arXiv subjects

W. Cassing

Publications and source records attributed to W. Cassing.

At least 199 records · Page 11Linked to original sources

arXiv1997

$ρ$-Meson Production and Decay in Proton-Nucleus Collisions

We analyze the production of $ρ$-mesons in $p + A$ reactions including both the production by proton-nucleon as well as pion-nucleon collisions within a coupled channel transport approach. The final state interactions of the $ρ$-meson with nucleons are evaluated from a resonance model which allows to extract elastic and inelastic cross sections. We include the latter final state interactions, the $ρ$-meson decay into two pions as well as the final pion-nucleon interactions within the transport approach. We find the invariant mass distribution of pion pairs to be sensitive to the $ρ$-meson properties in the nuclear medium. However, due to the strong final state interactions of pions only light targets like $^{12}C$ might be suited to extract a $ρ$-signal from the uncorrelated two pion background which carries information about the in-medium properties of the $ρ$-meson.

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arXiv1997

Probing the Rho Spectral Function in Hot and Dense Nuclear Matter by Dileptons

We present a dynamical study of $e^+e^-$ and $μ^+ μ^-$ production in proton-nucleus and nucleus-nucleus collisions at CERN-SPS energies on the basis of the covariant transport approach HSD employing a momentum-dependent $ρ$-meson spectral function that includes 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 CERES and HELIOS-3 Collaborations can be described equally well as within the dropping $ρ$-mass scenario. Whereas corresponding dilepton $q_T$-spectra are found to be very similar, the inclusive dilepton yield in the invariant mass range $0.85 \leq M \leq 1.0$ GeV should allow to disentangle the two scenarios experimentally.

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arXiv1997

Medium effects in the production and decay of vector mesons in pion-nucleus reactions

The $ω$-, $ρ$- and $ϕ$-resonance production and their dileptonic decay in $π^- A$ reactions at 1.1 - 1.7 GeV/c are calculated within the intranuclear cascade (INC) approach. The invariant mass distribution of the dilepton pair for each resonance can be decomposed in two components which correspond to their decay 'inside' the target nucleus and in the vacuum, respectively. The first components are strongly distorted by the nuclear medium due to resonance-nucleon scattering and a possible mass shift at finite baryon density. These medium modifications are compared to background sources in the dilepton spectrum from $πN$ bremsstrahlung as well as the Dalitz decays of $ω$ and $η$ mesons produced in the reaction. Detailed predictions for $π^- Pb$ reactions at 1.3 and 1.7 GeV/c are made within several momentum bins for the lepton pair.

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arXiv1997

Reanalysis of antiproton production in proton-nucleus and nucleus-nucleus reactions at subthreshold energies

We reanalyse the production of $p\bar{p}$ pairs in proton-nucleus and nucleus-nucleus collisions employing novel elementary cross sections for baryon-baryon and pion-baryon production channels based on a boson-exchange model. In contrast to previous transport studies performed in the literature the secondary pion induced channel is found to be most important in both p+A and A+A collisions at subthreshold energies. A detailed comparison with the experimental data available indicates that sizeable attractive $\bar{p}$ potentials in the order of - 100 to - 150 MeV at normal nuclear matter density are needed to reproduce the size and shape of the experimental spectra.

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arXiv1997

Hypernucleus formation and strangeness production in proton-nucleus reactions

We study the production of $Λ$ hyperons in $p + A$ reactions on the basis of a BUU transport approach from 1.1 to 1.9 GeV and evaluate the properties of the hypernuclei produced in particular with respect to their momentum distribution in the laboratory frame. Due to elastic $ΛN$ scattering large cross sections for the production of heavy hypernuclei in the order of 100 - 400 $μb$ are predicted for p + U at 1.5 - 1.9 GeV laboratory energy. Whereas the $K^+ Y$ production channels are expected to be only slightly modified in the nuclear medium, the antikaon production should be enhanced substantially due to large attractive $K^-$ selfenergies in dense matter. We predict an enhancement of the inclusive $K^-$ yield in p + $^{208}$Pb collisions of a factor of $\approx$ 10 at 2 GeV laboratory energy.

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arXiv1997

Convergence properties of the equal-time connected Green function approach for temporal gauge SU(2)_{2+1} Yang-Mills theory

The hierarchy of equations of motion for equal-time Green functions in temporal gauge SU(N) Yang-Mills theory is truncated using an expansion in terms of connected Green functions. A second hierarchy of constraint equations arises from Gauss law and can be truncated in a similar way. Within this approximation scheme we investigate SU(2) Yang-Mills theory on a torus in 2+1 spacetime dimensions in a finite basis of plane wave states and focus on infrared and ultraviolet properties of the approach. We study the consequences of restoring the hierarchy of Gauss law constraints and of different momentum cutoffs for the 2- and the 3-point functions. In all truncation schemes considered up to the 4-point level the connected Green function approach is found to be UV divergent and either violating gauge invariance and/or energy conservation. The problems associated with adiabatically generating a perturbed ground state are discussed as well.

hep-th↗

arXiv1997

Production and absorption of $c \bar{c}$ pairs in nuclear collisions at SPS energies

We study the production of $c \bar{c}$ pairs and dimuons from hard collisions in nuclear reactions within the covariant transport approach HSD, which describes successfully both hadronic and electromagnetic observables from p+A and A+A collisions from SIS to SPS energies. The production of $c \bar{c}$ and Drell-Yan pairs is treated perturbatively employing experimental cross sections while the interactions of $c\bar{c}$ pairs with hadrons are included by conventional cascade-type two-body collisions. Adopting 6mb for the $c \bar{c}$-baryon cross sections the data on $J/Ψ$ suppression in p+A reactions are reproduced in line with calculations based on the Glauber model. We study different models for $c \bar{c}$ dissociation on mesons in comparison with the experimental data of the HELIOS-3, NA38 and NA50 collaborations. Adopting absorption cross sections with mesons above the $D\bar{D}$ threshold in the order of 1.5 - 3mb we find that all data on $J/Ψ$ suppression from both proton-nucleus and nucleus-nucleus collisions can be described without assuming the formation of a quark-gluon plasma in these collisions.

hep-ph↗

arXiv1997

Analysis of Kaon Production at SIS Energies

We analyse the production and propagation of pions and kaons in heavy-ion reactions from 0.8 -- 1.8~A$\cdot$GeV within a coupled channel transport approach including the kaon production channels $BB \to K^+YN, πB\to K^+Y, BB \to NN K \bar{K}, πB\to N K\bar{K}, K^+B\to K^+B $ and $ππ\to K \bar{K}$. Assuming the hyperon selfenergy to be 2/3 of the nucleon selfenergy we find that all inclusive experimental $K^+$ spectra at SIS energies can be reproduced reasonably well without any selfenergies for the kaons although a slightly repulsive kaon potential cannot be excluded by the present data on kaon spectra and flow.

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arXiv1997

Dilepton Production from AGS to SPS Energies within a Relativistic Transport Approach

We present a nonperturbative dynamical study of $e^+e^-$ production in proton-nucleus and nucleus-nucleus collisions from AGS to SPS energies on the basis of the covariant transport approach HSD. For p + Be reactions the dilepton yield for invariant masses $M \leq 1.4$ GeV is found to be dominated by the decays of the $η, ρ, ω$ and $Φ$ mesons at all energies from 10 -- 450 GeV. For nucleus-nucleus collisions, however, the dilepton yield shows an additional large contribution from $π^+π^-$, $K^+K^-$ and $πρ$ channels. Systematic studies are presented for the 'free' meson mass scenario in comparison to a 'dropping' meson mass scenario at finite baryon density. We find that for 'dropping' meson masses the invariant dilepton mass range 0.35 GeV $\leq M \leq$ 0.65 GeV is increased in comparison to the 'free' meson mass scenario and that the data of the CERES-collaboration for nucleus-nucleus collisions can be described much better within the 'dropping' mass scheme. We study in detail the contributions from the various dilepton channels as a function of the transverse momentum and rapidity of the lepton pair as well as a function of the charged particle multiplicity. Furthermore, various direct photon channels for S + Au at 200 GeV/u are computed and found to be well below the upper bounds measured by the WA80-collaboration.

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arXiv1997

Particle Production from SIS to SPS Energies

The production and propagation of mesons ($π, η, ρ, ω, Φ, K, \bar{K}, J/Ψ$) in proton-nucleus and nucleus-nucleus collisions from 1 - 200 GeV/u is studied within the covariant transport approach HSD, which explicitly allows to investigate selfenergy effects of the hadrons at finite baryon density. Whereas the experimental pion and $K^+$ spectra can be described without introducing any selfenergies for the mesons, the $K^-$ yield in Ni + Ni collisions is underestimated by a factor of 5 - 7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon masses in line with effective chiral Lagrangians a satisfactory agreement with the data is achieved. A dropping of the $ρ$-meson mass with baryon density, as suggested by QCD sumrule studies, is proposed to explain the dilepton spectra for S + Au and Pb + Au at SPS energies, which indicates independently that a partial restoration of chiral symmetry might be found already in the present experiments.

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arXiv1997

Probing nuclear expansion dynamics with $π^-/π^+$-spectra

We study the dynamics of charged pions in the nuclear medium via the ratio of differential $π^-$- and $π^+$-spectra in a coupled-channel BUU (CBUU) approach. The relative energy shift of the charged pions is found to correlate with the pion freeze-out time in nucleus-nucleus collisions as well as with the impact parameter of the heavy-ion reaction. Furthermore, the long-range Coulomb force provides a 'clock' for the expansion of the hot nuclear system. Detailed comparisons with experimental data for $Au + Au$ at 1 GeV/A and $Ni + Ni$ at 2.0 GeV/A are presented.

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arXiv1996

Antikaon production in nucleon-nucleon reactions near threshold

The antikaon production cross section from nucleon-nucleon reactions near threshold is studied in a meson exchange model. We include both pion and kaon exchange, but neglect the interference between the amplitudes. In case of pion exchange the antikaon production cross section can be expressed in terms of the antikaon production cross section from a pion-nucleon interaction, which we take from the experimental data if available. Otherwise, a $K^*$-resonance exchange model is introduced to relate the different reaction cross sections. In case of kaon exchange the antikaon production cross section is related to the elastic $KN$ and $\bar KN$ cross sections, which are again taken from experimental measurements. We find that the one-meson exchange model gives a satisfactory fit to the available data for the $NN\to NNK\bar K$ cross section at high energies. We compare our predictions for the cross section near threshold with an earlier empirical parameterization and that from phase space models.

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arXiv1996

Medium effects in the production and decay of $ω$- and $ρ$-resonances in pion-nucleus interactions

The $ω$- and $ρ$-resonance production and their dileptonic decay in $π^- A$ reactions at GSI energies are calculated within the intranuclear cascade (INC) approach. The invariant mass distribution of the dilepton pair for each resonance is found to have two components which correspond to the decay of the resonances outside and inside the target nucleus. The latter components are strongly distorted by the nuclear medium due to resonance-nucleon scattering and a possible mass shift at finite baryon density. These medium modifications are compared to background sources in the dilepton spectrum from $πN$ bremsstrahlung and the Dalitz decays of $Δ$'s and $η$ mesons produced in the reaction.

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arXiv1996

Convergence properties of the cluster expansion for equal-time Green functions in scalar theories

We investigate the convergence properties of the cluster expansion of equal-time Green functions in scalar theories with quartic self-coupling in (0+1), (1+1), and (2+1) space-time dimensions. The computations are carried out within the equal-time correlation dynamics approach. We find that the cluster expansion shows good convergence as long as the system is in a single phase configuration and that it breaks down in a two phase configuration, as one would naively expect. In the case of dynamical calculations with a time dependent Hamiltonian we find two timescales determining the adiabaticity of the propagation; these are the time required for adiabaticity in the single phase region and the time required for tunneling into the non-localized lowest energy state in the two phase region.

hep-th↗

arXiv1996

Kaon versus Antikaon Production at SIS Energies

We analyse the production and propagation of kaons and antikaons in Ni + Ni reactions from 0.8--1.85 GeV/u within a coupled channel transport approach including the channels $BB \to K^+YN, πB\to K^+Y, BB \to NN K \bar{K}, πB\to N K\bar{K}, K^+B\to K^+B, \bar{K} B\to \bar{K}B, Y N\to \bar{K} NN, ππ\to K \bar{K}$ as well as $πY\to \bar{K}N$ and $\bar{K} N\to πY$ for the antikaon absorption. Whereas the experimental $K^+$ spectra can be reproduced without introducing any selfenergies for the mesons in Ni + Ni collisions from 0.8 to 1.8 GeV/u, the $K^-$ yield is underestimated by a factor of 5--7 at 1.66 and 1.85 GeV/u. However, introducing density dependent antikaon masses as proposed by Kaplan and Nelson, the antikaon spectra can be reasonably well described.

nucl-th↗

arXiv1996

$J/Ψ$ suppression in nuclear collisions at SPS energies

We have carried out a first calculation of $J/Ψ$ production from nuclear collisions within the covariant transport approach HSD, which has been very successful in describing both hadronic and electromagnetic observables from heavy-ion collisions at intermediate and high energies. The production of $J/Ψ$'s is based on the Lund string fragmentation model, while its interactions with hadrons are included by conventional cascade-type two-body collisions. Adopting 6 mb for the $J/Ψ$-baryon cross sections and 3 mb for the $J/Ψ$-meson cross sections above the $D\bar{D}$ threshold, we find that data on $J/Ψ$ suppression from both proton-nucleus and nucleus-nucleus collisions (including Pb + Pb) can be explained without assuming the formation of a quark-gluon plasma in these collisions. Our microscopic studies thus confirm the suggestion of Gavin et al. that $J/Ψ$ suppression observed in nuclear collisions is largely due to absorption by comovers, i.e., the produced mesons.

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arXiv1996

Antiproton production in $pp, dp$ and $dd$ collisions close to threshold

The production of antiprotons in $pp$ collisions is investigated close to threshold where experimental data about the total cross section are not available. We analyze the latter reaction within the LUND string model for inclusive $\bar{p}$ production and within the framework of a one-boson exchange model for the exclusive reaction $pp \rightarrow ppp\bar{p}$. The application of our new results to the analysis of subthreshold antiproton production in $d + p$ and $d + d$ collisions shows cross sections that are much lower than expected before. Nevertheless, the comparison of experimental $\bar{p}$ differential cross sections from $d +p$ and $d + d$ is expected to provide valuable information about a nonnucleonic component in the deuteron wavefunction.

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arXiv1996

Pion-Production in Heavy-Ion Collisions at SIS energies

We investigate the production of pions in heavy-ion collisions in the energy range of $1$ - $2$ GeV/A. The dynamics of the nucleus-nucleus collisions is described by a set of coupled transport equations of the Boltzmann-Uehling-Uhlenbeck type for baryons and mesons. Besides the $N(938)$ and the $Δ(1232)$ we also take into account nucleon resonances up to masses of $1.9 GeV/c^2$ as well as $π$-, $η$- and $ρ$-mesons. We study in detail the influence of the higher baryonic resonances and the $2π$-production channels ($NN\to NN ππ$) on the pion spectra in comparison to $π^-$ data from $Ar + KCl$ collisions at $1.8$ GeV/A and $π^0$-data for $Au+Au$ at 1.0 GeV/A. We, furthermore, present a detailed comparison of differential pion angular distributions with the BEVALAC data for Ar + KCl at 1.8 GeV/A. The general agreement obtained indicates that the overall reactions dynamics is well described by our novel transport approach.

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