Study of the b-quark properties with the first ATLAS data

In this doctoral thesis a study of the properties of the B+ meson is presented, analyzing the first pp collision data at sqrt{s}=7 TeV recorded with the ATLAS detector at the LHC at CERN in 2010-2011. The first data from the LHC provide an excellent opportunity for B-physics studies with two important goals: a) to validate the performance of the detector by reproducing well-known properties of heavy-flavor mesons and b) to provide the first physics results from the LHC. The B+ meson consists of a b anti-quark and a u quark, with the b being much heavier than the light u quark. The decay of B+ mesons to J/ψ K+, with J/ψ mesons decaying to two muons, is an ideal testbed for extended studies on b-quark properties. This thesis presents an analysis of the data recorded in 2010 (about 40 pb-1 in total) that led to the observation of the production of B mesons, demonstrating the ability of the ATLAS detector to reconstruct exclusive B --> J/ψ X decays. Next, a simultaneous measurement of the mass and lifetime of B mesons is presented, in good agreement with the world-average values. By reproducing these well-known properties, the excellent performance of the inner detector system and the vertex reconstruction algorithms is demonstrated already with the first ATLAS data and its proper functioning is validated in light of the physics measurements and discoveries that follow. The main physics result of this thesis is the measurement of the B+ production cross-section with 2.4 fb-1 of data collected in 2011. The cross-section was measured as a function of transverse momentum and rapidity of B+ mesons, extending recent measurements by the CMS and LHCb experiments at sqrt{s}=7 TeV to a higher transverse momentum region, up to about 100 GeV. The comparison of results with theoretical predictions of Quantum Chromodynamics is interesting in order to assess their validity at the new energy regime, especially after recent developments in the calculations that also showed good agreement with data from Tevatron. Moreover, comparisons are made in wider rapidity and transverse momentum ranges than what was previously studied. The measured cross-sections are compared with predictions obtained with the POWHEG and MC@NLO theoretical frameworks and with predictions of the FONLL approach, comprising next-to-leading-order calculations and resummation techniques. Good agreement is found with the measurements provided by this doctoral thesis, especially in the high-pT region, where the approximations used in the calculations within these frameworks are particularly sensitive. The measurements presented in this doctoral thesis provide points with total uncertainty smaller than that of the theory predictions, thus providing a motivation for theorists to improve the theoretical uncertainties.