Charged-particle Multiplicity Density Research Articles

K/πFluctuations at Relativistic Energies

We report K/pi fluctuations from Au + Au collisions at sqrt[s(NN)]= 19.6, 62.4, 130, and 200 GeV using the STAR detector at the Relativistic Heavy Ion Collider. K/pi fluctuations in central collisions show little dependence on incident energy and are on the same order as those from NA49 at the Super Proton Synchrotron in central Pb + Pb collisions at sqrt[s(NN)]=12.3 and 17.3 GeV. We report results for the collision centrality dependence of K/pi fluctuations and results for charge-separated fluctuations. We observe that the K/pi fluctuations scale with the charged particle multiplicity density.

Scaling properties in bulk andpT-dependent particle production near midrapidity in relativistic heavy ion collisions

The centrality dependence of the midrapidity charged-particle multiplicity density (|η| < 1) is presented for Au + Au and Cu + Cu collisions at RHIC over a broad range of collision energies. The multiplicity measured in the Cu + Cu system is found to be similar to that measured in the Au + Au system, for an equivalent Npart, with the observed factorization in energy and centrality still persistent in the smaller Cu + Cu system. The extent of the similarities observed for bulk particle production is tested by a comparative analysis of the inclusive transverse momentum distributions for Au + Au and Cu + Cu collisions near midrapidity. It is found that, within the uncertainties of the data, the ratio of yields between the various energies for both Au + Au and Cu + Cu systems are similar and constant with centrality, both in the bulk yields and as a function of pT ,u p to at least 4 GeV/c. The effects of multiple nucleon collisions that strongly increase with centrality and energy appear to only play a minor role in bulk and intermediate transverse momentum particle production.

Systematic measurements of identified particle spectra inpp,d+Au, andAu+Aucollisions at the STAR detector

Identified charged-particle spectra of pi(+/-), K(+/-), p, and (p) over bar at midrapidity (vertical bar y vertical bar < 0.1) measured by the dE/dx method in the STAR (solenoidal tracker at the BNL Relativistic Heavy Ion Collider) time projection chamber are reported for pp and d + Au collisions at root s(NN) = 200 GeV and for Au + Au collisions at 62.4, 130, and 200 GeV. Average transverse momenta, total particle production, particle yield ratios, strangeness, and baryon production rates are investigated as a function of the collision system and centrality. The transverse momentum spectra are found to be flatter for heavy particles than for light particles in all collision systems; the effect is more prominent for more central collisions. The extracted average transverse momentum of each particle species follows a trend determined by the total charged-particle multiplicity density. The Bjorken energy density estimate is at least several GeV/fm(3) for a formation time less than 1 fm/c. A significantly larger net-baryon density and a stronger increase of the net-baryon density with centrality are found in Au + Au collisions at 62.4 GeV than at the two higher energies. Antibaryon production relative to total particle multiplicity is found to be constant over centrality, but increases with the collision energy. Strangeness production relative to total particle multiplicity is similar at the three measured RHIC energies. Relative strangeness production increases quickly with centrality in peripheral Au + Au collisions, to a value about 50% above the pp value, and remains rather constant in more central collisions. Bulk freeze-out properties are extracted from thermal equilibrium model and hydrodynamics-motivated blast-wave model fits to the data. Resonance decays are found to have little effect on the extracted kinetic freeze-out parameters because of the transverse momentum range of our measurements. The extracted chemical freeze-out temperature is constant, independent of collision system or centrality; its value is close to the predicted phase-transition temperature, suggesting that chemical freeze-out happens in the vicinity of hadronization and the chemical freeze-out temperature is universal despite the vastly different initial conditions in the collision systems. The extracted kinetic freeze-out temperature, while similar to the chemical freeze-out temperature in pp, d + Au, and peripheral Au + Au collisions, drops significantly with centrality in Au + Au collisions, whereas the extracted transverse radial flow velocity increases rapidly with centrality. There appears to be a prolonged period of particle elastic scatterings from chemical to kinetic freeze-out in central Au + Au collisions. The bulk properties extracted at chemical and kinetic freeze-out are observed to evolve smoothly over the measured energy range, collision systems, and collision centralities.

Results from the First Physics Run of PHENIX at RHIC

First results from measurements of Au−Au collisions at \(\sqrt {s_{NN} } = 130\) GeV, performed with the PHENIX experiment during the initial physics run of Brookhaven National Laboratory’s Relativistic Heavy Ion Collider in summer 2000, are summarized. Global characterization of the collisions was obtained from charged-particle multiplicity and transverse energy densities. At mid-rapidity, dNch/dν and dET/dν rise steadily with the event centrality, while the ratio / remains constant. Transverse momentum spectra of charged particles up to 5 GeV/c yield a suppression of high-momentum particles in the most central events when compared to a simple scaling of binary nucleon-nucleon collisions. The scaling matches with peripheral collisions. Transverse momentum spectra of identified neutral pions from 1 to 4 GeV/c in peripheral collisions show consistency with point-like scaling with the average number of binary collisions. Yields in central collisions are significantly suppressed. Transverse momentum spectra of identified charged hadrons up to 3.5 GeV/c were measured in minimum bias events. The mass and centrality dependencies of inverse slope parameters and mean transverse momenta were determined. Inclusive spectra of electrons were measured for momenta from a few hundred MeV/c to 4 GeV/c. Inclusive photon spectra from a few hundred MeV/c to 3 GeV/c were obtained from conversion pair measurements.

Study of new scaling of direct photon production in pp collisions at high energies using MC simulation

The new scaling, z-scailing, of prompt proton production in pp collisions at high energies is studied: The scaling function H( z) is expressed via the inclusive cross section of photon production Ed 3 σ/ dq 3 and the multiplicity density of charged particles ρ( s) at pseudorapidity η = 0. Monte Carlo (MC) simulation based on the PYTHIA code is used to calculate the cross section and to study the scaling. The MC technique used to construct the scaling function is described. The H( z) dependence on the scaling variable z, the center-of-mass energy √ s at a produced angle of θ = 90 is investigated. The predictions of the Ed 3 σ/ dq 3 dependence on transverse momentum q⊥ at a colliding energy of √ s = 0.5, 5.0 and 14.0 TeV are made. The obtained results are compared with the experimental data and can be of interest for future experiments at RHIC (BNL), LHC (CERN), HERA (DESY) and Tevatron (Batavia).