Abstract
The ALICE detector at the LHC is used to study the properties of the Quark-Gluon Plasma produced in heavy-ion collisions. As a reference measurement, also the analysis of proton-proton (pp) collisions is very important. In the study presented here, event-by-event fluctuations of the mean transverse momentum are analysed in pp collisions at sqrt(s) = 0.9, 2.76 and 7 TeV, and Pb-Pb collisions at sqrt(s_NN) = 2.76 TeV as a function of the charged-particle multiplicity. In both systems, dynamical fluctuations beyond the statistical expectation are observed. In pp collisions, no significant dependence on collision energy is found, even in comparison to inclusive results at much lower collision energies. Likewise, central A-A collisions show only little dependence on collision energy. The multiplicity dependence observed in peripheral Pb-Pb data is in agreement with that in pp collisions. Going to more central Pb-Pb collisions, a clear deviation from this trend is found, reaching a significant reduction of the fluctuations in most central collisions. Comparisons to Monte Carlo event generators show good agreement in pp, but rather large differences in Pb-Pb collisions.
Highlights
At Large Hadron Collider (LHC) energies, heavy-ion (A–A) collisions, and pp collisions produce a large number of particles in the final state
The ALICE detector [1] is well suited for the investigation of both systems and can cope in particular with the very large number of particles produced in central Pb–Pb collisions
The results presented are based on data measured at the CERN Large Hadron Collider (LHC) [7] with the ALICE
Summary
At LHC energies, heavy-ion (A–A) collisions, and pp collisions produce a large number of particles in the final state. This phase transition may go along with critical fluctuations of thermodynamic quantities which could be observed via event-by-event fluctuation measurements These analyses may reveal collective effects like the onset of thermalization of a system and are proposed for the investigation of the hot and dense matter generated in heavy-ion collisions [2,3,4,5]. Many kinds of correlations among the transverse momenta of the final-state particles may lead to such fluctuations, e.g. jets, resonance decays and quantum correlations. These are already present in pp collisions, which can serve as a model-independent baseline of the investigation of additional effects√in A–A collisions.
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