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The measured particle ratios in central heavy-ion collisions at RHIC-BNL are investigated within a chemical and thermal equilibrium chiral SU(3) theta - omega approach. The commonly adopted noninteracting gas calculations yield temperatures close to or above the critical temperature for the chiral phase transition, but without taking into account any interactions. Contrary, the chiral SU(3) model predicts temperature and density dependent e ective hadron masses and e ective chemical potentials in the medium and a transition to a chirally restored phase at high temperatures or chemical potentials. Three di erent parametrizations of the model, which show di erent types of phase transition behaviour, are investigated. We show that if a chiral phase transition occured in those collisions, freezing of the relative hadron abundances in the symmetric phase is excluded by the data. Therefore, either very rapid chemical equilibration must occur in the broken phase, or the measured hadron ratios are the outcome of the dynamical symmetry breaking. Furthermore, the extracted chemical freeze-out parameters di er considerably from those obtained in simple noninteracting gas calculations. In particular, the three models yield up to 35 MeV lower temperatures than the free gas approximation. The in-medium masses turn out di er up to 150 MeV from their vacuum values.
We calculate p, ±,K± and (+ 0) rapidity distributions and compare to experimental data from SIS to SPS energies within the UrQMD and HSD transport approaches that are both based on string, quark, diquark (q, ¯q, qq, ¯q ¯q) and hadronic degrees of freedom. The two transport models do not include any explicit phase transition to a quark-gluon plasma (QGP). It is found that both approaches agree rather well with each other and with the experimental rapidity distributions for protons, s, ± and K±. In- spite of this apparent agreement both transport models fail to reproduce the maximum in the excitation function for the ratio K+/ + found experimen- tally between 11 and 40 A·GeV. A comparison to the various experimental data shows that this failure is dominantly due to an insu cient description of pion rapidity distributions rather than missing strangeness . The modest di erences in the transport model results on the other hand can be attributed to di erent implementations of string formation and frag- mentation, that are not su ciently controlled by experimental data for the elementary reactions in vacuum.
A canonical partition function for the two-component excluded volume model is derived, leading to two di erent van der Waals approximations. The one is known as the Lorentz-Berthelot mixture and the other has been proposed recently. Both models are analysed in the canonical and grand canonical ensemble. In comparison with the one-component van der Waals excluded volume model the suppression of particle densities is reduced in these two-component formulations, but in two essentially di erent ways. Presently used multi-component models have no such reduction. They are shown to be not correct when used for components with di erent hard-core radii. For high temperatures the excluded volume interaction is refined by accounting for the Lorentz contraction of the spherical excluded volumes, which leads to a distinct enhancement of lighter particles. The resulting e ects on pion yield ratios are studied for AGS and SPS data.
The amount of proton stopping in central Pb+Pb collisions from 20–160 A GeV as well as hyperon and antihyperon rapidity distributions are calculated within the UrQMD model in comparison to experimental data at 40, 80, and 160 A GeV taken recently from the NA49 collaboration. Furthermore, the amount of baryon stopping at 160A GeV for Pb+Pb collisions is studied as a function of centrality in comparison to the NA49 data. We find that the strange baryon yield is reasonably described for central collisions, however, the rapidity distributions are somewhat more narrow than the data. Moreover, the experimental antihyperon rapidity distributions at 40, 80, and 160 A GeV are underestimated by up to factors of 3—depending on the annihilation cross section employed—which might be addressed to missing multimeson fusion channels in the UrQMD model. Pacs-Nr.: 25.75.2q, 24.10.Jv, 24.10.Lx
Im Rahmen einer Zusammenschau werden im hier vorliegenden Bericht landwirtschaftliche und den ländlichen Raum betreffende Entwicklungen eingeschätzt und, in Verbindung hiermit, künftige Umweltentlastungen sowie mögliche neue Umweltgefährdungen dargestellt. Im Mittelpunkt der Betrachtungen steht die Nahrungsmittelproduktion in Deutschland, wobei die Integration der deutschen Landwirtschaft in die europäische Landwirtschaft und die Weltwirtschaft und die hiermit einhergehenden äußeren Einflüsse einbezogen werden. Der Zeithorizont der Betrachtungen ist der Zeitraum bis 2030. ...
The lightest supersymmetric particle, most likely the lightest neutralino, is one of the most prominent particle candidates for cold dark matter (CDM). We show that the primordial spectrum of density fluctuations in neutralino CDM has a sharp cut-off, induced by two different damping mechanisms. During the kinetic decoupling of neutralinos, non-equilibrium processes constitute viscosity effects, which damp or even absorb density perturbations in CDM. After the last scattering of neutralinos, free streaming induces neutralino flows from overdense to underdense regions of space. Both damping mechanisms together define a minimal mass scale for perturbations in neutralino CDM, before the inhomogeneities enter the non- linear epoch of structure formation. We find that the very first gravitationally bound neutralino clouds ought to have masses above 10-6M , which is six orders of magnitude above the mass of possible axion miniclusters.
We study properties of compact stars with the deconfinement phase transition in their interiors. The equation of state of cold baryon-rich matter is constructed by combining a relativistic mean-field model for the hadronic phase and the MIT Bag model for the deconfined phase. In a narrow parameter range two sequences of compact stars (twin stars), which differ by the size of the quark core, have been found. We demonstrate the possibility of a rapid transition between the twin stars with the energy release of about 10 ^52 ergs. This transition should be accompanied by the prompt neutrino burst and the delayed gamma-ray burst.
A novel mechanism of H0 and strangelet production in hadronic interactions within the Gribov-Regge approach is presented. In contrast to traditional distillation approaches, here the production of multiple (strange) quark bags does not require large baryon densities or a QGP. The production cross section increases with center of mass energy. Rapidity and transverse momentum distributions of the H 0 are predicted for pp collisions at E_lab = 160 AGeV (SPS) and \sqrt s = 200 AGeV (RHIC). The predicted total H 0 multiplicities are of order of the Omega-baryon yield and can be accessed by the NA49 and the STAR experiments.
The production of multiple strange baryons in pp interactions is studied. Here one can directly probe the microscopic decay of color flux tubes, allowing to differentiate between different string models and a statistical description of the hadronization. To analyse the different stages of a heavy ion collision the time evolution of the elastic and inelastic collision rates in central Pb+Pb interactions are studied. The microscopic simulation supports the idea of separated phases (non-equilibrium -> chemical freeze-out -> kinetic freeze-out) in the evolution of the system. The spectra and abundances of Lambda(1520), K 0(892) and other resonances are used to study the break-up dynamics of the source between chemical and thermal freeze-out.