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The low mass Drell-Yan di-muon process is investigated with the ATLAS detector at the LHC, in order to provide information that advances our knowledge of the Parton Density Functions in a region of phase space unaccessible at previous experiments. A cut-based selection of di-muon events is performed, using 2010 data with a centre of mass energy of the proton-proton collisions of 7TeV, and an integrated luminosity of 36 pb−1. The analysis probes the region of low muon transverse momentum (pT > 6GeV), and low di-muon mass region 12 < M < 66GeV. The main challenges of the analysis are the understanding of the muon isolation and the trigger efficiency. In order to reject the large QCD background the analysis relies on stringent isolation criteria. The efficiency of the chosen selection is presented in detail. The second main part of the analysis is the measurement of the trigger efficiency for low pT threshold muon triggers. This is an important aspect of the cross section measurement, since the pT spectrum of leptons from the low mass Drell-Yan process are soft and populate the trigger threshold region. In order to measure the differential cross section in mass d dM in the fiducial region of |η| < 2.4 and pT,μ1 > 9GeV and pT,μ2 > 6GeV (asymmetric analysis) or pT,μ1 > 6GeV and pT,μ2 > 6GeV (symmetric analysis), a one dimensional bin-by-bin unfolding is adopted to account for detector reconstruction and resolution effects; all the associated uncertainties are also presented. The fiducial and extrapolated differential cross sections are measured at the Born level. Dressed level corrections are also given. The measured cross sections are shown to agree with theoretical predictions within the margin of error. A precision of 9.7% is achieved in the asymmetric analysis in the lowest invariant mass bin between 12 and 17GeV; the statistical and systematic uncertainties in the same bin are 4.2% and 8.7% respectively. In the remaining mass region the total uncertainty is smaller. The luminosity error during the 2010 data taking period is estimated to be 3.4%. In addition to the Drell-Yan cross section measurement, the thesis describes the study performed in order to extract the Lorentz angle value in the ATLAS Semiconductor Tracker. The Lorentz angle is computed through the study of the SCT cluster width from both cosmic and collision data and comparison with simulation is shown. The track selection on collision data is defined and the fitting range is optimised to give robust results. Throughout this thesis the convention c = 1 is adopted.

The properties of hadronic and quark matter are studied as a function of density using a chiral model based on quark degrees of freedom. Nucleons are described as quark - diquark states in the Faddeev approach and this description is extended to infinite nuclear matter in the mean field approximation. We calculate the properties of two flavour quark matter, allowing for the possibility of colour superconductivity in the form of a spin zero condensate ( i.e. the 2SC phase ). These calculations are performed using the proper - time regularisation method. We find that the phase diagrams for asymmetric matter in this description can have charge neutral phase transitions from the hadronic phase to the decon - fined phase, depending on the pairing strength for quarks in the 2SC phase. We study the evolution of the phase diagrams as a function of the pairing strength. The properties of nuclear matter are significantly improved once we take into account the self - energy of the nucleon. We also find that the structure of the nucleon has important consequences for the phase diagram. The charge neutral equations of state are used to produce compact star configurations by solving the Tolman - Oppenheimer - Volkoff ( TOV ) equations. We use these solutions to investigate the possibility of hybrid stars. / Thesis (Ph.D.)--School of Chemistry and Physics, 2006.

One unresolved question in hadronic physics is the origin of large transverse single-spin asymmetries, AN, observed in hadron production from high-energy polarized-proton collisions. Collinear perturbative Quantum Chromodynamics (pQCD) predicts that AN should scale with the quark mass, however, experiments have since reported large AN for inclusive hadron production. Recent measurements from RHIC experiments show examples of these asymmetries at forward angles in a kinematic region where pQCD cross-section calculations reasonably agree with measured cross-sections. Disentangling dynamical contributions to AN from hadro-production requires moving beyond inclusive measurements. One possibility is to investigate asymmetries in two-particle correlations due to Interference Fragmentation Functions (IFF) and the Sivers effect. In 2008, RHIC dedicated a portion of the run to transversely polarized proton collisions at sqrt(s) = 200 GeV. STAR was equipped with a Foward Meson Spectrometer (FMS) and a Forward Time Projection Chamber (FTPC), overlapping in the pseudorapidity range of 2.5 < eta &lt; 4. By analyzing neutral pions with the FMS correlated with charged particles from the FTPC, correlation asymmetries can be measured at kinematics where large inclusive asymmetries have been measured. Correlations are measured for pi^0's with 2 &lt; pT, pi^0 &lt; 5 GeV/c and associated charged particles in two ranges of transverse momentum: 1 &lt; pT, ch &lt; 2 GeV/c and 0.5 &lt; pT, ch &lt; 1 GeV/c. IFF and Sivers asymmetries manifest themselves through the correlation of two particles from the same jet. These events are selected through a cut on the pair radius, delta R. Gain non-uniformities and electronics failures have resulted in large holes in trigger acceptance and associated particle acceptance, respectively. This non-uniform acceptance allows the Sivers and IFF effects to mix and distort the raw asymmetries. Techniques are developed to measure this leak-through by means of unpolarized yields and event weighting. They result in small corrections to the asymmetries. IFF and Sivers asymmetries both for xF > 0 and for xF &lt; 0 are reported for forward-angle pi^0-charged particle correlations from polarized-proton collisions at sqrt(s) = 200 GeV. Asymmetries are shown corrected for full underlying-event and pileup backgrounds, as well as corrected only for pile-up contributions. It appears the asymmetries are less sensitive to delta R when corrected for the full underlying-event background. Unfortunately, statistics limitations preclude a firm conclusion.

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Thesis supervisor: Yasar Onel. Includes bibliographic references (p. 97-98).