows physicists to experimentally access regions of the phase-space that were previously unexplored.In this thesis, the cross-section of the top-antitop quark pair was measured, namely how often the top-antitop quark pair is produced, by identifying those topologies with a single-lepton in their final state.The top quark is the heaviest elementary particle known to date, and due to its mass it is expected to play an important role in understanding the electroweak symmetry breaking mechanism, the procedure with which particles gain mass. The top-antitop quark pair cross-section is the first step towards gaining a better understanding of the top quark physics at the energies provided by the LHC. To achieve the goal, the thesis also addresses the issue of selecting the proper events real-time and subsequently further identifying them offline over the huge background of other processes. The result presented in this thesis, using data of only 35 pb-1 of integrated luminosity, is found to be: 178.1 (stat.: +- 13.5) (syst.: +23.0 -24.6) (lumi.: +6.2 -5.9) pico-barns, which is considered to be in good agreement with the theoretical expectations

Charged-particle multiplicities in pp interactions at √s = 900 GeV measured with the ATLAS detector at the LHC

The first measurements from proton-proton collisions recorded with the ATLAS detector at the LHC are presented. Data were collected in December 2009 using a minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV. The charged-particle multiplicity, its dependence on transverse momentum and pseudorapidity, and the relationship between mean transverse momentum and charged-particle multiplicity are measured for events with at least one charged particle in the kinematic range |η|500 MeV. The measurements are compared to Monte Carlo models of proton-proton collisions and to results from other experiments at the same centre-of-mass energy. The charged-particle multiplicity per event and unit of pseudorapidity at η=0 is measured to be 1.333 ± 0.003(stat.) ± 0.040(syst.), which is 5-15% higher than the Monte Carlo models predict