arXiv subjects
Publications and source records attributed to J. Cleymans.
Ratios of integrated particle yields provide the best method for determining the temperature and the chemical potential. The chemical freeze-out parameters obtained at CERN/SPS, BNL/AGS and GSI/SIS energies all correspond to a unique value of 1 GeV per hadron in the local rest frame of the system, independent of the beam energy and of the target and beam particles.
Attention is drawn to the role played by the size of the system in the thermodynamic analysis of particle yields in relativistic heavy ion collisions at SIS energies. This manifests itself in the non-linear dependence of K+ and K- yields in $AA$ collisions at 1 -- 2 A.GeV on the number of participants. It is shown that this dependence can be quantitatively well described in terms of a thermal model with a canonical strangeness conservation. The measured particle multiplicity ratios (pi+/p, pi-/pi+, d/p, K+/pi+ and K+/K- but not eta/pi0) in central Au-Au and Ni-Ni collisions at 0.8 -- 2.0 A.GeV are also explained in the context of a thermal model with a common freeze-out temperature and chemical potential. Including the concept of collective flow a consistent picture of particle energy distributions is derived with the flow velocity being strongly impact-parameter dependent.
The production of hadrons in relativistic heavy ion collisions is studied using a statistical ensemble with thermal and chemical equilibrium. Special attention is given to exact conservation laws, i.e. certain charges are treated canonically instead of using the usual grand canonical approach. For small systems, the exact conservation of baryon number, strangeness and electric charge is to be taken into account. We have derived compact, analytical expressions for particle abundances in such ensemble. As an application, the change in $K/π$ ratios in AGS experiments with different interaction system sizes is well reproduced. The canonical treatment of three charges becomes impractical very quickly with increasing system size. Thus, we draw our attention to exact conservation of strangeness, and treat baryon number and electric charge grand canonically. We present expressions for particle abundances in such ensemble as well, and apply them to reproduce the large variety of particle ratios in GSI SIS 2 A GeV Ni-Ni experiments. At the energies considered here, the exact strangeness conservation fully accounts for strange particle suppression, and no extra chemical factor is needed.
It is shown that the chemical freeze-out parameters obtained at CERN/SPS, BNL/AGS and GSI/SIS energies all correspond to a unique value of 1 GeV per hadron in the local rest frame of the system, independent of the beam energy and of the target and beam particles.
The production of hadrons in Ni-Ni at the GSI is considered in a hadronic gas model with chemical equilibrium. Special attention is given to the abundance of strange particles which are treated using the exact conservation of strangeness. It is found that all the data can be described using a temperature T = 70 pm 10 MeV and a baryon chemical potential mu_B = 720 pm 20 MeV.
We compare the predictions of different models for transverse momentum spectra in relativistic heavy ion collisions. Particular emphasis is given to hydrodynamic flow with different assumptions for the radial expansion, and to models based on a superposition of fireballs along the transverse direction (random walk). It is shown that present experimental data cannot distinguish between the models under consideration.
Relativistic heavy ion collisions are studied assuming that particles can be described by a hadron gas in thermal and chemical equilibrium. The exact conservation of baryon number, strangeness and charge are explicitly taken into account. For heavy ions the effect arising from the neutron surplus becomes important and leads to a substantial increase in e.g. the $π^-/π^+$ ratio. A method is developed which is very well suited for the study of small systems.
Strangeness plays an important role in the study of quark matter since it indicates the rate at which new particles are being produced and therefore provides information about the degree of chemical equilibration reached in heavy ion collisions. It is shown that ratios provide an excellent method for determining the temperature and the chemical potential since many theoretical uncertainties cancel out. All indications are that particle abundances are very close to chemical equilibrium. The amount of strangeness present in $S-S$ collisions is very close to the one corresponding to chemical equilibrium. Deviations are at the 30% level.
Particle and photon production is analyzed in the presence of transverse flow using two approximations to describe the properties of the hadronic medium, one containing only $π, ρ, ω$, and $η$ mesons (simplified equation of state) and the other containing hadrons and resonances from the particle data table. Both are considered with and without initial quark gluon plasma formation. In each case the initial temperature is fixed by requiring $dN_{ch}/dy \sim$ 550 in the final state. It is shown that most observables are very sensitive to the equation of state. This is particularly evident when comparing the results of the simplified equation of state in the scenarios with and without phase transition. The hadronic gas scenario leads to a substantially higher rate for the $p_T$-distribution of all particles. In the complete equation of state with several hundreds of hadronic resonances, the difference between the scenarios with and without phase transition is rather modest. Both photon and particle spectra, in a wide $p_T$ range, show very similar behavior. It is therefore concluded that from the $p_T$ spectra it will be hard to disentangle quark gluon plasma formation in the initial state. It is to be stressed however, that there are conceptual difficulties in applying a pure hadronic gas equation of state at SPS-energies. The phase transition scenario with a quark gluon plasma present in the initial state seems to be the more natural one.
The most abundantly produced hadron species in $Si\!-\!Au$ collisions at the BNL-AGS (nucleons, pions, kaons, antikaons and hyperons) are shown to be in accord with emission from a thermal resonance gas source of temperature $T\simeq 110$ MeV and baryochemical potential $μ_B \simeq 540$ MeV, corresponding to about 1/3 standard nuclear density. Our analysis takes the isopin asymmetry of the initial state fully into account.
A calculation of thermal gluon decay shows that this process contributes significantly to strangeness production in a quark-gluon plasma. Our analysis does not support recent claims that this is the dominant process. In our calculations we take into account the resummed form of the transverse and longitudinal parts of the gluon propagator following the Braaten-Pisarski method. Our results are subject to the uncertainty concerning the estimate of the damping rate entering the effective gluon propagator.
We review the relationship between the pion decay constant $f_π$, the chiral symmetry restoration temperature $T_c$ and the phenomenology of low energy chiral symmetry breaking in view of the recent confirmation of the existence of a sigma meson with a mass of 600-700 MeV.
Particle production in central S-A collisions at 200 GeV/A energy is analysed within a thermal model. Present data imply that the strange particles freeze out at a higher temperature than the non-strange particles and that the strangeness saturation is incomplete.
An estimate is made of the Landau-Pomeranchuk effect on the production of dileptons in a hadronic gas and in a quark-gluon plasma. For low mass dilepton pairs this effect reduces the production rate for bremsstrahlung by an order of magnitude. For high invariant masses its influence is negligible. This behaviour is of importance for the theoretical analysis of low mass dilepton pairs produced in relativistic heavy ion collisions.
Deep inelastic scattering is considered in a statistical model of the nucleon. This incorporates certain features which are absent in the standard parton model such as quantum statistical correlations which play a role in the propagation of particles when considering Feynman diagrams containing internal lines. The inclusion of the ${\cal O}(α_{s})$ corrections in our numerical calculations allows a good fit to the data for $x\geq 0.25$. The fit corresponds to values of temperature and chemical potential of approximately $T=0.067$ GeV and $μ=0.133$ GeV. The latter values of parameters, however, give rise, for all $x$, to a large value for $R=σ_{L}/σ_{T}$.
The temperature dependence of the kinetic coefficients is obtained in the non-perturbative region with the help of Green-Kubo-type formulae in the model of massive gluon gas motivated by numerical results from simulations of lattice QCD. The entropy production rate is estimated using scaling hydrodynamics. It is shown that the increase in the viscosity coefficients leads to entropy generation in heavy ion collision processes which could be big, especially for temperatures close to the critical one.
The effect of a sharp front separating the quark-gluon plasma phase from the hadronic phase is investigated. Energy-momentum conservation and baryon number conservation constrain the possible temperature jump across the front. If one assumes that the temperature in the hadronic phase is $T\simeq$ 200 MeV , as has been suggested by numerous results from relativistic ion collisions, one can determine the corresponding temperature in the quark phase with the help of continuity equations across the front. The calculations reveal that the quark phase must be in a strongly supercooled state. The stability of this solution with respect to minor modifications is investigated. In particular the effect of an admixture of hadronic matter in the quark phase (e.g. in the form of bubbles) is considered in detail. In the absence of admixture the transition proceeds via a detonation transition and is accompanied by a substantial super-cooling of the quark-gluon plasma phase. The detonation is accompanied by less supercooling if a small fraction of bubbles is allowed. By increasing the fraction of bubbles the supercooling becomes weaker and eventually the transition proceeds via a smoother deflagration wave.
We propose a mechanism which naturally contains the relation $μ_{B} = 3μ_{S}$ of the hadronic gas produced in heavy-ion collisions at CERN. Our starting assumption is the existence of a sharp front separating the quark-gluon plasma phase from the hadronic phase. Energy-momentum conservation across the front leads to the following consequences for an adiabatic process a) The baryon chemical potential, $μ_{B}$, is approximately continuous across the front. b) The temperature in the hadronic gas is higher than the phase transition temperature due to superheating. c) In the region covered by the experiments the velocity of the hadronic gas approximately equals the speed of sound in the hadronic gas.