GLOBALHIATLAS · Measurement of the global characteristics of Relativistic Heavy Ion Collisions with the ATLAS detector at LHC

Measurement of the global characteristics of relativistic heavy ion collisions with the ATLAS detector at LHC

Relativistic heavy ion collisions (RHI) provide a direct way to test quantum chromodynamics under extreme conditions in the laboratory environment. The worldwide effort to study the interactions of RHI led to the discovery of the strongly coupled quark-gluon plasma at Super Proton Synchrotron (SPS) and Relativistic HIC (RHIC), which was reconfirmed at the Large Hadron Collider (LHC). Study of the global observables in RHI collisions covers a variety of measurements, which play central role in understanding the observed phenomena. An advent of the LHC RHI programme, which was delayed by one year, nevertheless proven itself to be a great success and led to new discoveries, such as observation of the centrality dependent dis-balance in the di-jet energies, long-pseudorapidy-range particle correlations in small systems, and suppression of the excited Upsilon states. A deeper understanding of many previously known phenomena is emerging form the data analysed by the LHC experiment. This grant directly contributed to some of new discoveries and allowed to significantly deepen the understanding of the RHI. Working on the project supported by this grant the following objectives outlined in the original proposal were achieved. Determination of the event centrality in lead (Pb)+Pb collisions using the sum of transverse energy measured by the ATLAS forward calorimeter. Pb-Pb collisions measured in interactions at LHC are classified according to the number of nucleons, which actually took part in teach single interaction. Using the response of the ATLAS forward calorimeter it is possible to provide an estimate of the number of participating nucleons, number of collisions between them, and some other parameters, such as the trigger efficiency. Centrality parameters are used in almost all heavy ion publications. ATLAS measured the centrality dependence of the charged particle multiplicity in Pb-Pb collisions. The total number of charged particles produced at mid-rapidity is approximately 2.15 times more than at RHIC, while many theoretical models, based on RHIC data predicted significantly lesser increase. At the same time the shape of centrality dependence is very similar to that measured at RHIC. Techniques worked out to measure charged particle multiplicity in Pb-Pb interactions are now used to measure the proton-Pb data using the data obtained by ATLAS during short run in September 2013. The study of inclusive charged hadron production at high transverse momentum is a way to understand the properties of the hot dense matter created in RHI collisions at high energy. The ATLAS experiment at the LHC has measured the production of charged hadrons in Pb+Pb collisions as a function of transverse momentum in the range 0.5 - 150 GeV and in the pseudorapidity range up to 2.5. The charged particle spectra measured at different pseudorapidities are consistent with each other within the statistical and systematic uncertainties. The ratio of the central to peripheral events measured in the 0 - 5 % and 60 - 80 % centrality classes and scaled by the number of binary nucleon-nucleon collisions shows a suppression of charged hadrons by a factor of 5 at transverse momentum of 7 GeV. Above that the ratio increases showing suppression, consistent to that measured with inclusive jets, arguing that the jets fragmentation function is not strongly modified at very high transverse momentum. Understanding the performance of the ATLAS detector tracking system in the dense environment of the Pb-Pb collisions, which was performed as part of the spectra measurements played important role in several analyses published by the ATLAS experiment. ATLAS currently uses technical approaches used in the analyses of the Pb-Pb data for the analysis of the first proton-Pb data obtained in the fall 2012.

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