119186084
Bimodality and Coulomb effects with a canonical thermodynamic model
Abstract The effect of the Coulomb interaction on the phase diagram of finite nuclei is studied within the Canonical Thermodynamic Model. If Coulomb effects are artificially switched off, this model shows a phenomenology consistent with the liquid–gas phase transition. The inclusion of Coulomb does not significantly affect the phase diagram but it drastically modifies the nature and order parameter of the transition. A clear understanding of the phenomenon can be achieved looking at the distribution of the largest fragment produced in each fragmentation event. Possible connections with experimental observations are outlined.
122137392
A case study in intermittency: The Thermodynamic Model for intermediate energy heavy ion collisions
Abstract The Thermodynamic Model for heavy ion collisions explains, at least semi-quantitatively, many, though not all, features of the experimental data at Bevalac energies. We use this model as a case study for intermittency. A composite with n nucleons and total momentum P will be registered at the detector as n nucleons each with momentum P / n . We find that in this model the scaled factorial moments grow indefinitely with M where M is the number of divisions in the p z window. In so far as composites are droplets of various sizes (free nucleons constitute a gas), intermittency is seen to be linked to the existence of droplets. The role of resonances is examined by including excited “deuterons” whose mass determines the width of the momentum distribution of the decay nucleons. For nucleons and resonances only, the scaled factorial moments saturate with M when the bin size becomes smaller than the width. These results do not depend upon the size of the colliding systems thus the model cannot explain why, for example, e + e − shows stronger intermittency than the currently available heavy ion data.
42042854
Fluctuations and clustering in heavy-ion collisions.
We propose a new theory to treat fluctuation phenomena in heavy-ion reactions. In practical terms, the method is an extension of the theories of the one-body density based on mean-field plus collisional dynamics. In an exploratory study of the Ne+ Ne reaction, we find considerable fragmentation with a rapidly falling mass spectrum. There has been a large amount of theoretical work recently devoted to the study of the fragmentation of highly excited nuclei. The models discussed employ a broad variety of physical assumptions. They involve such concepts as thermodynamic phase transitions, ' statistical equilibrium, percolation, molecular dynamics, and more complicated hybrid models that use mean-field dynamics. By itself mean-field theory lacks enough fluctuation to produce realistic fragmentation. Up to now, the important fluctuations were introduced by ad hoc assumptions. In the work of Knoll and Strack the initial compressional phase of the reaction was assumed to produce a local equilibrium, and only the expansion phase was simulated by time-dependent Hartree-Fock (TDHF) theory. In another class of hybrid model, the Boltzmann-Uehling-Uhlenbeck (BUU) theory was used for the initial stage of the reaction. At some point in the time evolution, the single-particle density matrix is replaced by particle wave packets. Here we shall present the beginnings of a theory that includes the important quantum eflects and also has sufhcient fluctuations to show qualitatively realistic fragmentation. Theories of nuclear dynamics are founded on various approximations to the wave function or propagator of the system. The TDHF theory follows when the wave function is approximated by a single Slater determinant. The extended TDHF theory, which is the quantum parent of the classical BUU theory, evolves the single-particle density, including the mixing of many Slater determinants perturbatively. However, the potential field is calculated from the single-particle density, and so does not have any additional fluctuations. As in the extended TDHF theory, we assume that the mixing of Slater determinants can be calculated perturbatively. We also assume that any coherence between the determinants can be ignored. The neglect of coherence is a crucial assumption which allows the evolution of the systern to be calculated stochastically on a Markov chain, jumping from one configuration to another. Thus only one determinant need be considered at a time, and the mean field will be correct for that state. The major ambiguity with this approach is the choice of single-particle basis for the determinants. The basis aAects the evolution of the system if the coherence between determinants is neglected. Obviously, the basis should be chosen to minimize the error in this fundamental approximation. Couplings between configurations are minimized by our choosing orbitals that localize the particles as much as possible in coordinate space. Of course, the perturbative
2820708
Transverse momenta, nuclear equation of state, and momentum-dependent interactions in heavy-ion collisions.
We investigate the generation of transverse momentum in high-energy heavy-ion collisions and its relation to the nuclear equation of state. We find that streamer chamber data can be fitted by Boltzmann-Uehling-Uhlenbeck calculations with a momentum-dependent potential that closely models realistic nuclear matter interactions and yields an equation of state with {ital K}=215 MeV.
14530230
Temperature determination from the lattice gas model.
Determination of temperature from experimental data has become important in searches for critical phenomena in heavy-ion collisions. Widely used methods are ratios of isotopes (which rely on chemical and thermal equilibrium), population ratios of excited states, etc. Using the lattice gas model we propose a new observable: {ital n}{sub ch}/{ital Z} where {ital n}{sub ch} is the charge multiplicity and {ital Z} is the charge of the fragmenting system. We show that the reduced multiplicity is a good measure of the average temperature of the fragmenting system. {copyright} {ital 1996 The American Physical Society.}
118634033
Comparison of statistical multifragmentation model simulations with canonical thermodynamic model results : A few representative cases
The statistical multifragmentation model (SMM) has been widely used to explain experimental data of intermediate energy heavy ion collisions. A later entrant in the field is the canonical thermodynamic model (CTM) which is also being used to fit experimental data. The basic physics of both the models is the same, namely that fragments are produced according to their statistical weights in the available phase space. However, they are based on different statistical ensembles, and the methods of calculation are different: while the SMM uses Monte Carlo simulations, the CTM solves recursion relations. In this Letter we compare the predictions of the two models for a few representative cases.
35353810
Speed and adaptability of overlap fermion algorithms
We compare the efficiency of four different algorithms to compute the overlap Dirac operator, both for the speed, i.e. time required to reach a desired numerical accuracy, and for the adaptability, i.e. the scaling of speed with the condition number of the (square of the) Wilson-Dirac operator. Although orthogonal polynomial expansions give good speeds at moderate condition number, they are highly non-adaptable. One of the rational function expansions, the Zolotarev approximation, is the fastest and is adaptable. The conjugate gradient approximation is adaptable, self-tuning, and nearly as fast as the ZA.
120756008
Supercooling in viscous hydrodynamics for QCD phase transition
First order quark hadron phase transition is considered in scaling hydrodynamics including the Csernai-Kapusta model of nucleation of hadronic bubbles leading to supercooling. The effect of viscosity on the entropy production is studied in ideal as well as in viscous hydrodynamics with and without supercooling. It is found that the excess entropy produced due to supercooling depends weakly on viscosity.
119408212
Lattice QCD results on strangeness and quasi-quarks in heavy-ion collisions
Fluctuations of conserved quantities in heavy-ion collisions have been proposed as diagnostic tools for the nature of the produced phase. These can be related to the predictions of quark number susceptibilities (QNS) from lattice QCD. Using the diagonal QNS, we extracted the Wroblewski parameter in a dynamical QCD computation. Our results on the cross-correlations χBQ, χBY, χBS and χQY allow us to explore the charge and baryon number of objects that carry flavour. We present evidence that in the high-temperature phase of QCD the different flavour quantum numbers are excited in linkages which are exactly the same as one expects from quarks.
119419540
Analyticity and the phase diagram of QCD
Some consequences of the analyticity of the free energy (pressure) of QCD at finite chemical potential are deduced. Lattice simulations at imaginary chemical potential are possible due to the positivity of the measure. We show that such a theory cannot be interpreted as a thermodynamic system with a phase diagram in the usual sense. However, simulations at imaginary chemical potential are useful because the free energy can be analytically continued and information on the thermodynamic system at real chemical potential can be extracted by two means-either by the usual method of Taylor expansions, or by using this data to extract Yang-Lee zeroes of the grand-canonical partition function. We also comment on the structure of zeroes expected for the usual picture of the phases of QCD, following a discussion of the physics of imaginary chemical potential.
119206892
Strange freezeout
We argue that known systematics of hadron cross sections may cause different particles to freeze out of the fireball produced in heavy-ion collisions at different times. We find that a simple model with two freezeout points is a better description of data than that with a single freezeout, while still remaining predictive. The resulting fits seem to present constraints on the late stage evolution of the fireball, including the tantalizing possibility that the QCD chiral transition influences the yields at √