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star focus: Collision Energy Dependence of pt Correlations in Au+Au Collisions at RHIC

The STAR Collaboration recently has had the paper “Collision-energy dependence of pt correlations in Au+Au collisions at energies available at the BNL Relativistic Heavy Ion Collider” accepted for publication in Physical Review C.

The study of event-by-event correlations and fluctuations in global quantities can provide insight into the properties of the hot and dense matter created in Au+Au collisions at RHIC. Correlations of transverse momentum, pt, have been proposed as a measure of thermalization and as a probe for the critical point of QCD. A detailed study of the of dependence of two-particle pt correlations on collision energy and centrality may demonstrate the effects of thermalization. If the matter produced in collisions at RHIC passes through the QMD critical point, the fluctuations are predicted to increase with respect to a baseline of uncorrelated emission. A possible signature of the critical point could be non-monotonic behavior of the two-particle correlations as a function of the collision energy in central collisions.

This paper reports two-particle transverse-momentum correlations from Au+Au collisions taken during the RHIC Beam Energy Scan at center of mass energies ranging from 7.7 GeV to 200 GeV. These measurements are compared to previous measurements from the CERES Collaboration at the Super Proton Synchrotron and from ALICE at the Large Hadron Collider. The data are compared with UrQMD model calculations and with a model based on a Boltzmann-Langevin approach incorporating effects from thermalization.


Left: The relative dynamical correlation for 7.7 GeV and 200 GeV Au+Au collisions compare with similar results fron 2.76 TeV Pb+Pb collision. The dashed line represents a fit to the data at 200 GeV given by 22.3%/(Npart)1/2. Statistical and systematic errors are shown. Right: The relative dynamical correlation for Au+Au collisions as a function of collision energy for the 0-5% centrality bin along with results for Pb+Pb collisions from ALICE along with UrQMD calculations and results from Boltzmann-Langevin model calculations. The solid line is drawn to guide the eye. Statistical and systematic errors are shown.

The covariance, <Δpt,i Δpt,j>, is calculated as a function of centrality at each of the eight collision energies along with the event-averaged pt, <<pt>>. The two-particle pt correlations are then characterized by the relative dynamical correlation, (<Δpt,i Δpt,j>)1/2/<<pt>>, which represents the magnitude of the dynamic fluctuations of the average pt.

The left panel shows the relative dynamical correlation for Au+Au collisions at 7.7 and 200 GeV compared with similar results from Pb+Pb collisions at 2.76 TeV. The results for Au+Au collisions at 200 GeV agree well with the results for Pb+Pb collisions at 2.76 TeV. The dashed line represents a power law fit to the STAR data at 200 GeV of the form 22.3%/(Npart)1/2. This fit also reproduces the ALICE data for Pb+Pb collisions at 2.76 TeV. The fact that the relative dynamical correlation scales as 1/(Npart)1/2 lends credence to the idea that the observed particle production comes from uncorrelated sources. As the collision energy is lowered to 7.7 GeV, the power-law scaling of the relative dynamical correlation breaks down.

The right panel show the relative dynamical correlation as a function collision energy for the most central bin. Also shown are similar results from CERES and ALICE. Also shown are UrQMD calculations. The data from CERES are for Pb+Pb collisions at 8.7, 12.3, and 17.3 GeV. The STAR data show that the relative dynamical correlation decreases at lower collision energies although the STAR data are in reasonable agreement with the CERES data. This figure also shows the relative dynamical correlation for the 5% most central collisions from Pb+Pb collisions at 2.76 TeV from ALICE. This result seems to show that the relative dynamical correlation plateaus above 200 GeV. The relative dynamical correlation at 2.76 TeV is somewhat lower than the value at 200 GeV. This difference could be partially due to the fact that the 0-5% centrality bin for Pb+Pb collisions at 2.76 TeV is associated with a somewhat larger value of Npart than the value for 200 GeV Au+Au collisions, leading to a lower value of the relative dynamical correlation assuming a 1/(Npart)1/2 scaling.

In conclusion, we observe a power law scaling of the form 1/(Npart)1/2 for the relative dynamical correlation in Au+Au collisions at 200 GeV. A similar power law scaling had been previously observed in Pb+Pb collisions at 2.76 TeV except in the most central collisions. As the collision energy for Au+Au collisions is decreased to 7.7 GeV, the power law scaling observed at 200 GeV breaks down. For the most central Au+Au collisions, the relative dynamical correlations increase with collision energy up to 200 GeV showing no evidence of non-monotonic behavior in this range of Au+Au collision energies.

Posted April 29, 2018

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