R.M. Quick

Chemical equilibration in heavy-ion collisions

Abstract We examine the question of chemical equilibration in heavy-ion collisions by comparing hadron production ratios from the NA35 and WA85 Collaborations with those calculated in an equilibrium thermodynamic model of hadron product. This model includes hadron interactions in a mean field approximation, hadron resonance decays and the effects of finite system size. We find that the mean values obtained from the calculation are in good agreement with experiment, which suggests that the system formed in the heavy-ion collision was in chemical equilibrium. We find that the NA35 and WA85 data are compatible with a freeze-out temperature and baryon number density of 170 MeV and 0.1 fm −3 respectively. In addition, we calculate the statistical fluctuations in the hadron production rates obtained from the model.

The specific heat of hadronic matter at large baryon number densities

Abstract We have calculated the specific heat per unit volume for a relativistic hadron gas in the grand canonical ensemble using the experimental hadron spectrum as input. In the physical situation of a conserved baryon number density, structure in this quantity is apparent in the baryon sector at a temperature of approximately 140 MeV. This temperature is notably insensitive to both the baryon number density and the strength of the hadron-hadron interactions. We find that this structure occurs only in the baryon sector and is due to the change in the level density present in the experimental mass spectrum. As the baryon number density is increased to 5–10 times normal nuclear density, this structure also becomes gradually visible in the specific heat per unit volume for the hadron gas as a whole, and may provide a “signature” for the change of the hadron gas from baryon-to meson-dominated.

Correlated finite temperature mean field approximations

Abstract A correlated finite temparature mean field approximation is compared with the exact canonical, the conventional finite temperature Hartree-Fock (grand canonical) and canomical finite temperature Hartree-Fock results in Ne for the first time with a realistic interaction. The thermal behavior obtained in the correlated approach differs substantially from that given by mean field calculations. There are no sharp transitions and the system remains deformed at all temperatures, in agreement with exact canonical results.

Finite size effects in the deconfinement transition

The concepts of quark confinement and asymptotic freedom inherent in models of strongly interacting matter at the sub-hadronic level have lead to a great deal of interest in the associated deconfinement phase transition, i.e. the transition from a gas of Hadrons to a hot plasma of deconfined quarks and gluons. The possibility of obtaining energy densities which are large enough to cause deconfinement in ultra-relativistic heavy ion collisions has acted as one of the main stimuli to interest in such collisions, from both the experimental and theoretical points of view.

Time-like shock hadronization of a supercooled quark-gluon plasma

Abstract We study the energy-momentum and baryonic number conservation laws for quark-gluon plasma discontinuity transitions into hadron matter states. We find that the time-like shock hadronization of a supercooled quark-gluon plasma (when the normal vector to the discontinuity hypersurface is time-like) should take place. We consider some properties of this process, which is different from the standard space-like shock hadronization.

The observation of nuclear shape transitions at fixed angular momentum

Abstract The low temperature behaviour of 24Mg for fixed values of the angular momentum is examined in both the finite temperature mean field approximation and the canonical ensemble. The canonical ensemble calculations provide evidence that the shape transitions predicted by mean field calculations do occur. In both calculations the shape transitions result in a peak in the specific heat and a corresponding sudden increase in the many-body energy level density and we conclude that a relevant order parameter for these transitions is a function of the latter quantity. In addition, the results indicate that shape transitions are due to the thermal excitation from collective to non-collective portions of the nuclear energy eigenspectrum. This appears to be a general result, and should also apply to shape transitions in heavier nuclei.

BCS and polycrystalline high-Tc materials

Abstract We examine the behavior of small superconducting grains using a BCS-like theory, in which the effects of the small size of the system are taken into account. Such effects are expected to be important in the analysis of polycrystalline high- T c materials, where the individual grains are weakly coupled. Both the shift in T c and the sharpening of the peak in the specific heat observed during the sintering process of high- T c materials can be qualitatively understood by taking into account finite size effects. Furthermore, it is shown that the average of the gap parameter remains finite above the critical temperature resulting in a tail in the upper bound for the critical current. This is in qualitative agreement with experiment, where a tail in the critical current and a smeared-out transition have been observed in polycrystalline Y 1.2 Ba 0.8 Cu 2 O x bulk samples and in sputtered Bi-Sr-Ca-Cu-O films.

Canonical hadron gas interpretation ofp-A E802 data

Hadron gas models have proved successful in predicting particle production in relativistic nucleus-nucleus collisions. The extension of these models to the smaller systems formed in proton-nucleus collisions requires that the finite size of the system be considered. We study two features introduced by the finite size: the need to conserve strangeness and baryon number exactly by performing calculations in the canonical ensemble, and the inclusion of a finite size geometrical correction term in the single particle density of states. We find significant differences between the grand canonical and canonical ensembles and a strong dependence on the baryon number of the system.

Thermodynamics of hadronic matter at high density

Abstract We calculate thermodynamics observables for an interacting relativistic hadron gas. Hadronic states are taken into account by the use of a sizeable portion of the experimental hadron spectrum, supplemented in some cases by an exponentially rising continuum. Calculations with non-zero baryon number densities, subject to the additional requirement of zero net strangeness, show structure in the heat capacity per unit volume of the baryon sector at a temperature of approximately 140 MeV. This structure also becomes visible in the total heat capacity per unit volume at large baryon number densities, and provides a signature for the change of the thermal response of the hadron gas from baryon- to meson-dominated, even though the meson number density is lower than the baryon number density. Furthermore, this structure is not seen in calculations with a massless hadron gas. Its origin is therefore associated with information contained in the hadronic mass spectrum, and thus with the sub-hadronic degrees of freedom of the hadrons.

Baryon number projection for an interacting hadron gas

Calculations of hadronic matter usually enforce conservation of the average baryon number density using the grand canonical ensemble. We have performed calculations for an interacting system in the canonical ensemble with fixed baryon numberNb, as appropriate for a finite fireball of the type produced in ultra relativistic heavy ion collisions. These results are compared with those obtained from calculations in the grand canonical ensemble. For an interacting nucleon gas the two ensembles yield free energies which differ by approximately 5%.