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H. Stoecker

Publications and source records attributed to H. Stoecker.

140 records · Page 8Linked to original sources
arXiv1997

Hadron and hadron-cluster production in a hydrodynamical model including particle evaporation

We discuss the evolution of the mixed phase at RHIC and SPS within boostinvariant hydrodynamics. In addition to the hydrodynamical expansion, we also consider evaporation of particles off the surface of the fluid. The back-reaction of the evaporation process on the dynamics of the fluid shortens the lifetime of the mixed phase. In our model this lifetime of the mixed phase is <12 fm/c in Au+Au at RHIC and <6.5 fm/c in Pb+Pb at SPS, even in the limit of vanishing transverse expansion velocity. Strangeness separation occurs, especially in events (or at rapidities) with relatively high initial net baryon and strangeness number, enhancing the multiplicity of MEMOs (multiply strange nuclear clusters). If antiquarks and antibaryons reach saturation in the course of the pure QGP or mixed phase, we find that at RHIC the ratio of antideuterons to deuterons may exceed 0.3 and even anti-helium to helium>0.1. Due to fluctuations, at RHIC even negative baryon number at midrapidity is possible in individual events, so that the antibaryon and antibaryon-cluster yields exceed those of the corresponding baryons and clusters.

nucl-th↗
arXiv1997

Probes for the early reaction dynamics of heavy-ion collisions at AGS and SPS

We discuss the early evolution of ultrarelativistic heavy-ion collisions within a multi-fluid dynamical model. In particular, we show that due to the finite mean-free path of the particles compression shock waves are smeared out considerably as compared to the one-fluid limit. Also, the maximal energy density of the baryons is much lower. We discuss the time scale of kinetic equilibration of the baryons in the central region and its relevance for directed flow. Finally, thermal emission of direct photons from the fluid of produced particles is calculated within the three-fluid model and two other simple expansion models. It is shown that the transverse momentum and rapidity spectra of photons give clue to the cooling law and the early rapidity distribution of the photon source.

nucl-th↗
arXiv1997

Hadron production in relativistic nuclear collisions: thermal hadron source or hadronizing quark-gluon plasma?

Measured hadron yields from relativistic nuclear collisions can be equally well understood in two physically distinct models, namely a static thermal hadronic source vs.~a time-dependent, nonequilibrium hadronization off a quark-gluon plasma droplet. Due to the time-dependent particle evaporation off the hadronic surface in the latter approach the hadron ratios change (by factors of $<\approx 5$) in time. Final particle yields reflect time averages over the actual thermodynamic properties of the system at a certain stage of the evolution. Calculated hadron, strangelet and (anti-)cluster yields as well as freeze-out times are presented for different systems. Due to strangeness distillation the system moves rapidly out of the T, $μ_q$ plane into the $μ_s$-sector. Strangeness to baryon ratios f_s=1-2 prevail during a considerable fraction (50%) of the time evolution (i.e. $Λ$-droplets or even $Ξ^-$-droplets form the system at the late stage: The possibility of observing this time evolution via HBT correlations is discussed). The observed hadron ratios require $T_c\approx 160 MeV$ and $B^{1/4}>\approx 200 MeV$. If the present model is fit to the extrapolated hadron yields, metastable hypermatter can only be produced with a probability $p< 10^{-8}$ for $A \ge 4$.

nucl-th↗
arXiv1996

Particle production by time-dependent meson fields in relativistic heavy-ion collisions

Collective bremsstrahlung of vector meson fields caused by mutual deceleration of the colliding nuclei is considered within the time-dependent Walecka theory. Excitation functions and rapidity distributions of particles produced by this mechanism are calculated under some simplifying assumptions about the space-time variation of the baryon current in a nuclear collision. In the case of central Au+Au collisions the bremsstrahlung mechanism becomes comparable with particle production in incoherent hadron--hadron collisions above the AGS energies.

hep-ph↗
arXiv1996

Collective flow in heavy ion reactions and the properties of excited nuclear matter

Quantum Molecular Dynamics (QMD) calculations of central collisions between heavy nuclei are used to study fragment production and the creation of collective flow. It is shown that the final phase space distributions are compatible with the expectations from a thermally equilibrated source, which in addition exhibits a collective transverse expansion. However, the microscopic analyses of the transient states in the intermediate reaction stages show that the event shapes are more complex and that equilibrium is reached only in very special cases but not in event samples which cover a wide range of impact parameters as it is the case in experiments. The basic features of a new molecular dynamics model (UQMD) for heavy ion collisions from the Fermi energy regime up to the highest presently available energies are outlined.

nucl-th↗
arXiv1996

Extracting the equation of state from a microscopic non-equilibrium model

We study the thermodynamic properties of infinite nuclear matter with the Ultrarelativistic Quantum Molecular Dynamics (URQMD), a semiclassical transport model, running in a box with periodic boundary conditions. It appears that the energy density rises faster than $T^4$ at high temperatures of $T\approx 200-300$~MeV. This indicates an increase in the number of degrees of freedom. Moreover, We have calculated direct photon production in Pb+Pb collisions at 160~GeV/u within this model. The direct photon slope from the microscopic calculation equals that from a hydrodynamical calculation without a phase transition in the equation of state of the photon source.

nucl-th↗
arXiv1996

Dynamics of Strangeness Production and Strange Matter Formation

The dynamics of a hadronizing quark matter drop is investigated with a hybrid-like model, which takes into account equilibrium as well as non-equilibrium features of the process. We study the the particle rates from the hadronizing plasma, which have to be put in relation to the whole time evolution of the system. The impact of finite-size effects on the distillation process is considered. $H^0$-Dibaryon yields are estimated. Microscopic calculations of hypercluster formation at AGS and SPS are presented.

nucl-th↗
arXiv1996

Baryon Stopping and Strangeness Production in Ultra-Relativistic Heavy Ion Collisions

The stopping behaviour of baryons in massive heavy ion collisions (at SPS, RHIC and LHC) is investigated within different microscopic models. At SPS-energies the predictions range from full stopping to virtually total transparency. Experimental data are indicating strong stopping. The initial baryo-chemical potentials and temperatures at collider energies and their impact on the formation probability of strange baryon clusters and strangelets are discussed.

nucl-th↗
arXiv1996

Neural Networks for Impact Parameter Determination

An accurate impact parameter determination in a heavy ion collision is crucial for almost all further analysis. The capabilities of an artificial neural network are investigated to that respect. A novel input generation for the network is proposed, namely the transverse and longitudinal momentum distribution of all outgoing (or actually detectable) particles. The neural network approach yields an improvement in performance of a factor of two as compared to classical techniques. To achieve this improvement simple network architectures and a 5 by 5 input grid in (p_t,p_z) space are sufficient.

nucl-th↗
arXiv1995

Distillation of Strangelets for low initial mu/T

We calculate the evolution of quark-gluon-plasma droplets during the hadronization in a thermodynamical model. It is speculated that cooling as well as strangeness enrichment allow for the formation of strangelets even at very high initial entropy per baryon $S/A^{\rm init}\approx 500$ and low initial baryon numbers of $A_{\rm B}^{\rm init}\approx 30$. It is shown that the droplet with vanishing initial chemical potential of strange quarks and a very moderate chemical potential of up/down quarks immediately charges up with strangeness. Baryon densities of $\approx 2ρ_0$ and strange chemical potentials of $μ_s>350$~MeV are reached if strangelets are stable. The importance of net--baryon and net--strangeness fluctuations for the possible strangelet formation at RHIC and LHC is emphasized.

nucl-th↗
arXiv1995

Antibaryons in massive heavy ion reactions: Importance of potentials

In the framework of RQMD we investigate antiproton observables in massive heavy ion collisions at AGS energies and compare to preliminary results of the E878 collaboration. We focus here on the considerable influence of the *real* part of an antinucleon--nucleus optical potential on the antiproton momentum spectra.

nucl-th↗
arXiv1995

The Disappearance of Flow

We investigate the disappearance of collective flow in the reaction plane in heavy-ion collisions within a microscopic model (QMD). A systematic study of the impact parameter dependence is performed for the system Ca+Ca. The balance energy strongly increases with impact parameter. Momentum dependent interactions reduce the balance energies for intermediate impact parameters $b\approx4.5$ fm. Dynamical negative flow is not visible in the laboratory frame but does exist in the contact frame for the heavy system Au+Au. For semi-peripheral collisions of Ca+Ca with $b\approx6.5$ fm a new two-component flow is discussed. Azimuthal distributions exhibit strong collectiv flow signals, even at the balance energy.

nucl-th↗
arXiv1995

Azimuthal correlations of pions in relativistic heavy ion collisions at 1 GeV/nucl.

Triple differential cross sections of pions in heavy ion collisions at 1 GeV/nucl. are studied with the IQMD model. After discussing general properties of $Δ$ resonance and pion production we focus on azimuthal correlations: At projectile- and target-rapidities we observe an anticorrelation in the in-plane transverse momentum between pions and protons. At c.m.-rapidity, however, we find that high $p_t$ pions are being preferentially emitted perpendicular to the event-plane. We investigate the causes of those correlations and their sensitivity on the density and momentum dependence of the real and imaginary part of the nucleon and pion optical potential.

nucl-th↗