George S. F. Stephans

arXiv2014

Energy and centrality dependence of particle multiplicity in heavy ion collisions from $\sqrt{s_{_{NN}}}$ = 20 to 2760 GeV

The centrality dependence of midrapidity charged-particle multiplicities at a nucleon-nucleon center-of-mass energy of 2.76 TeV from CMS are compared to PHOBOS data at 200 and 19.6 GeV. The results are first fitted with a two-component model which parameterizes the separate contributions of nucleon participants and nucleon-nucleon collisions. A more direct comparison involves ratios of multiplicity densities per participant pair between the different collision energies. The results support and extend earlier indications that the influences of centrality and collision energy on midrapidity charged-particle multiplicities are to a large degree independent.

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arXiv2010

Centrality Dependence of Two-Particle Correlations in Heavy Ion Collisions

Data from the PHOBOS detector have been used to study two-particle correlations over a broad range of pseudorapidity. A simple cluster model parameterization has been applied to inclusive two-particle correlations over a range of centrality for both Cu+Cu and Au+Au collisions at sqrt(s_NN)=200GeV. Analysis of the data for Au+Au has recently been extended to more peripheral collisions showing that the previously-observed rise in cluster size with decreasing system size eventually reaches a maximum value. Model studies have been used to quantify the significant effect of limited detector acceptance on the extracted cluster parameters. In the case of Au+Au, correlations between a trigger particle with pT>2.5GeV and inclusive associated particles have also been studied. These reveal the presence of a `ridge' at small relative azimuthal angle which extends with roughly constant amplitude out to the largest relative pseudorapidity studied. The large phase-space coverage of the PHOBOS detector has enabled a quantitative understanding of the so-called `ZYAM' parameter used in the subtraction of the contribution of elliptic flow to these triggered correlations.

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