Energy dependence of transverse momentum correlations and an exploration of the color string percolation model in AuAu collisions

David M Garand, Purdue University


The Relativisic Heavy Ion Collider (RHIC) conducted a two year beam energy scan (BES I) to provide data for AuAu collisions at 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV. The motivation for this scan was to search for the phase transition critical point between normal matter and the quark gluon plasma (QGP). This thesis reports on two analyses that attempt to directly search for such a critical point. Data from the Solenoidal Tracker at RHIC (STAR) was analyzed for this study, and included pp collisions at 62 and 200 GeV, and AuAu collisions at 7.7, 11.5, 19.6, 27, 39, 62.4, and 200 GeV. ^ The Color String Percolation Model (CSPM) provides a method for extracting an initial temperature for collisions by ?tting the transverse momentum (pT) spec trum created from a sample of events of a given energy and centrality. In order to search for fluctuations in temperature, this thesis explores applying the CSPM to single events in order to create a distribution of temperatures for collisions of a given energy. Applying this method to the BES I data allows a search for an increase in temperature fluctuations indicative of the critical point. To gauge the accuracy of these measurements, the first systematic study of applying the CSPM to data is reported. ^ The two particle transverse momentum correlator has previously been used to search for fluctuations in pT that could be indicative of the critical phase transition point. The BES I energies were speci?cally studied in reference [1]. An interesting behavior of the pT spectrum was noted at lower energies in that work, and in this thesis. It was proposed that this could be the effect of the increase in the ratio of protons to pions at lower energies. In order to gauge if this a?ects the pT correlations reported at these lower energies, the analysis was attempted with the inclusion of particle identi?cation in order to remove protons from the analysis. This thesis applies the correlation function to AuAu collisions at 7.7, 11.5, and 19.6 GeV, and adds an additional cut on the protons in order to see if any signal may have been masked. ^ The CSPM method was found to represent the [special characters omitted] of the data, and the extracted temperature was found to share a behavior similar to that observable. Self consistency was not found when applied to a Monte Carlo simulation, and systematic analyses of the method led to larger uncertainties than one would expect from the traditional variation of cuts. Results were found to be consistent with prior publications except for the reported uncertainty, which was smaller than this systematic study observes. Results also demonstrate that the CSPM is not able to produce a temperature that is sensitive enough to make a clear statement on how the temperature will vary with decreasing energy, nor how the fluctuations in temperature will behave. The two particle transverse momentum correlation study was found to be consistent with earlier works. The removal of protons from the data led to a di?erent magnitude of correlation, but the behavior in general was found to remain the same. There was no non-monotonic behavior with changing center of mass energy observed, which would have indicated the phase transition critical point.^




Andrew Hirsch, Purdue University.

Subject Area

Quantum physics|Condensed matter physics

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