N.J. Davidson

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.

A semi-empirical determination of the properties of nuclear matter

Abstract The temperature dependence of the coefficients in the semi-empirical mass formula has been determined from a least squares fit to the canonical ensemble average of the excitation energy for nuclei throughout the periodic table. The low temperature behavior in the leading coefficient, the bulk energy density, is in disagreeement with current parameterizations used in the equations of state for symmetric nuclear matter. Peaked structure in the specific heat occurs at a temperature of approximately 0.5 MeV in agreement with theoretical predictions of the critical temperature of a pairing phase transition in nuclear matter.

Hadronic gas interpretation of NA35 and WA85 results

Abstract Particle ratios, as recently measured by the NA35 and WA85 Collaborations, are interpreted within the framework of chemically equilibrated hadronic gas models. The lower and upper experimental bounds of the NA35s Λ /Λ and Ks0/Λ ratios are compatible with a hadronic gas at temperatures T⩾174 MeV, and freeze-out baryon densities nB⩾0.16 fm−3. The Ξ−/Λ, Ξ − / Λ , Λ /Λ and Ξ − /Ξ − ratios, as reported by WA85, can be fitted using the same hadronic gas model with T⩾178 MeV and nB⩾0.16 fm−3.

Freeze-out in the E802 and NA35 experiments

Abstract We compare the thermodynamic conditions at freeze-out obtained from the results of E802 and NA35 Collaborations using an equilibrium thermodynamic model of hadron production. This model includes hadron interactions in a mean field approximation, hadron resonance decays and the effects of finite system size. The large discrepancies in freeze-out temperatures and densities may be qualitatively understood in terms of a simple kinetic model, and are the result of the very different meson to baryon ratios in the two experiments. This arises from the different incident energies, and thus initial compositions of the hadron gas, in the two experiments.

Kaon condensation in hadron gas models

Abstract We investigate the possibility of kaon condensation in dense hadronic matter within the context of equilibrium hadron gas models, where different approximations for the hadron hard-core repulsive interaction are made. We find that models which approximate the interactions by an excluded volume formalism always lead to a kaon condensate, while the occurrence of such a condensate in models which include the interactions via a mean field approximation is related to the number of hadron species which interact. Moreover, hadron gas models which are consistent with the presently available experimental data do not allow for kaon condensation in a physically realistic region.

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.

Model dependence of kaon condensation in dense hadronic matter

Abstract We investigate the possibility of kaon condensation in dense hadronic matter, and show that the occurence of this phenomenon is strongly model dependent. In particular, we show that a kaon condensation does not occur in a thermodynamically consistent model in which the baryon-baryon and meson-meson interactions are included in a mean field approximation.

Variational results for the Rabi Hamiltonian

Abstract We present simple two- and three-parameter variational calculations for the Rabi Hamiltonian. The importance of symmetry in the ansatze is stressed. The numerical results indicate that our ansatze provide accurate approximations both to the ground-state energy and wavefunction and to the first excited state if the two-boson energy significantly exceeds the level splitting.

Thermal gas models and particle production in heavy ion collisions

A systematic study of particle production in nuclear S−S and S−W collisions at 200 GeV/A is presented within the context of an equilibrium interacting hadron gas model. It is shown that the results for strange particle multiplicities and for non-strange baryons obtained in the NA35 and WA85 experiments can be well described in terms of the considered model.

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.