Hadron production in heavy relativistic systems

We investigate particle production in heavy-ion collisions at RHIC energies as function of incident energy, and centrality in a three-sources Relativistic Diffusion Model. Pseudorapidity distributions of produced charged hadrons in Au + Au and Cu + Cu collisions at √ sNN = 19.6 GeV, 62.4 GeV, 130 GeV and 200 GeV show an almost equilibrated midrapidity source that tends to increase in size towards higher incident energy, and more central collisions. It may indicate quark-gluon plasma formation prior to hadronization. Introduction. – The precise calculation and prediction of transverse momentum and rapidity distributions of produced particles is of basic importance in relativistic heavy-ion physics. In this Letter we propose nonequilibrium-statistical methods [1] to investigate analytically the gradual thermalization in rapidity space occuring in the course of particle production at the highest available energies. The approach is tailored to identify the fraction of produced particles in local thermal equilibrium from their pseudorapidity distribution functions in heavy systems, with a focus on Cu + Cu and Au + Au. It may yield indirect evidence for the extent and energy dependence of a locally equilibrated parton plasma. There exist other theoretical approaches that allow to compute rapidity distribution functions for produced particles, albeit with less precision. Some of them are based on QCD, such as calculations within the framework of the Parton Saturation Model [2]. Ideal hydrodynamics is well developed in its applications to relativistic collisions, but more realistic dissipative hydrodynamic approaches are still in the early stage of theoretical development [3]. Thermal models are outstanding in their ability to correctly predict particle abundance ratios at midrapidity, or momentum integrated [4, 5]. But since these approaches do not deal with nonequilibrium-statistical effects, one can not expect precise results for distribution functions whenever hadronic or partonic thermalization processes through multiple collisions on an event-by-event basis are important [5]. Within a thermal model, such effects could be simulated to some extent by different values of local temperature and chemical potential when investigating particle production at different rapidities. Hence, nonequilibrium statistics is the natural choice for a detailed description of the gradual approach to statistical equilibrium in relativistic collisions of heavy systems. Our Relativistic Diffusion Model (RDM) underlines the nonequilibrium-statistical features of high-energy heavy-ion collisions, but it also encompasses kinetic (thermal) equilibrium of the system for times that are sufficiently larger than the relaxation times of the relevant variables.