Abstract
To assess the properties of the quark–gluon plasma formed in ultrarelativistic ion collisions, the ATLAS experiment at the LHC measures a correlation between the mean transverse momentum and the flow harmonics. The analysis uses data samples of lead–lead and proton–lead collisions obtained at the centre-of-mass energy per nucleon pair of 5.02 TeV, corresponding to total integrated luminosities of 22~upmu text {b}^{-1} and 28~text {nb}^{-1}, respectively. The measurement is performed using a modified Pearson correlation coefficient with the charged-particle tracks on an event-by-event basis. The modified Pearson correlation coefficients for the 2nd-, 3rd-, and 4th-order flow harmonics are measured in the lead–lead collisions as a function of event centrality quantified as the number of charged particles or the number of nucleons participating in the collision. The measurements are performed for several intervals of the charged-particle transverse momentum. The correlation coefficients for all studied harmonics exhibit a strong centrality evolution, which only weakly depends on the charged-particle momentum range. In the proton–lead collisions, the modified Pearson correlation coefficient measured for the 2nd-order flow harmonics shows only weak centrality dependence. The lead-lead data is qualitatively described by the predictions based on the hydrodynamical model.
Highlights
The large azimuthal anisotropy observed for particles produced in heavy-ion collisions at RHIC [1,2,3,4] and the LHC [5,6,7,8] is one of the main signatures of the formation of strongly interacting matter called quark–gluon plasma (QGP)
This paper reports on the first measurement of the ρ coefficient with the ATLAS detector in Pb+Pb and p+Pb collisions at a centre-of-mass energy per nucleon pair of 5.02 TeV
The Pb+Pb results are presented as a function of collision centrality expressed by the average number of nucleons participating in the collision, Npart, to allow comparison with theoretical predictions [37]
Summary
The large azimuthal anisotropy observed for particles produced in heavy-ion collisions at RHIC [1,2,3,4] and the LHC [5,6,7,8] is one of the main signatures of the formation of strongly interacting matter called quark–gluon plasma (QGP). The values of the respective variances are increased, especially for [ pT] The magnitude of this broadening depends on the choice of kinematic region and on detector performance, making direct comparisons between experimental results and with theoretical calculations difficult. To overcome this problem, a modified correlation coefficient ρ, less sensitive to the charged-particle multiplicity than R, was suggested in. To reduce the auto-correlation effects and those due to the finite charged-particle multiplicity in an event, the variances of the vn{2}2 and [ pT] distributions are replaced by corresponding dynamical variables, which are more sensitive to intrinsic initial-state fluctuations.
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