Multifragmentation and dynamics in heavy ion collisions

A midrapidity zone formed in heavy-ion collisions has been investigated through special selections of light particles and intermediate mass fragments detected in the reaction ³⁵Cl on ¹²C at 43 MeV/nucleon and the reactions ⁵⁸Ni on ¹²C, ²⁴Mg, and ¹⁹⁷Au at 34.5 MeV/nucleon, and of neutron energy spectra measured in the reaction ³⁵Cl on ^natTa. Properties of the observables have been examined to characterize the neck-like structure formed between the two reaction partners. For the HERACLES collaboration.     

Influence of then-n cross section on the observables in dynamical process of heavy ion collisions

The influence of then- n cross section on the observables in dynamical process of heavy ion collisions was investigated by means of the BUU (Boltzmann-Uehling-Uhlenbeck) equation. The influence of then-n cross section upon transverse momentum, flow angle and quadrupole moment of momentum distribution with increasing incident energy was successfully illustrated according to the variation law ofn-n cross section with and without the medium effects with increasing bombarding energy. This investigation also shows a close relationship between the medium effects of then- n cross section and the nuclear equation of state (EOS).     

Preequilibrium emission in heavy ion collisions: A dynamical approach

A coupled treatment of both dynamics and G.D.H. Model is proposed to treat “prior equilibration” phase in heavy ion collisions. Assumptions and coupling effects are discussed in details, and a numerical procedure is developped. Comparison with²⁷A1(¹⁴N,X) at energies standing from 7–30 MeV/nucleon is presented. Proton and helium 4 productions, angle integrated spectra and angular distributions are compared. Incomplete fusion to fusion ratio is correctly reproduced in the intermediate incident energy region. A part of the linear momentum lost can be explained and a non negligeable transverse momentum is predicted for the fusion events. Differents types of experimental data are needed to test the validity of this approach and further developments are discussed.     

Phase transition dynamics in ultra-relativistic heavy ion collisions

We investigate various problems related to the dynamics of a first-order phase transition from quarkgluon plasma to hadronic matter in ultra-relativistic heavy ion collisions. These include nucleation, growth and fusion of hadronic bubbles in either the Bjorken longitudinal hydrodynamic expansion model or the Cooper-Frye-Schönberg spherical hydrodynamic expansion model. With reasonable input parameters the conversion of one phase into the other is relatively close to the idealized adiabatic Maxwell construction, although one can choose parameters such that the conversion is strongly out of equilibrium.     

On the dynamics of statistical fluctuations in heavy ion collisions

We show how statistical fluctuations can be treated within the collective approach to heavy ion reactions. In the classical limit, the equation of motion for the distribution d in the collective variables Q_μ and their conjugate momenta P_μ turns out to be a Fokker-Planck equation. We briefly describe the connection of this equation to one of the Smoluchowski type for a distribution in Q_μ only, often used in heavy ion physics. For anharmonic motion our general Fokker-Planck equation is simplified to be linear in the deviations of the Q_μ mand P_μ from their mean values. The solution of this equation is discussed in terms of a simple Gaussian. The parameters of this Gaussian are determined completely by the first and second moments in Q_μ mand P_μ. The equations for the first moments are identical to the Newton equations including frictional forces. Those for the second moments are linear differential equations of first order and hence easily solvable. The whole derivation is completely analogous to that for the Newton equation reported recently. Here the starting point is the quantum mechanical von Neumann equation rather than the Heisenberg equations. As an intermediate result we obtain and discuss briefly a quantal equation for the reduced density operator d which includes frictional effects.     

Effect of two-body collisions on heavy ion dynamics

Two-body collisions are included in TDHF by the time-relaxation method. Calculations are made in a one-dimensional slab-geometry at $\text{E}{}_{\mathrm{c.m}}\text{A}\text{= 20}\text{MeV}$. The relaxation time is assumed to be temperature dependent. Results arc shown for two functions of the temperature as well as for ordinary TDHF (infinite relaxation time). Temperature and the two components of the pressuretensor (perpendicular and parallel to the beam direction) are shown. The approach towards the hydrostatic limit (isototropic pressure) as the relaxation time decreases is shown. Relaxation time is expected to decrease with temperature, i.e. beam energy. The energy at which the pressure becomes isotropic is important for understanding the dynamics of heavy ion collisions.     

The process γ+γ→ℓ+ℓ in coherent heavy ion collisions

Using a covariant formalism we calculate to leading order the 2-photon contribution to lepton pair production in coherent heavy ion collisions with-in the framework of EPA. The most significant result of our calculation is the presence of a prominent narrow threshold enhancement in thee ⁺ e ^? invariant mass distribution. The position of this peak is essentially energy independent, however, its height and width tend to increase with increasing energy, mass and charge of the colliding ions. Results are presented for configurations at the Bevalac and SPS machines which are then extrapolated down to the GSI energy region.     

Mass exchange and angular distribution in a dynamical treatment of heavy ion collisions

We present a first numerical computation of the absolute value of the double differential cross section as a function of mass asymmetry and detection angle including a dynamical coupling between relative motion and mass asymmetry. We apply it to the⁶³Cu+¹⁹⁷Au experiment at two different energies. The equation of motion used is a Fokker Planck equation for the distribution function in classical phase space. The coefficients needed are those known from classical model calculations, besides a friction coefficient introduced for the mass asymmetry degree. We find encouraging agreement between the calculated and experimental curves.     

The Ψ′ in the relativistic heavy ion collision

We have investigated the temperature influence on the hadronic decay width of Ψ′. Contrary to the divergence of the decay width ofρ-meson,π-meson at critical temperature as suggested in some papers, we find that the decay width of Ψ′ decreases with increasing temperature. Thus the leptonic decay can be used to measure the number of produced Ψ′ at finite temperature just as at zero temperature.     

Equilibrium in heavy ion collisions

We discuss the question of equilibration in heavy ion collisions and how it can be addressed in experiment     

Phase transitions in high energy heavy ion collisions within fluid dynamics

Recent advances in Fluid Dynamical modeling of heavy ion collisions are presented, with particular attention to mesoscopic systems, QGP formation in the pre FD regime and QGP hadronization coinciding with the final freeze-out.     

Femtoscopy in heavy ion collisions

Non-trivial space-time geometrical effects are at the core of bulk-sector heavy ion physics, and two-particle correlations at low relative velocity are the most direct probe of this geometry at the femtometer scale. I present a brief overview of the wealth of femtoscopic measurements from the past two decades of heavy ion experiments. Essentially every conceivable “knob” at our disposal has been turned; the response of two-particle correlations to these variations has revealed much about the space-momentum substructure of the hot source created in the collisions. I discuss the present status of the femtoscopic program and questions which remain, and point to new efforts which aim to resolve them. PACS 25.75.Ld; 25.75.Gz; 24.10.Nz     

Nonperturbative dynamics of an expanding flux-tube in the theory of heavy ion collisions

The considered variant of the flux-tube model is based on the joint action of the same mechanisms of the vacuum particle creation as the standard Schwinger mechanism and the Casimir effect. It is assumed that pre-equilibrium quark-gluon plasma (QGP) is created in ultrarelativistic heavy-ion collision. According to the considered model, these two mechanisms act at different time scales and lead to qualitatively different momentum distributions. The text was submitted by the authors in English.     

The high-density symmetry energy in heavy ion collisions

The nuclear symmetry energy as a function of density is rather poorly constrained theoretically and experimentally both below saturation density, but particularly at high density, where very few relevant experimental data exist. We discuss observables which could yield information on this question, in particular, proton-neutron flow differences, and the production of pions and kaons in relativistic heavy ion collisions. For the meson production we investigate particularly ratios of the corresponding isospin partners π⁻/π⁺ and K⁰/K⁺, where we find that the kaons are an interesting probe to the symmetry energy. In this case we also discuss the influence of various choices for the kaon potentials or in-medium effective masses.     

Bulk-viscosity-driven freeze-out in heavy ion collision

We give a review on the HBT puzzle, and argue that its resolution requires the introduction of new physics close to the phase transition scale. We argue that a candidate for this new physics is bulk viscosity, recently postulated to peak, and even diverge, close to the phase transition temperature. We show that such a viscosity peak can force the system created in heavy ion collisions to become unstable, and filament into fragments whose size is weakly dependent on the global size of the system, thereby triggering freeze-out.     

Low mass lepton pair production and pion dynamics in ultrarelativistic heavy ion collisions

The production of lepton pairs with low invariant mass as probes for the pion dynamics in relativistic nuclear collisions is discussed in some detail. We emphasize the implications of a possible large positive pion chemical potential for the production rate of lepton pairs. We find a strong enhancement in the low invariant mass region, \(2m_\pi \leqslant M_{\mu ^ + \mu ^ - } \leqslant 600\,MeV\) , for lepton pairs which are produced via the annihilation of pions in the hot and dense collision zone. Various sources of background which could mask this enhancement are discussed in some detail. We present and compare calculations for dilepton mass spectra based on different nuclear collision scenarios. It is found that the di-muon mass spectrum can serve as a useful probe to distinguish between these different scenarios.     

Kaon flow in heavy ion collisions

Using transport models that include explicitly the kaon degree of freedom, we have studied kaon flow, i.e., the average kaon transverse momentum as a function of rapidity, and the associated flow parameter in heavy ion collisions at both SIS and AGS energies. It is found that the pattern of kaon flow in heavy ion collisions is sensitive to the kaon potential used in the model and thus provides a useful means for studying the kaon properties in nuclear medium.     

Event-by-event fluctuations in heavy ion collisions

We discuss the physics underlying event-by-event fluctuations in relativistic heavy ion collision. We will emphasize how the fluctuations of particle ratios can be utilzed to explore the properties of the matter created in these collisions. In particular we will argue that the fluctutions of the ratio of positively over negatively charged particles may serve as a unique signature for the Quark-Gluon Plasma.     

Nuclear flow in heavy ion collisions

Short overview of the flow phenomenon from the low upto relativistic heavy ion energies is given. Strict difference between spectator and participant flow is emphased. The latter is described by the modification of the nuclear blast model. Thermal source expanding in a plane perpendicular to the line connecting the centers of projectile and target is assumed. Observed dependences of squeeze-out, side-splash and of mean transverse-energy in relativistic heavy-ion collisions on the particle's mass are reproduced. Partioning of initial projectile energy into thermal and compressional parts in²⁰⁹Bi(1 GeV/u)+²⁰⁸Pb and¹⁹⁷Au(150–800 MeV/u)+¹⁹⁷Au collisions is discussed.Invited lecture given at the International School-Workshop Relativistic Heavy-Ion Physics, Prague (Czech Republic), 19–23 September 1994.Experimental data obtained at Bi+Pb collisions [6] are the result of the group effort. The contribution of V. Wagner, M. Pachr, Yu. G. Sobolev, R. Simon, S. Hlava, M. umbera as well as of my student R. Pleska, who carried out most of the fits, is gratefully acknowledged. This work was supported by the Granting Agency of the Czech Republic under contract No. 202/93/1144. A.K. gratefully acknowledges support from EC during his stay at KVI Groningen, where part of this work was done.