ATLAS Results on Pb+Pb Collisions

A wide research program provided by heavy ion collisions is ongoing at the Large Hadron Col-lider with the aim of studying the properties of QCD matter at extreme temperatures and densities. The large acceptance and high granularity of the ATLAS Detector is well suited to perform detailed analyses on bulk phenomena, jets and leptonic probes. Measurements of these observables provided by the 9 µb −1 of Pb+Pb collision data collected during the Fall 2010 Pb+Pb run at the nucleon-nucleon center of mass energy of 2.76 TeV are presented. Evidence from SPS/CERN and RHIC/BNL data suggest that in extreme conditions of temperature and density, as those provided by ultra-relativistic Pb+Pb collisions, matter undergoes a phase transition from ordinary hadronic matter to a plasma of quarks and gluons, the QGP. Later, under its own pressure, the QGP expands and cools recombining the quarks and gluons into hadrons that carry information about the deconfined phase. Pb+Pb collisions at the LHC are expected to produce a QGP with energy densities at least 3 times larger than at RHIC, with higher initial temperatures and longer lifetimes. After one year of smooth running with p+p collisions, the LHC switched to lead beams in order to carry out the Heavy Ion Program. The first Pb+Pb collisions, at the center of mass energy of 2.76 TeV, took place in November 2010. ATLAS [1] has shown an excellent performance. The full detector was operational and the fraction of data passing quality criteria in each sub-detector was almost 100%. A total integrated luminosity of 9 µb −1 , which corresponds to more than 95% of the luminosity provided by the LHC, has been recorded. The collision centrality, characterized by the impact parameter of two colliding nuclei, has been estimated using the total transverse energy deposited in the forward calorimeters (FCal, 3.2 < |η| < 4.9) and compared to a Glauber Monte Carlo model [2], convoluted with p+p data taken at the same beam energy. The fraction of the total inelastic cross section measured after all trigger and selection cuts, is f = 98 ± 2%. Using this fraction events are categorized into centrality classes and the numbers of binary collisions and nucleon participants N part are calculated for these classes using the same Glauber MC model. The charged particle multiplicity density is one of the most basic characteristics of the HI collisions. The top plot of Figure 1 shows the …