Charged particle multiplicity fluctuations and correlations in heavy-ion collisions in the PHOBOS experiment at RHIC

Abstract

The Relativistic Heavy Ion Collider is the first accelerator in which beams of heavy ions collide at an unprecedented energy of p sNN = 200 GeV. Collisions of Au nuclei result in creation of a system with extremely high energy density in which a new phase of matter, the strongly interacting Quark-Gluon Plasma, is formed. It manifests itself in suppression of partons with very high transverse momenta and large elliptic flow. A general overview of experimental signals of QGP creation is presented with an emphasis on results from fluctuation and correlation studies. In a system crossing a phase transition or passing near a critical point enhanced fluctuations that can be visible also in various correlation studies may appear. This work describes in detail the analyzes of multiplicity fluctuations and correlations performed by the PHOBOS Collaboration. They include a search for events with unusually high multiplicities or very large fluctuations of the angular particle density distribution. The results of correlation studies can be explained by production of particles in clusters, which are large and very wide in pseudorapidity. In the analysis of reconstruction of cluster parameters and in calculations of acceptance effects Simple Cluster Model is applied. The centrality dependence of correlations suggests that they can not be described as decays of heavy resonances only, thus theWounded Nucleon Model is used to explain these correlations. It is shown that in this model the correlations are partly due to fluctuations of the number of wounded nucleons and that the Wounded Nucleon Model predicts them better than other models. The more technical aspects of the heavy-ion collisions studies are also described. There is some information on the RHIC accelerator complex and the experiments measuring Au + Au collisions, with a detailed discussion of the PHOBOS detector. In addition to results on multiplicity and charged particle pseudorapidity distributions, the multiplicity reconstruction methods are presented. Several methods of the vertex reconstruction applied in the PHOBOS experiment are discussed, including a novel algorithm based on hits in a single layer of silicon sensors. The system of PHOBOS Monte Carlo simulations and a report on its outcome are presented.