Energy and centrality dependence of mid-rapidity charged particle multiplicity in relativistic heavy-ion collisions

Abstract

The properties of quantum chromodynamics (QCD), the modern theory of the strong interaction, can be investigated through the study of relativistic nucleus-nucleus collisions. Recently, the Relativistic Heavy-Ion Collider (RHIC) was completed and started taking data at ten times higher center-of-mass energies than the previous most energetic heavy-ion collisions. This thesis presents some of the first measurements at RHIC from any experiment. The PHOBOS detector is used to measure the charged particle pseudo-rapidity density at mid-rapidity (i.e., in [eta] < 1) as a function of collision energy and centrality. The multiplicity is measured by counting short tracks in the silicon spectrometer; the centrality measurement uses two scintillator detectors covering 3 < [eta] < 4.5. The charged particle multiplicity at mid-rapidity for the 6% most central collisions is 379 +/- 9(stat.) +/- 42(syst.), 555 +/- 3(stat.) +/- 35(syst.) and 661 +/- 4(stat.) +/- 40(syst.) for [the square root of] SNN=56 GeV, 130 GeV and 200 GeV respectively. The charged particle multiplicity is compared to results obtained in heavy-ion experiments at lower energies and to results obtained in pp(p̄) collisions. When normalizing the multiplicity to the number of participant pairs, the charged particle production is found to be up to 55% larger than in pp(p̄) collisions at the same collision energy, precluding any simple nucleon-nucleon superposition model. A perturbative QCD model (HIJING) and two parton saturation type models are presented and compared to the data.

(cont.) Multiplicities obtained for these three models are found to broadly agree with the measurements. The shape of the evolution of the normalized charged particle density with centrality is very similar for the [square root of] SNN=130 GeV and 200 GeV data, with 14% more charged particles produced at the higher energy at all centralities. The theoretical uncertainties associated with converting an interval in total cross section to the number of participating nucleons are investigated in the Glauber framework. The popular optical limit approximation is found to underestimate the number of participants by up to 15% in peripheral collisions.