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 $\Delta$ 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.Comment: 40 pages, 18 eps-figures, uses psfig.sty; complete postscript file available at ftp://th.physik.uni-frankfurt.de/pub/bass/GSI-preprint_95-7.ps.

Production of light nuclei, hypernuclei and their antiparticles in relativistic nuclear collisions

We present, using the statistical model, an analysis of the production of light nuclei, hypernuclei and their antiparticles in central collisions of heavy nuclei. Based on these studies we provide predictions for the production yields of multiply-strange light nuclei.Comment: 9 pages, 6 figues; v2: final versions accepted for publication (Phys. Lett. B

Predictions of the pseudo-complex theory of Gravity for EHT observations- II. Theory and predictions

We present a resum\'e on the modified theory of gravity, called pseudo-complex General Relativity (pc-GR). It is the second in a series of papers, where the first one (Boller et al. 2019, referred to as paper I) discussed the observational consequences of pc-GR. In this paper, we concentrate on the underlying theory. PC-GR involves an algebraic extension of the standard theory of GR and it depends on two phenomenological parameters. An element included in pc-GR that is not present in standard GR is the energy-momentum tensor corresponding to an anisotropic ideal fluid, which we call dark energy. The two parameters are related to the coupling of mass to the dark energy and its fall-off as a function of r. The consequences and predictions of this theory will be discussed in the context of the observational results of the Even Horizon Telescope, expected soon. Our main result is that due to the accumulation of dark energy near a large mass, the modified theory predicts a dark ring followed by a bright ring in the emission profile of the accretion disc. We also discuss the light ring in the equatorial plane.Comment: 2 figure

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.Comment: Phys. Rev. C in print. Postscript-file also available at http://www.th.physik.uni-frankfurt.de/~bass/pub.htm

J/psi Suppression and Enhancement in Au+Au Collisions at the BNL RHIC

We consider the production of the J/psi mesons in heavy ion collisions at RHIC energies in the statistical coalescence model with an exact (canonical ensemble) charm conservation. The c\bar{c} quark pairs are assumed to be created in the primary hard parton collisions, but the formation of the open and hidden charm particles takes place at the hadronization stage and follows the prescription of statistical mechanics. The dependence of the J/psi production on both the number of nucleon participants and the collision energy is studied. The model predicts the J/psi suppression for low energies, whereas at the highest RHIC energy the model reveals the J/psi enhancement.Comment: 14 pages, REVTeX, 3 PS-figure

Charm coalescence at relativistic energies

The J/psi yield at midrapidity at the top RHIC (relativistic heavy ion collider) energy is calculated within the statistical coalescence model, which assumes charmonium formation at the late stage of the reaction from the charm quarks and antiquarks created earlier in hard parton collisions. The results are compared to the new PHENIX data and to predictions of the standard models, which assume formation of charmonia exclusively at the initial stage of the reaction and their subsequent suppression. Two versions of the suppression scenario are considered. One of them assumes gradual charmonium suppression by comovers, while the other one supposes that the suppression sets in abruptly due to quark-gluon plasma formation. Surprisingly, both versions give very similar results. In contrast, the statistical coalescence model predicts a few times larger $J/\psi$ yield in the most central collisions.Comment: 5 pages, RevTeX4, 1 PS-figure. V2: The final PHENIX data are analyze

Statistical Coalescence Model Analysis of J/psi Production in Pb+Pb Collisions at 158 A GeV

Production of J/\psi mesons in heavy ion collisions is considered within the statistical coalescence model. The model is in agreement with the experimental data of the NA50 Collaboration for Pb+Pb collisions at 158 A GeV in a wide centrality range, including the so called ``anomalous'' suppression domain. The model description of the J/psi data requires, however, strong enhancement of the open charm production in central Pb+Pb collisions. This model prediction may be checked in the future SPS runs.Comment: 13 pages, REVTeX, 2 PS-figure

Charmonium chemistry in A+A collisions at relativistic energies

Charmonium production and suppression in heavy-ion collisions at relativistic energies is investigated within different models, i.e. the comover absorption model, the threshold suppression model, the statistical coalescence model and the HSD transport approach. In HSD the charmonium dissociation cross sections with mesons are described by a simple phase-space parametrization including an effective coupling strength $|M_i|^2$ for the charmonium states $i=\chi_c, J/\psi, \psi^\prime$. This allows to include the backward channels for charmonium reproduction by $D \bar{D}$ channels -- which are missed in the comover absorption and threshold suppression model -- employing detailed balance without introducing any new parameters. It is found that all approaches yield a reasonable description of $J/\psi$ suppression in S+U and Pb+Pb collisions at SPS energies. However, they differ significantly in the $\psi^\prime/J/\psi$ ratio versus centrality at SPS and especially at RHIC energies. These pronounced differences can be exploited in future measurements at RHIC to distinguish the hadronic rescattering scenarios from quark coalescence close to the QGP phase boundary.Comment: 13 pages, 8 figures, to be published in Phys. Rev.

On the Observation of Phase Transitions in Collisions of Elementary Matter

We investigate the excitation function of directed flow, which can provide a clear signature of the creation of the QGP and demonstrate that the minimum of the directed flow does not correspond to the softest point of the EoS for isentropic expansion. A novel technique measuring the compactness is introduced to determine the QGP transition in relativistic-heavy ion collisions: The QGP transition will lead to higher compression and therefore to higher compactness of the source in coordinate space. This effect can be observed by pion interferometry. We propose to measure the compactness of the source in the appropriate principal axis frame of the compactness tensor in coordinate space.Comment: LaTeX, 8 pages, 6 figures, Conference proceedings to CRIS 2000, 3rd Catania Relativistic Ion Studie