arXiv subjects

C. Greiner

Publications and source records attributed to C. Greiner.

89 records · Page 5Linked to original sources

arXiv1998

Strangeness Production in the HSD Transport Approach from SIS to SPS energies

We study systematically the production of strangeness in nuclear reactions from SIS to SPS energies within the covariant hadronic transport approach HSD. Whereas the proton and pion rapidity distributions as well as pion transverse momentum spectra are well described in the hadronic transport model from 2-200 AGeV, the $K^+$ and $K^-$ spectra are noticeably underestimated at AGS energies while the $K^+$ spectra match well at SIS and SPS energies with the experimental data. We conclude that the failure of the hadronic model at AGS energies points towards a nonhadronic phase during the collision of heavy systems around 10 AGeV.

nucl-th↗

arXiv1998

The influence of medium effects on the gross structure of hybrid stars

We investigate the influence of medium effects on the structure of hybrid stars, i.e. neutron stars possessing a quark matter core. We found that medium effects reduce the extent of a pure quark matter phase in the interior of a hybrid star significantly in favor of a mixed phase of quark and hadronic matter. Over a wide range of the strong coupling constant - which parameterizes the influence of medium effects - quark matter is able to exist at least in a mixed phase in the interior of neutron stars.

astro-ph↗

arXiv1998

Charmonium Suppression with cc~ Dissociation by Strings

We study the production of cc~ pairs in nuclear reactions at SPS energies within the covariant transport approach HSD. The production of cc~ is treated perturbatively employing experimental cross sections while the interactions of cc~ pairs with baryons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the cc~-baryon cross sections the data on J/Ψsuppression in p+A reactions are reproduced in line with calculations based on the Glauber model. Additionally the dissociation of the cc~ pairs by strings is included in a purely geometrical way. We find good agreement with experimental data from the NA38 and NA50 collaboration with an estimate for the string radius of ~0.2-0.3 fm.

nucl-th↗

arXiv1998

Medium effects and the structure of neutron stars in the effective mass bag model

One of the most intriguing consequence of the extreme conditions inside neutron stars is the possibility of the natural existence of a deconfined strange quark matter phase in the high density interior of the star. The equation of state (EOS) of strange quark matter (SQM) was recently improved in the framework of the MIT bag model by including medium effects. It was found that medium effects increase the energy per baryon of SQM and therefore lower the stability of this phase. In this work we investigate the influence of medium effects on the structure of hybrid stars within this model. We found that the medium effects reduce the extent of a pure SQM phase in the interior of an hybrid star significantly in favor of a mixed phase of quark and hadronic matter.

astro-ph↗

arXiv1997

Phase transition of a finite quark-gluon plasma

The deconfinement transition region between hadronic matter and quark-gluon plasma is studied for finite volumes. Assuming simple model equations of state and a first order phase transition, we find that fluctuations in finite volumes hinder a sharp separation between the two phases around the critical temperature, leading to a rounding of the phase transition. For reaction volumes expected in heavy ion experiments, the softening of the equation of state is reduced considerably. This is especially true when the requirement of exact color-singletness is included in the QGP equation of state.

hep-ph↗

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.

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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 $A \ge 4$.

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arXiv1997

Dynamical Properties of Flux Tubes in the Friedberg-Lee Model

A dynamical model of confinement based on a microscopic transport description of the Friedberg-Lee model is extended to explicit color degrees of freedom. The string tension is reproduced by an adiabatic string formation from the nucleon ground state. As a particular application, we address the question of how a charmonium state might be dissociated by the strong color electric fields when moving through a color electric flux tube and speculate on the importance of such an effect with respect to the issue of $J/ψ$--suppression observed in ultrarelativistic heavy ion collisions. Furthermore, we show the dynamical breakup of flux tubes via $q-\bar{q}$-particle production and the disintegration into mesons. There we encounter some problems within the Vlasov-type realization of describing the motion of the quarks which can be resolved by a molecular dynamical approach.

hep-ph↗

arXiv1997

J/ψ-dissociation by a color electric flux tube

We adress the question of how a $c-\bar{c}$-state (a $J/ψ$) can be dissociated by the strong color electric fields when moving through a color electric flux tube. The color electric flux tube and the dissociation of the heavy quarkonia state are both described within the Friedberg-Lee color dielectric model. We speculate on the importance of such an effect with respect to the observed $J/ψ$-suppression in ultrarelativistic heavy ion collisions.

nucl-th↗

arXiv1997

Dynamical fragmentation of flux tubes in the Friedberg-Lee model

We present two novel dynamical features of flux tubes in the Friedberg-Lee model. First the fusion of two (anti-)parallel flux tubes, where we extract a string-string interaction potential which has a qualitative similarity to the nucleon-nucleon potential in the Friedberg-Lee model obtained by Koepf et al. Furthermore we show the dynamical breakup of flux tubes via $q\bar{q}-$particle production and the disintegration into mesons. We find, as a shortcoming of the present realization of the model, that the full dynamical transport approach presented in a previous publication fails to provide the disintegration mechanism in the semiclassical limit. Therefore, in addition, we present here a molecular dynamical approach for the motion of the quarks and show, as a first application, the space-time development of the quarks and their mean-fields for Lund-type string fragmentation processes.

hep-ph↗

arXiv1997

Strange quark matter with effective quark masses

The properties of strange quark matter at zero temperature are investigated including medium effects. The quarks are considered as quasiparticles which acquire an effective mass generated by the interaction with the other quarks of the dense system. Within this approach we find that these medium effects reduce the binding energy of strange quark matter with respect to $^{56}Fe$.

hep-ph↗

arXiv1996

Medium effects in strange quark matter and strange stars

We investigate the properties of strange quark matter at zero temperature including medium effects. The quarks are considered as quasiparticles which acquire an effective mass generated by the interaction with the other quarks of the dense system. The effective quark masses are derived from the zero momentum limit of the dispersion relations following from an effective quark propagator obtained from resumming one-loop self energy diagrams in the hard dense loop approximation. This leads to a thermodynamic selfconsistent description of strange quark matter as an ideal Fermi gas of quasiparticles. Within this approach we find that medium effects reduce the overall binding energy with respect to $^{56}Fe$ of strange quark matter. For realistic values of the strong coupling constant strange quark matter is not absolutely stable. The application to pure strange quark matter stars shows that medium effects have, nevertheless, no impact on the mass-radius relation of the stars. However, a phase transition to hadronic matter at the surface of the stars becomes more likely.

hep-ph↗

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

Hypermatter - Properties and Formation in Relativistic Nuclear Collisions

The extension of the Periodic System into hitherto unexplored domains - antimatter and hypermatter - is discussed. Starting from an analysis of hyperon and single hypernuclear properties we investigate the structure of multi-hyperon objects (MEMOs) using an extended relativistic meson field theory. These are contrasted with multi-strange quark states (strangelets). Their production mechanism is studied for relativistic collisions of heavy ions from present day experiments at AGS and SPS to future opportunities at RHIC and LHC. It is pointed out that absolutely stable hypermatter is unlikely to be produced in heavy ion collisions. New attention should be focused on short lived metastable hyperclusters ($τ\propto 10^{-10}$s) and on intensity interferometry of multi-strange-baryon correlations.

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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↗

arXiv1993

Some remarks on pion condensation in relativistic heavy ion collisions

Recently it was pointed out that coherent or condensated states of pions may account for the explanation of the Centauro events observed in cosmic ray showers. We argue that an occurrence of condensed pions requires that the system evolves far out of thermal equilibrium. Besides an unusual charge ratio distribution we show that such a produced state also would lead to strong isospin correlations.

hep-ph↗