Searcharxiv⌕ Search

arXiv subjects

Carsten Greiner

Publications and source records attributed to Carsten Greiner.

At least 163 records · Page 9Linked to original sources
arXiv2003

Chromofields of Strings and Baryons

We calculate color electric fields of quark/antiquark ($\bar{q}q$) and 3 quark ($qqq$) systems within the chromodielectric model (CDM). We explicitly evaluate the string tension of flux tubes in the $\bar{q}q$--system and analyze their profile. To reproduce results of lattice calculations we use a bag pressure $B = (320 MeV)^4$ from which an effective strong coupling constant $α_s \approx 0.3$ follows. With these parameters we get a $Y$ shaped configuration for large $qqq$--systems.

hep-ph↗
arXiv2002

Collective effects in microscopic transport models

We give a reminder on the major inputs of microscopic hadronic transport models and on the physics aims when describing various aspects of relativistic heavy ion collisions at SPS energies. We then first stress that the situation of particle ratios being reproduced by a statistical description does not necessarily mean a clear hint for the existence of a fully isotropic momentum distribution at hadrochemical freeze-out. Second, a short discussion on the status of strangeness production is given. Third we demonstrate the importance of a new collective mechanism for producing (strange) antibaryons within a hadronic description, which guarantees sufficiently fast chemical equilibration.

nucl-th↗
arXiv2002

Strangeness in strongly interacting matter

This talk is devoted to review the field of strangeness production in (ultra-)relativistic heavy ion collisions within our present theoretical understanding. Historically there have been (at least) three major ideas for the interest in the production of strange hadronic particles: (1) mass modification of the kaons in a (baryon-)dense environment; (2) (early) K+ - production probes the nuclear equation of state (EoS); (3) enhanced strangeness production especially in the (multi-)strange (anti-)baryon channels as a signal of quark gluon plasma (QGP) formation. As a guideline for the discussion I employ the extensive experience with microscopic hadronic transport models. In addition, I elaborate on the recent idea of antihyperon production solely by means of multi-mesonic fusion-type reactions.

nucl-th↗
arXiv2000

Signatures of the quark gluon plasma: a personal overview

We first discuss, on a no-QGP basis, the two prominent indications of (a) enhanced strangeness production and of (b) anomalous $J/ψ$-suppression: We elaborate in particular on a recent idea of antihyperon production solely by multi-mesonic reactions. As a possible source for an enhanced dissociation of $c \bar{c}$ pairs we summarize the findings within the 'early'-comover absorption scenario of prehadronic string excitations. As an exotic candidate we then finally adress the stochastic formation of so called disoriented chiral condensates: An experimentally feasible DCC, if it does exist, has to be a rare event following an unusual and nontrivial distribution on an event by event basis.

nucl-th↗
arXiv2000

Do chemically saturated antihyperon abundancies signal the quark gluon plasma?

We first review the production and the possible chemical equilibration of strange particles at CERN-SPS energies within a microscopic hadronic transport calculation. It is shown in particular that the strange quarks are produced initially via string excitations in the primary, secondary and ternary interactions. We then further elaborate on a recent idea of antihyperon production by multi-mesonic reactions like $n_1π+ n_2 K \to \bar{Y}+p $ corresponding to the inverse of the strong binary baryon-antibaryon annihilation process. It is argued that by these reactions the (rare) antihyperons are driven towards local chemical equilibrium with pions, nucleons and kaons on a timescale of 1--3 fm/c in the still moderately baryon-dense initial hadronic environment after the termination of the prehadronic string phase. Accordingly this mechanism can provide a convenient explanation for the antihyperon yields at CERN-SPS energies without any need of a deconfined quark gluon plasma phase.

nucl-th↗
arXiv2000

Antihyperon-Production in Relativistic Heavy Ion Collision

Recently it has been shown that the observed antiproton yield in heavy-ion collisions at CERN-SpS energies can be understood by multi-pionic interactions which enforce local chemical equilibrium of the antiprotons with the nucleons and pions. Here we show that antihyperons are driven towards local chemical equilibrium with pions, nucleons and kaons on a timescale of less than 3 fm/c when applying a similar argument for the antihyperons by considering the inverse channel of annihilation reactions anti-Y + p to pions + kaons. These multi-mesonic reactions easily explain the antihyperon yields at CERN-SpS energies as advertised in pure thermal, hadronic models without the need of a quark gluon plasma phase. In addition, the argument also applies for AGS energies.

nucl-th↗
arXiv2000

Stochastic treatment of Disoriented Chiral Condensates within a Langevin description

Applying a microscopically motivated semi-classical Langevin description of the linear sigma model we investigate for various different scenarios the stochastic evolution of a disoriented chiral condensate (DCC) in a rapidly expanding system. Some particular emphasize is put on the numerical realisation of colored noise in order to treat the underlying dissipative and nonmarkovian stochastic equations of motion. A comparison with an approximate markovian (i.e. instantaneous) treatment of dissipation and noise will be made in order to identify the possible influence of memory effects in the evolution of the chiral order parameter. Assuming a standard Rayleigh cooling term to simulate a D-dimensional scaling expansion we present the probability distribution in the low momentum pion number stemming from the relaxing zero mode component of the chiral field. The best DCC signal is expected for initial conditions centered around $<σ> \approx 0 $ as would be the case of effective light `pions' close to the phase transition. By choosing appropriate idealized global parameters for the expansion our findings show that an experimentally feasible DCC, if it does exist in nature, has to be a rare event with some finite probability following a nontrivial and nonpoissonian distribution on an event by event basis. DCCs might then be identified experimentally by inspecting higher order factorial cumulants $θ_m$ ($m\ge 3$) in the sampled distribution.

hep-ph↗
arXiv1998

Stochastic interpretation of Kadanoff-Baym equations

We show that the nonperturbative transport equations, the so called `Kadanoff-Baym equations', within the non-equilibrium real time Green's function description can be be understood as the ensemble average over stochastic equations of Langevin type. For this we couple a free scalar boson quantum field to an environmental heat bath with some given temperature T. The inherent presence of noise and dissipation related by the fluctuation-dissipation theorem guarantees that the modes or particles become thermally populated on average in the long-time limit. This interpretation leads to a more intuitive physical picture of the process of thermalization and of the interpretation of the Kadanoff-Baym equations. One also immediately understands that the emerging wave equations for long wavelength modes with momenta much smaller than temperature behave nearly as classical fields. We also demonstrate how the problem of so called pinch singularities is resolved by a clear physical necessity of damping within the one-particle propagator. The occurrence of such ill-defined terms arising solely in a strictly perturbative expansion in out of equilibrium quantum field theory has a natural interpretation in analogy to Fermi's golden rule.

hep-ph↗
arXiv1998

Interpretation and resolution of pinch singularities in non-equilibrium quantum field theory

Ill-defined pinch singularities arising in a perturbative expansion in out of equilibrium quantum field theory have a natural analogue to standard scattering theory. We explicitly demonstrate that the occurrence of such terms is directly related to Fermi's golden rule known from elementary scattering theory and is thus of no mystery. We further argue that within the process of thermalization of a plasma one has to resum such contributions to all orders as the process itself is of non-perturbative nature. In this way the resummed propagators obtain a finite width. Within the Markov approximation of kinetic theory the actual phase space distribution at a given time of the evolution enters explicitly.

hep-ph↗
arXiv1998

Physics of Strange Matter

Relativistic heavy ion collisions offer the possibility to produce exotic metastable states of nuclear matter containing (roughly) equal number of strangeness compared to the content in baryon number. The reasoning of both their stability and existence, the possible distillation of strangeness necessary for their formation and the chances for their detection are reviewed. In the later respect emphasize is put on the properties of small lumps of strange quark matter with respect to their stability against strong or weak hadronic decays. In addition, implications in astrophysics like the properties of neutron stars and the issue of baryonic dark matter will be discussed.

hep-ph↗
arXiv1998

Stochastic interpretation of Kadanoff-Baym equations and their relation to Langevin processes

In this more pedagogical study we want to elucidate on stochastic aspects inherent to the (non-)equilibrium real time Green's function description (or `closed time path Green's function' -- CTPGF) of transport equations, the so called `Kadanoff-Baym equations'. As a toy model we couple a free scalar boson quantum field to an exemplaric heat bath with some given temperature T. It will be shown in detail that the emerging transport equations have to be understood as the ensemble average over stochastic equations of Langevin type. This corresponds to the equivalence of the influence functional approach by Feynman and Vernon and the CTP technique. The former, however, gives a more intuitive physical picture. In particular the physical role of (quantum) noise and the connection of its correlation kernel to the Kadanoff-Baym equations will be discussed. The inherent presence of noise and dissipation related by the fluctuation-dissipation theorem guarantees that the modes or particles become thermally populated on average in the long-time limit. For long wavelength modes with momenta much less than the temperature the emerging wave equation do behave nearly as classical. On the other hand, a kinetic transport description can be obtained in the semi-classical particle regime. Including fluctuations, its form resembles that of a phenomenological Boltzmann-Langevin description. However, we will point out some severe discrepancies in comparison to the Boltzmann- Langevin scheme. As a further byproduct we also note how the occurrence of so called pinch singularities is circumvented by a clear physical necessity of damping within the one-particle propagator.

hep-ph↗
arXiv1998

Physics of Strange Matter

The properties of strange quark matter and strange hadronic matter and their finite counterparts (strangelets and MEMOs) are reviewed. The production of strange matter in heavy-ion collision and the appearance of strange matter in neutron stars are discussed.

nucl-th↗
arXiv1997

Properties of Exotic Matter for Heavy Ion Searches

We examine the properties of both forms of strange matter, small lumps of strange quark matter (strangelets) and of strange hadronic matter (Metastable Exotic Multihypernuclear Objects: MEMOs) and their relevance for present and future heavy ion searches. The strong and weak decays are discussed separately to distinguish between long-lived and short-lived candidates where the former ones are detectable in present heavy ion experiments while the latter ones in future heavy ion experiments, respectively. We find some long-lived strangelet candidates which are highly negatively charged with a mass to charge ratio like a anti deuteron (M/Z=-2) but masses of A=10 to 16. We predict also many short-lived candidates, both in quark and in hadronic form, which can be highly charged. Purely hyperonic nuclei are bound and have a negative charge while carrying a positive baryon number. We demonstrate also that multiply charmed exotics (charmlets) might be bound and can be produced at future heavy ion colliders.

nucl-th↗
arXiv1997

Dissipation and Fluctuation at the Chiral Phase Transition

Utilizing the Langevin scenario for the linear sigma model we investigate the effect of friction and white noise on the evolution and stability of collective pionic fields in heavy ion collisions. We find that the smaller the volume the more stable pionic fluctuations become (the avergae mass increases). On the other hand the variance of the mass square increases even more, so for a system preheated in 10 fm3 volume individual trajectories do become unstable during a rapid one dimensional expansion in contrast to the ensemble averaged solution. This result supports the idea of looking for disoriented chiral condensate (DCC) formation in individual events.

hep-ph↗
arXiv1996

Detectability of Strange Matter in Heavy Ion Experiments

We discuss the properties of two distinct forms of hypothetical strange matter, small lumps of strange quark matter (strangelets) and of hyperon matter (metastable exotic multihypernuclear objects: MEMOs), with special emphasis on their relevance for present and future heavy ion experiments. The masses of small strangelets up to A = 40 are calculated using the MIT bag model with shell mode filling for various bag parameters. The strangelets are checked for possible strong and weak hadronic decays, also taking into account multiple hadron decays. It is found that strangelets which are stable against strong decay are most likely highly negative charged, contrary to previous findings. Strangelets can be stable against weak hadronic decay but their masses and charges are still rather high. This has serious impact on the present high sensitivity searches in heavy ion experiments at the AGS and CERN facilities. On the other hand, highly charged MEMOs are predicted on the basis of an extended relativistic mean-field model. Those objects could be detected in future experiments searching for short-lived, rare composites. It is demonstrated that future experiments can be sensitive to a much wider variety of strangelets.

nucl-th↗
arXiv1996

Classical Fields Near Thermal Equilibrium

We discuss the classical limit for the long-distance (``soft'') modes of a quantum field when the hard modes of the field are in thermal equilibrium. We address the question of the correct semiclassical dynamics when a momentum cut-off is introduced. Higher order contributions leads to a stochastic interpretation for the effective action in analogy to Quantum Brownian Motion, resulting in dissipation and decoherence for the evolution of the soft modes. Particular emphasis is put on the understanding of dissipation. Our discussion focuses mostly on scalar fields, but we make some remarks on the extension to gauge theories.

hep-th↗
arXiv1996

Shell-like Structures in an expanding Quark-Antiquark Plasma

The particle density distribution emerging from the solution of the Vlasov equation for relativistic, non-interacting particles with spherically symmetric initial conditions is shown to exhibit a shell-like structure for late fixed times in the center-of-mass (CM) frame of the system. A similar phenomenon was recently observed employing the test-particle method to solve the Vlasov equation for quarks with Nambu-Jona-Lasinio-type interactions, and was attributed to the attractive forces among the particles. Contrary to this claim, it is demonstrated here that this effect is of purely relativistic origin and is sensitive only to the mass of the particles and their initial phase-space distribution.

nucl-th↗