arXiv subjects

W. Cassing

Publications and source records attributed to W. Cassing.

At least 181 records · Page 10Linked to original sources

arXiv1998

Analysis of flow effects in relativistic heavy-ion collisions within the CBUU approach

We study flow phenomena in relativistic heavy-ion collisions, both in transverse and radial direction, in comparison to experimental data. The collective dynamics of the nucleus-nucleus collision is described within a transport model of the coupled channel BUU type (CBUU). This recently developed version includes all nucleonic resonances up to 1.95 GeV in mass and mean-field potentials both of the Skyrme and momentum dependent MDYI type. We find that heavy resonances play an important role in the description of transverse flow above 1 AGeV incident energy. For radial flow we analyse reaction times and equilibration and extract the parameters $T$ and $β$ for temperature and collective flow velocity within different prescriptions. Furthermore, we apply a coalescence model for fragment production and check the mass dependence of the flow signals.

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arXiv1998

The role of the $P_{11}(1710)$ in the $NN{\to}NΣK$ reaction

Using the resonance model, which was successfully applied for the study of the $pp{\to}pΛK^+$ reaction, we investigate $NN{\to}NΣK$ reactions that should provide cleaner information about resonance excitations and meson exchange contributions. For this purpose we demonstrate that the invariant mass distribution for the $ΣK$ system, as well as the Dalitz plot for the $NN{\to}NΣK$ reaction, provide direct information about the $ΣK$ production mechanism, which can be tested in the near future by experiments at COSY.

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arXiv1998

Isospin equilibration in relativistic heavy-ion collisions

We study the mixing and the kinetic equilibration of projectile and target nucleons in relativistic heavy-ion collisions in the energy regime between 150~AMeV and 2~AGeV in a coupled-channel BUU (CBUU) approach. We find that equilibrium in the projectile-target degrees of freedom is in general not reached even for large systems at low energy where elastic nucleon-nucleon collisions dominate. Inelastic nucleon excitations are more favorable for equilibration and their relative abundance increases both with energy and mass. Experimentally, the projectile/target admixture can be determined by measuring the degree of isospin equilibration in isospin asymmetric nuclear collisions. For one of the most promising systems currently under investigation, ${}^{96}_{44}{\rm Ru}+{}^{96}_{40}{\rm Zr}$, we investigate the influence of the equation of state and the inelastic in-medium cross section.

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arXiv1998

$e^+e^-$ pairs from $π^-$A reactions

We present a dynamical study of $e^+e^-$ production in $π^-$A reactions at 1.3 and 1.7 GeV on the basis of the Coupled-Channel-BUU approach. The contributions from vector mesons ($ρ,ω$) are calculated taking into account the collisional broadening effect and are compared to background sources in the dilepton spectrum from the Dalitz decays of $ω$ and $η$ mesons produced in the reaction. Two possible scenarios for the medium modifications of the vector mesons are investigated, i.e. the 'dropping mass' scheme for the $ρ$ and $ω$ and a momentum dependent $ρ$-meson spectral function that includes the polarization of the $ρ$-meson due to resonant $ρ-N$ scattering.

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arXiv1998

On the possibility of $f_0$ observation in low energy $pp$ collisions

Within the meson-exchange model we calculate $f_0$-meson production cross section in $\piN$ and $NN$ reactions and investigate the possibility for $f_0$ observation via the $K\bar{K}$ decay mode in $pp$ collisions. Our studies indicate that an extraction of the $f_0$ signal is unlikely due to the large background from other reaction channels.

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arXiv1998

J/Psi Absorption Scenarios in Nuclear Collisions

We study the production of $c \bar{c}$ pairs and dimuons from hard collisions in nuclear reactions within the covariant transport approach HSD. Adopting 6 mb for the $c \bar{c}$-baryon cross section the data on $J/Ψ$ suppression in p + A reactions are reproduced in line with calculations based on the Glauber model. Furthermore, using $J/Ψ$ absorption cross sections with mesons above the $D\bar{D}$ threshold in the order of 1.5 - 3 mb we find that all data on $J/Ψ$ suppression from NA38/NA50 can be described without assuming the formation of a quark-gluon plasma. Alternatively, we also investigate an 'early'-comover absorption scenario where the $c \bar{c}$ pairs are dissociated in the color electric fields of neighboring strings. Again we find good agreement with the experimental data with an estimate for the string radius of $R_s \approx 0.2-0.25 fm$.

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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.

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arXiv1998

$η^{\prime}$ Production in Proton-Proton Collisions near Threshold

Within the one-pion exchange model we calculate the near-threshold $η^{\prime}$ production from $pp$ collisions including the final state interaction between the protons. Since the description of the data is quite well we conclude that $η, ρ$ or $ ω$ exchange currents either play no role or cancel each other to a large extent in this reaction.

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arXiv1998

Meson Production and Meson Properties at Finite Nuclear Density

The properties of $π, η, K^+$ and $K^-$ mesons are studied in nuclear reactions from SIS to SPS energies within the covariant transport approach HSD in comparison to the experimental data. Whereas the pion, $η$ and kaon abundancies and spectra indicate little or vanishing selfe-nergies for these mesons in the medium, antikaons (as well as antiprotons) are found to experience strong attractive potentials in nucleus-nucleus collisions at SIS energies. However, even when including these potentials the $K^+$ and $K^-$ spectra at AGS energies are noticeably underestimated showing an experimental excess of strangeness that points towards a nonhadronic phase in these reactions. On the other hand the $K^+, K^-$ production at SPS energies is well described by the hadronic approach without incorporating any parton degrees of freedom.

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arXiv1998

Correlation Dynamics of Yukawa-theory in 1+1 dimensions

Using the method of correlation dynamics we investigate the properties of a field-theory for fermions and scalar bosons coupled via a Yukawa interaction. Within this approach, which consists in an expansion of full equal-time Green functions into connected equal-time Green functions and a corresponding truncation of the hierarchy of equations of motion we carry out calculations up to 4th order in the connected Green functions and evaluate the effective potential of the theory in 1+1 dimensions on a torus. Comparing the different approximations we find a strong influence of the connected 4-point functions on the properties of the system.

hep-th↗

arXiv1998

$ρ$-meson properties at finite nuclear density

We calculate the momentum dependence of the $ρ$-meson selfenergy based on the dispersion relation for the $ρN$ scattering amplitude $f(ω$) at low nuclear density. The imaginary part of $f(ω)$ is determined from the optical theorem, while the total $ρN$ cross section is obtained within the VDM at high energy and within the resonance model at low energy. Our numerical results indicate a sizeable broadening of the $ρ$-meson width in the medium especially for low relative momenta $p$ while the real part of the $ρ$ selfenergy is found to change its sign and becomes repulsive already at momenta above 100 MeV/c. Extrapolating to nuclear saturation density $ρ_0$ we find a dropping of the $ρ$-mass for $p \approx$ 0 in line with the QCD sumrule analysis of Hatsuda while at high energy an increase of the $ρ$-mass in line with the prediction by Eletsky and Joffe is obtained. However, when including a broadening of the baryonic resonances in the medium, the $ρ$-meson mass shift at $p \approx$ 0 becomes slightly repulsive whereas the width increases substantially.

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arXiv1998

Antikaon Production in Proton-Nucleus Reactions and the $K^-$ properties in nuclear matter

We calculate the momentum-dependent potentials for $K^+$ and $K^-$ mesons in a dispersion approach at nuclear density $ρ_0$ using the information from the vacuum $K^+ N$ and $K^- N$ scattering amplitudes, however, leaving out the resonance contributions for the in-medium analysis. Whereas the $K^+$ potential is found to be repulsive ($\approx$ + 30 MeV) and to show only a moderate momentum dependence, the $K^-$ selfenergy at normal nuclear matter density turns out to be $\approx$ - 200 MeV at zero momentum in line with kaon atomic data, however, decreases rapidly in magnitude for higher momenta. The antikaon production in p + A reactions is calculated within a coupled transport approach and compared to the data at KEK including different assumptions for the antikaon potentials. Furthermore, detailed predictions are made for $p + ^{12}C$ and $p + ^{207}Pb$ reactions at 2.5 GeV in order to determine the momentum dependent antikaon potential experimentally.

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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.

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arXiv1998

Final State Interactions in the Near-Threshold Production of Kaons from Proton-Proton Collisions

We analyse the $pp \to p ΛK$ cross section recently measured at COSY arguing that the enhancement of the production cross section at energies close to the reaction threshold should be due to the $Λp$ final state interaction. We find that the experimental $Λp$ elastic scattering data as well as the predictions from the Jülich-Bonn model are in reasonable agreement with the new results on $K^+$-meson production. We propose to study directly the final state interaction by measurements of the cross section as a function of the hyperon momentum in the $Λp$ cm system.

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arXiv1998

Strangeness production in proton-proton collisions

The cross sections of the reaction $pp \to N K Y$ for $K^+$ or $K^0$ mesons and $Λ$ or $Σ$ hyperons are calculated within the boson exchange model including pion and kaon exchange diagrams. We analyze the dependence of the results on the accuracy of the input $πN$ amplitude. By fixing the $πNN $ coupling constant and the cut-off parameter $Λ_π$ at the $πNN$ vertex we calculate the contribution from the kaon exchange diagram and obtain the ratio of the $KN Λ$ and $KN Σ$ coupling constants by a fit to the experimental data. This ratio is in a good agreement with the SU(6) prediction. Our calculated total cross sections for the different reaction channels are fitted by simple expressions and compared with other parameterizations used in the literature. Furthermore, the gross features of the production cross section close to threshold are discussed.

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arXiv1998

Dilepton production and $m_T$-scaling at BEVALAC/SIS energies

We present a dynamical study of $e^+e^-$ production in C + C and Ca + Ca collisions at BEVALAC/SIS energies on the basis of the covariant transport approach HSD employing momentum-dependent $ρ$-meson spectral functions that include the pion modifications in the nuclear medium as well as the polarization of the $ρ$-meson due to resonant $ρ- N$ scattering. We find that the experimental data from the DLS collaboration cannot be described within the $ρ$-meson spectral function approach. A dropping $η$-mass scenario leads to a good reproduction of the DLS dilepton data, however, violates the $m_T$-scaling of $π^0$ and $η$ spectra as observed by the TAPS collaboration as well as $η$ photoproduction on nuclei.

nucl-th↗

arXiv1998

Baryon flow at SIS energies

We calculate the baryon flow $< P_x/A(y) >$ in the energy range from .25 to $\le 2.5 AGeV$ in a relativistic transport model for $Ni+Ni$ and $Au+Au$ collisions employing various models for the baryon self energies. We find that to describe the flow data of the FOPI Collaboration the strength of the vector potential has to be reduced at high relative momentum or at high density such that the Schrödinger- equivalent potential at normal nuclear density decreases above 1 GeV relative kinetic energy and approaches zero above 2 GeV.

nucl-th↗

arXiv1997

Meson $m_T$-scaling in heavy-ion collisions at SIS energies

We perform systematical studies on transverse-mass spectra of $π^0$, $η$, $ω$, $ϕ$, $K^+$ and $K^-$ mesons for C + C, Ni + Ni, Au + Au collisions at SIS energies within the HSD transport approach. We find that the $m_T$-spectra sensitively reflect the in-medium properties of the mesons. The 'bare mass' scenario leads to a general scaling behaviour for the meson $m_T$-spectra when including a mass shift due to the associate strangeness production threshold for kaons and antikaons whereas a 'dropping' mass scheme violates this $m_T$-scaling. The relative slope parameters, however, provide valuable information about the average meson potentials in the nuclear medium.

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