Scaling of hadron masses and widths in thermal models for ultrarelativistic heavy-ion collisions

By means of a simple rescaling, modifications of hadron masses and widths are incorporated into the thermal analysis of particle ratios in ultrarelativistic heavy-ion collisions. We find that moderate, up to 20%, changes of hadron masses do not spoil the quality of the fits, which remain as good as those obtained without modifications. Larger changes are not likely. The fits with the modified masses yield modified values of the optimal temperature and baryon chemical potential. In particular, with decreasing masses of all hadrons (except for pseudo-Goldstone bosons) the fitted values of the temperature and the baryon chemical potential are lowered, with the change approximately proportional to the scaling of masses. In addition, we find that the broadening of the hadron widths by less than a factor of two practically does not affect the fits.     

Similarity of hadron production inpp and heavy-ion collisions

The characteristic features of pseudorapidity distributions of hadron production are investigated in terms of a geometrical parameterh≡(dn/dη)₀/Φ, Φ being the phase space covered by the η distribution. It is found thath is about the same for the heavy-ion (HI) andpp collisions at the same energy, the same for π andK production and behaves ash～E _cm ^?1/3 . This similarity property yieldsK ^?/π^?=0.041±0.004 andK ⁺/K ^?=4.5±0.6 for HI collisions at 14.6 GeV/A in agreement with BNL experiments.     

Freeze-out conditions in ultrarelativistic heavy-ion collisions

We present recent results on single particle transverse momentum distributions of pions, kaons and protons, measured in CERN Experiment NA44, of 200 AGeV/c S+S and 158 AGeV/c Pb+Pb central collisions. By comparing these data with thermal and transport models, freeze-out parameters like the temperatureT _fo and the chemical potentials (μ _q ,μ _s ) are extracted and discussed.     

Photon production in ultrarelativistic heavy-ion collisions at 200 GeV/u

A calculation of sources of inclusive photons in ultrarelativistic O-W and Pb-Pb collisions at 200 GeV/u is presented. The production of thermal photons in a quark-gluon plasma, a deconfined state of nuclear matter at high energy density, is addressed. In events with a large amount of energy deposited in the target the thermal contribution to the inclusive spectrum is found to be ～5%, quadratically increasing with particle multiplicity, but only weakly dependent on the size of the projectile nucleus. These photons are predominantly produced in a coexistence phase of plasma and hadronic matter. Hadronic bremsstrahlung is investigated including the spatial and temporal coherence due to the large space-time extension of heavy-ion systems. For small photon momenta there is a coherent signal in the fragmentation regions proportional to the squared number of participant protons.     

Scaling of hadron production bypp,p-nucleus and heavy-ion collisions

The rest-frame of secondaries frompp collisions determined by the covariant Boltzmann factor is found to be independent of the massm of the secondary. In this frame, the hadron production behaves like bremsstrahlung:n(m)～W ^* (m)/m ²,W ^* (m) being the available energy. This multiplicity law fitsp+p→m+? at 400 GeV/c of NA 27 Collaboration without free-parameters. Extended top-nucleus and heavy-ion collisions, assuming a mean-free-path of collision between the incident and the target nucleon to be ～2.80 fm and 1.63 fm respectively, it accounts forn(K ^?) and \(n(\bar \Lambda )\) of CERN-SPS experiments and theK ^?/π^? ratio of BNL experiments.     

Universal flow-driven conical emission in ultrarelativistic heavy-ion collisions

The double-peak structure observed in soft-hard hadron correlations is commonly interpreted as a signature for a Mach cone generated by a supersonic jet interacting with the hot and dense medium created in ultrarelativistic heavy-ion collisions. We show that it can also arise due to averaging over many jet events in a transversally expanding background. We find that the jet-induced away-side yield does not depend on the details of the energy-momentum deposition in the plasma, the jet velocity, or the system size. Our claim can be experimentally tested by comparing soft-hard correlations induced by heavy-flavor jets with those generated by light-flavor jets.     

Production of hadron resonances in heavy-ion collisions

The role of microscopic kinetics in the production of short-lived (broad) hadron resonances from subhadronic nuclear matter is considered. Anew approach to calculating the multiplicity of broad meson resonances is proposed. This approach takes explicitly into account the possibility that massive constituent quarks play a decisive role at the last stage of the expansion and cooling of matter produced in the central collisions of relativistic heavy nuclei. The resulting theoretical estimates are comparedwith available experimental data, and some quantitative and qualitative predictions are made.     

Mass dependence of hadron distributions in ultrarelativistic nucleus-nucleus collisions

(Anti-) baryon and kaon distributions in nucleus-nucleus reactions at 200 AGeV are studied in the framework of the transport model RQMD. Production mechanisms for strangeness and baryon pairs are tested by comparing their projectile and target mass dependence to available experimental data. RQMD contains two collective production processes, fusion of overlapping strings into highly charged chromoelectric ropes and hadronic rescattering. It turns out that both rope formation and hadronic rescattering are of importance for creation-and annihilation-of strangeness and antibaryons.     

Study of ultrarelativistic heavy-ion collisions in the PHENIX experiment

The latest physics results obtained in the PHENIX experiment by studying dAu, CuCu, and AuAu collisions at various interaction energies are reviewed. Attention is given primarily to the energy loss of hard-scattered partons and to the production of soft and high-transverse-momentum direct photons.     

Convective stability of hot matter in ultrarelativistic heavy-ion collisions

The convective stability of strongly interacting matter undergoing hydrodynamic flow in ultrarelativistic heavy-ion collisions is studied in both the quark-gluon plasma and hadron gas phases. We find that this stability depends on the form of the initial conditions assumed for the hydrodynamic flow. In the case of initial conditions corresponding to partial transparency the flow of the quark-gluon plasma is stable whereas the flow of the hadron gas is convectively unstable. The timescale for hydrodynamic oscillations around the (stable or unstable) equilibrium state is found to be larger than the expected lifetime of the system, suggesting that the flow in either case is close to neutral convective equilibrium.     

Significance of the fragmentation region in ultrarelativistic heavy-ion collisions

We present measurements of the pseudorapidity distribution of primary charged particles produced in Au+Au collisions at three energies, sqrt[s(NN)]=19.6, 130, and 200 GeV, for a range of collision centrali-ties. The distribution narrows for more central collisions and excess particles are produced at high pseudorapidity in peripheral collisions. For a given centrality, however, the distributions are found to scale with energy according to the "limiting fragmentation" hypothesis. The universal fragmentation region described by this scaling grows in pseudorapidity with increasing collision energy, extending well away from the beam rapidity and covering more than half of the pseudorapidity range over which particles are produced. This approach to a universal limiting curve appears to be a dominant feature of the pseudorapidity distribution and therefore of the total particle production in these collisions.