We present a measurement of the top anti-top quark (ttbar)production cross section in the dilepton final states from proton-proton collisions at a center of mass energy at 7 TeV at the LHC. A b-tagging algorithm based on tracks displaced from the event interaction vertex is applied to identify bottom quark jets from top quark decay and reject background events. Given the relatively pure sample of bottom quark jets in ttbar dilepton final states, a new technique to measure in-situ the b-tagging efficiency is introduced that uses the distribution of the number of observed b-tagged jets. We present results with data collected at the ATLAS detector in 2010 with an integrated luminosity of 35 pb-1. The measured ttbar cross section is 176 +22/-21 (stat.) ± 20 (syst.) ± 6 (lum.) pb in the dilepton channel. We will also discuss the future prospects of this measurement.

J/psi production is considered a very important probes for studying the properties of quark-gluon plasma (QGP). At the PHENIX experiment at Brookhaven National Laboratory, a large suppression of J/psi production in Au+Au collisions at 200 GeV center of mass energy as compared to the binary collision scaled p+p collisions was observed. The level of suppression is similar to that observed at other energies at CERN's SPS and LHC experiments. This work addresses the PHENIX J/psi measurements at sqrt{s_{NN}}= 39 and 62 GeV Au+Au collisions. These allow for the energy dependent J/psi suppression measurements in order to disentangle the important contributing factors of J/psi production. J/psi results over a wide range of center of mass energies (39-200 GeV) from PHENIX are discussed, in addition to a comprehensive comparison with other experiments.

Thèse de doctorat : Physique Subatomique : Strasbourg 1 : 2006. / Titre provenant de l'écran-titre. Bibliogr. 2 p.

Reproduction de : Thèse doctorat : Physique : Lyon 1 : 2003. Reproduction de : Thèse doctorat : Physique : Stony Brook University (New-York) : 2003. / Thèse soutenue en co-tutelle. Titre provenant de l'écran titre. 292 Réf. bibliogr.

In this dissertation, the phenomenology of electromagnetic radiation and high transverse momentum jets in relativistic heavy ion collisions is investigated. These are two very important probes to study the strongly interacting matter at extreme temperatures and/or densities and to investigate the possibility of a phase transition between hadronic matter and quark-gluon plasma (QGP). First, a new channel of direct photon production from a charge-asymmetric QGP is explored in the effective theory of quantum chromodynamics (QCD) at high temperature. The photon production from this new channel is found to be suppressed compared to QCD annihilation process and Compton scattering at low baryon density, but might assume significance in baryon-rich matter. Second, the radiative jet energy loss in a three-dimensional ideal hydrodynamical medium is studied for Au+Au collisions at Relativistic Heavy Ion Collider (RHIC). A systematic analysis of the nuclear modification factor RAA is presented for pi0 production at high pT in central and non-central collisions, at mid and forward rapidity. Third, jet energy loss by elastic collisions is consistently incorporated in the same formalism and applied to the study of jet quenching at RHIC. It is found that the nuclear modification factor RAA for pi0 in relativistic heavy ion collisions is sensitive to both collisional and radiative energy loss, while the average energy loss is less affected by the inclusion of collisional energy loss. Last, the nuclear suppression of photon-tagged jets at high pT is studied by incorporating not only direct photons, but the additional sources from fragmentation and jet-plasma interaction. We find that these additional sources are very important for a complete study of the correlations between hard photons and hadrons and even dominate in some kinetic regime.

J/psi production is considered a very important probes for studying the properties of quark-gluon plasma (QGP). At the PHENIX experiment at Brookhaven National Laboratory, a large suppression of J/psi production in Au+Au collisions at 200 GeV center of mass energy as compared to the binary collision scaled p+p collisions was observed. The level of suppression is similar to that observed at other energies at CERN's SPS and LHC experiments. This work addresses the PHENIX J/psi measurements at sqrt{s_{NN}}= 39 and 62 GeV Au+Au collisions. These allow for the energy dependent J/psi suppression measurements in order to disentangle the important contributing factors of J/psi production. J/psi results over a wide range of center of mass energies (39-200 GeV) from PHENIX are discussed, in addition to a comprehensive comparison with other experiments.

Nuclei at very high energy, characterized by a saturation scale, can be described by an effective theory of Quantum ChromoDynamics (QCD) called Color Glass Condensates. The earliest phase of the collision of two nuclei is modeled as the collision of two sheets of color glass. The classical field resulting from the collision then decays and equilibrates to a plasma of quarks and gluons. Using a recursive solution of the Yang-Mills equations, we calculate analytic expressions for the gluon field created in ultra-relativistic heavy ion collisions at small times τ. We have worked out explicit solutions for the fields and the energy momentum tensor up to 4^th order in an expansion in τ . We generalize the existing calculations to go beyond the limit of large homogenous nuclei. This allows us to calculate radial and elliptic flow of gluon fields. The resulting transverse and longitudinal structure of the Poynting vector field has a rich phenomenology. Besides the well known radial and elliptic flow in transverse direction, classical quantum chromodynamics predicts a rapidity-odd transverse flow that tilts the fireball for non-central collisions, and it implies a characteristic flow pattern for collisions of non-symmetric systems A + B. The rapidity-odd transverse flow translates into a directed particle flow v_1 which has been observed at RHIC and LHC. The global flow fields in heavy ion collisions could be a powerful check for the validity of classical Yang-Mill dynamics in high energy collisions. We also propose a procedure to calculate the energy momentum tensor of gluon fields on an event-by-event basis. The matching of the initial field energy momentum tensor to viscous hydrodynamic initial conditions is discussed and some preliminary results of a subsequent hydrodynamic evolution are shown. Our results can provide event-by-event initial conditions for hydrodynamic simulations of nuclear collisions that include initial flow and initial shear stress.

The Large Hadron Collider (LHC) at the international particle physics laboratory CERN in Switzerland is currently the most powerful particle accelerator on earth. This thesis presents analyses of proton-proton collisions at the energy √s = 7 TeV, recorded by ATLAS, one of the detectors at the LHC. The goal of the LHC and its detectors is to find new phenomena not described by the Standard Model (SM) of particle physics.The top quark is the heaviest known elementary particle and it is produced in very large numbers at the LHC. Measuring the production cross-section of top pairs (ttbar) is important for many reasons: for validating the strong production mechanism of the SM, for commissioning and calibration of the detector and analysis software and because several scenarios for physics beyond the SM predict changes to the ttbar production cross-section.Five different measurements of the ttbar cross-section will be presented in this thesis. The first three are measurements of the total cross-section, the fourth is a simultaneous measurement of the ttbar, Zττ and WW cross-sections and the fifth is a measurement of the relative differential ttbar cross-section. The most accurate measurement of the total cross-section is 176 pb with a total uncertainty of 9%, and the relative differential cross-section for ttbar-masses above ~ 1 TeV is 0.007 1/TeV with an uncertainty of 43%. Both values agree with the SM predictions.Measurements or searches in particle physics often have to be conducted in the presence of uninteresting background processes.  Reducing and providing estimates of these backgrounds is one of the main analysis tasks. Many backgrounds can be simulated with sufficiently good accuracy. However, the background due to mis-identified leptons cannot be accurately simulated. This thesis presents and evaluates a method for estimating this background from data, and this is then used in the total ttbar cross-section measurements. / LHC (Large Hadron Collider) vid det internationella partikelfysiklaboratoriet CERN i Schweiz är för närvarande världens mest kraftfulla partikleaccelerator. I den här avhandlingen presenteras anlyser av proton-protonkollisioner vid $\sqrt{s}=7\TeV{}$, registrerade med ATLAS som är en av detektorerna vid LHC. Målet med LHC och dess detektorer är att upptäcka nya fenomen som inte kan beskrivas av partikelfysikens nuvarande standardmodell (SM). Toppkvarken är den tyngsta elementarpartikeln man känner till och den produceras i stort antal vid LHC. Att mäta produktionstvärsnittet för par av topkvarkar (\ttbar{}) är viktigt av flera skäl: för att validera den starka produktionsmekanismen i SM, och för att testa och kalibrera detektorn och analysverktygen. Dessutom förutsäger flera teoretiska utvidgningar av SM ändringar av tvärsnittet för \ttbar{}. Fem olika mätningar av tvärsnittet för \ttbar{} presenteras i den här avhandlingen. De första tre är mätningar av det totala tvärsnittet, den fjärde är en simultan mätning av tvärsnitten för produktion av \ttbar{}, $WW$ och \Ztt{} och den femte är en mätning av det \emph{relativa differentiella} tvärsnittet. Det mest noggranna mätningen av det totala tvärsnittet är $176 $ pb med $9 \%$ total osäkerhet, och det relativa differentiella tvärsnittet för \ttbar{}-massor över $\sim 1 \TeV{}$ är $0.007 \TeV^{-1}$ med $43\%$ osäkerhet. Båda mätresultaten stämmer med respektive förutsägelse från standardmodellen. Mätningar eller sökningar inom partikelfysiken måste ofta utföras i närvaro av ointressanta bakgrundsprocesser. Att reducera och skatta dessa bakgrunder är en av huvuduppgifterna inom en dataanalys. Många bakgrunder kan simuleras med tillräcklig precision. Men bakgrunden av felidentifierade leptoner kan däremot inte simuleras tillräckligt noggrant. Den här avhandlingen presenterar och utvärderar en metod för att skatta bakgrunden av felidentifierade leptoner från data, och resultatet används i mätningarna av det totala tvärsnittet för \ttbar{}. / <p>At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 5: Manuscript.</p>

No description available.

The interactions of heavy quarks within the Quark Gluon Plasma (QGP) are interpreted utilizing an elastic, thermodynamic, 2-body T-matrix in order to calculate drag coefficients of heavy-quark systems derived from a Fokker-Planck equation. A spacelike momentum constraint is employed and produces an effective, color dependent potential with the addition of relativistic factors motivated by the appropriate Feynman diagrams. Hard Thermal Loop (HTL) corrections are interpreted in the context of a finite temperature quark-gluon system, allowing a non-perturbative determination of the gluon's contribution to the drag coefficient. An enhancement of the relaxation rate of ~2 is observed at low momenta, leading to an enhancement of the overall relaxation rate of 20%, while the high-p limit approaches a perturbative level. The importance of a nonperturbative treatment of the QGP to reproduce the dynamical drag coefficient is illustrated.