Direct hadron production in ultrarelativistic heavy-ion collisions

Hadrons emitted by the pre-surface layer of a quark-gluon plasma (QGP) before the phase transition into a hadronic gas are considered as possible sources of direct information about QGP. It is shown that if QGP is created in ultrarelativistic heavy-ion collisions, then these hadrons strongly contribute at soft p_t at SpS energy and dominate up to an order of magnitude at LHC energy.     

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.     

Re-hardening of hadron transverse mass spectra in relativistic heavy-ion collisions

We analyze the spectra of pions and protons in heavy-ion collisions at relativistic energies from 2 A GeV to 65+65 A GeV by using a jet-implemented hadron-string cascade model. In this energy region, hadron transverse mass spectra first show softening until SPS energies, and re-hardening may emerge at RHIC energies. Since hadronic matter is expected to show only softening at higher energy densities, this re-hardening of spectra can be interpreted as a good signature of the quark-gluon plasma formation     

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.     

Production of secondaries in high-energy heavy-ion collisions

In the framework of the quark-gluon string model, we calculate the inclusive spectra of secondaries produced in heavy-ion collisions at intermediate (CERN SPS) and at much higher (RHIC) energies. We demonstrate that the mechanism of secondary production changed drastically in the energy interval √s = 20–60 GeV and that it is in agreement with qualitative estimates of Glauber-Gribov theory. The results of numerical calculations at intermediate energies are in reasonable agreement with the data without change of the model parameters. At RHIC energies, numerically large inelastic screening correlations should be accounted for in calculations. The text was submitted by the authors in English.     

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.     

Saturation of transverse energy per charged hadron and freeze-out criteria in heavy-ion collisions

The effective (non-universal) nature of Sivers function reflects the process dependence of the imaginary phase required for T-odd Single Spin asymmetry. The explicit account for the phase allows to relate T-odd and T-even Spin asymmetries. The soft-gluon twist 3 contributions to single-spin asymmetries (SSA) in hard processes may be expressed in the form of effective T-odd Sivers distributions, whose signs and scales are modified by process-dependent colour factors. The Sivers mechanism is applied at large transverse momenta and the emission of balancing gluons provide the colour flow explaining this factor.     

Production mechanism of quark-gluon plasma in heavy-ion collisions

A general scheme is proposed here to describe the production of semihard and soft quarks and gluons that form the bulk of the plasma in ultrarelativistic heavy-ion collisions. We show how to obtain the production rates in the extended phase space, including the color part, as a function of time in a consistent manner and without having to make ad hoc assumptions. All the required features-the back reaction on QCD vacuum, the non-Markovian nature of the production, and the quasi particle nature of the partons-are naturally incorporated. We illustrate the results with a realistic albeit toy model and also show how physically tenable source terms may be obtained.     

Meson condensation and two photon emission in hadron capture by nuclei and heavy-ion collisions

New laboratory probes for the occurence of meson condensation in nuclei are proposed. They involve experiments which detect the simultaneous emission of two photons. Pion condensation stimulates a new mechanism for such emission which depends essentially upon the ratio of the condensate amplitude to the vacuum expectation value of the effective scalar meson field. We exhibit the special features of this mechanism for the photon energy spectrum and angular correlation, and compare with other mechanisms.     

Identified hadron production in √s = 130 GeV Au-Au collisions at relativistic heavy-ion collider

Identified π^±,K ^±, p and -p transverse momentum spectra at mid-rapidity in √^sNN = 130 GeV Au-Au collisions were measured by the PHENIX experiment at RHIC as a function of collision centrality. Average transverse momenta increase with the number of participating nucleonsN _part similarly for all particle species. The multiplicity densities scale faster thanN _part. TheK ^± andp ^±yields per participant increase faster than the π^± yields. We combine the PHENIX neutral and charged pion measurement and find that in central collisions forp _T >-2 GeV/c,-p andp yields are comparable to or even exceed the pion yields.     

Bremsstrahlung in heavy-ion collisions

The bremsstrahlung background emission of γ-rays in equal-mass heavy-ion collisions is calculated. The cross section is found to be about an order of magnitude smaller than preliminary upper limits of ¹²C-¹²C experiments in progress.     

Identification of baryon resonances in central heavy-ion collisions at energies between 1 and 2 AGeV

The mass distributions of baryon resonances populated in near-central collisions of Au on Au and Ni on Ni are deduced by defolding the p_t spectra of charged pions by a method which does not depend on a specific resonance shape. In addition the mass distributions of resonances are obtained from the invariant masses of (p, π^±) pairs. With both methods the deduced mass distributions are shifted by an average value of −60 MeV/c² relative to the mass distribution of the free Δ(1232) resonance, the distributions descent almost exponentially towards mass values of 2000 MeV/c². The observed differences between (p, π⁻) and (p, π⁺) pairs indicate a contribution of isospin I = 1/2 resonances. The attempt to consistently describe the deduced mass distributions and the reconstructed kinetic energy spectra of the resonances leads to new insights about the freeze out conditions, i.e. to rather low temperatures and large expansion velocities. Received: 26 June 1998 / Revised version: 2 September 1998     

Bremsstrahlung in heavy-ion molecular resonances

The bremsstrahlung cross section for the quadrupole photon emission in a heavy-ion collision is formulated on the basis of the coupled-channel method. For the branching ratio of the radiative decay width to the total width for the transition between the structures at 25.6 MeV and 19.3 MeV in the ¹²C + ¹²C collision, we predict (3–7) × 10^?7, which is roughly comparable with existing measurements. The radiation from the mutual 2⁺ excitation channel is found to yield 70% of the total branching ratio.