arXiv subjects

W. Cassing

Publications and source records attributed to W. Cassing.

At least 163 records · Page 9Linked to original sources

arXiv2000

Excitation functions of hadronic observables from SIS to RHIC energies

We calculate excitation functions for various dynamical quantities as well as experimental observables from SIS to RHIC energies within the HSD transport approach which is based on string, quark, diquark ($q, \bar{q}, qq, \bar{q}\bar{q}$) and hadronic degrees of freedom without including any explicit phase transition to a quark-gluon plasma (QGP). It is argued that the failure of this more 'conventional' approach in comparison to experimental data should indicate the presence of a different phase which might be either attributed to space-time regions of vanishing scalar quark condensate ($ $ = 0) or to the presence of a QGP phase with strongly interacting partons. We study the $K/π$ ratio, the low mass dilepton enhancement in the invariant mass regime from 0.2 -- 1.2 GeV as well as charmonium suppression for central Au + Au collisions as a function of the bombarding energy and present predictions for these observables as well as hadron rapiditiy distributions at RHIC energies. Whereas all observables studied within HSD show smooth increasing/decreasing excitation functions, the experimental $K^+/π^+$ ratio indicates a maximum at 11 A$\cdot$GeV (or above) which is interpreted as a signature for a chirally restored phase in the course of the reaction.

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arXiv2000

Equation of state of nucleonic matter

The nuclear equation of state (EoS) is investigated by flow phenomena in relativistic heavy-ion collisions, both in transverse and radial direction, in comparison to experimental data from 150 A MeV to 11 A GeV. To this aim the collective dynamics of the nucleus-nucleus collision is described within a transport model of the coupled channel RBUU type. There are two factors which dominantly determine the baryon flow at these energies: the momentum dependence of the scalar ($U_S$) and vector potentials ($U_μ$) for baryons and the resonance/string degrees of freedom for energetic hadron excitations. We fix the explicit momentum dependence of the nucleon-meson couplings by the nucleon optical potential up to 1 GeV and extrapolate to higher energy. When assuming the optical potential to vanish identically for $E_{kin} \geq 3.5$ GeV we simultaneously describe the sideward flow data of the PLASTIC BALL, FOPI, EoS and E877 collaborations, the elliptic flow data of the E895 and E877 collaborations and approximately the rapidity and transverse mass distribution of protons at AGS energies without employing any {\it explicit} assumption on a phase transition in the EoS. However, the gradual change from hadronic to string degrees of freedom with increasing bombarding energy can be viewed as a transition from {\it hadronic} to {\it string} matter, i.e. a dissolution of hadrons at high energy density.

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arXiv1999

(p,π^+-) correlations in central heavy-ion collisions at 1-2 AGeV

The proton - charged pion correlated emission is studied in the reactions Au (1.06 AGeV) + Au, Ni (1.06 and 1.93 AGeV) + Ni and Ni (1.97 AGeV) + Cu within the BUU approach. The associated invariant mass distributions are shifted to smaller energies with respect to the free $Δ(1232)$ mass distribution due to kinematical reasons. We find that the existing and partly conflicting experimental data do not allow to draw definite conclusions on the in-medium modification of the $Δ(1232)$.

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arXiv1999

Baryon flow from SIS to AGS energies

We analyze the baryon sideward and elliptic flow from SIS (0.25 $\sim$ 2 A GeV) to AGS (2 $\sim 11.0A$GeV) energies for Au + Au collisions in the relativistic transport model RBUU that includes all baryon resonances up to a mass of 2 GeV as well as string degrees of freedom for the higher mass continuum. There are two factors which dominantly determine the baryon flow at these energies: the momentum dependence of the scalar and vector potentials and the resonance-string degrees of freedom. We fix the explicit momentum dependence of the nucleon-meson couplings within the NL3 parameter set by the nucleon optical potential up to 1 GeV of kinetic energy. When assuming the optical potential to vanish identically for $E_{kin} \geq 3.5$ GeV we simultaneously reproduce the sideward flow data of the FOPI, EOS, E895 and E877 collaborations, the elliptic flow data of the EOS, E895 and E877 collaborations, and approximately the rapidity and transverse mass distribution of protons at AGS energies. The gradual change from hadronic to string degrees of freedom with increasing bombarding energy can be viewed as a transition from {\it hadronic} to {\it string} matter, i.e. a dissolution of hadrons.

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arXiv1999

Interactions of charmed mesons with nucleons in the $\bar p d$ reaction

We study the possibility to measure the elastic $ΦN$ ($Φ\equiv J/ψ, ψ(2S), ψ(3770),χ_{2c}$) scattering cross section in the reaction $\bar p{+}d{\to}Φ{+}n_{sp}$ and the elastic $D(\bar D) N$ scattering cross section in the reaction $\bar p{+}d \to D^- D^0 p_{sp}$. Our studies indicate that the elastic scattering cross sections can be determined for $Φ$ momenta about 4-6 GeV/c and $D/ \bar D$ momenta 2 -- 5 GeV/c by selecting events with $p_t \geq 0.4$ GeV/c for $Φ$'s and $p_t(p_{sp})\geq$ 0.5 GeV/c for $D/\bar D$-meson production.

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arXiv1999

Studying the $ωN$ elastic and inelastic cross section with nucleons

We explore the possibility to measure the elastic and inelastic $ωN$ cross section in $p{+}d{\to}d{+}ω{+}p_{sp}$ and $p{+}A$ reactions. Our studies indicate that the elastic scattering cross sections can be determined for $ω$ momenta above 1 GeV/c in $p{+}d$ reactions by gating on high proton spectator momenta whereas the $ωN$ absorption cross section down to low relative $ω$ momenta is most effectively studied in $p{+}A$ reactions at beam energies 2.0--2.7 GeV

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arXiv1999

Last Call for RHIC Predictions

This paper contains the individual contributions of all speakers of the session on 'Last Call for RHIC Predictions' at Quark Matter 99, and a summary by the convenor.

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arXiv1999

Semiclassical transport of particles with dynamical spectral functions

The conventional transport of particles in the on-shell quasiparticle limit is extended to particles of finite life time by means of a spectral function A(X,\vec{P},M^2) for a particle moving in an area of complex self-energy Σ^{ret}_X = Re Σ^{ret}_X -i Γ_X/2. Starting from the Kadanoff-Baym equations we derive in first order gradient expansion equations of motion for testparticles with respect to their time evolution in \vec{X}, \vec{P} and M^2. The off-shell propagation is demonstrated for a couple of model cases that simulate hadron-nucleus collisions. In case of nucleus-nucleus collisions the imaginary part of the hadron self-energy Γ_X is determined by the local space-time dependent collision rate dynamically. A first application is presented for A + A reactions up to 95 A MeV, where the effects from the off-shell propagation of nucleons are discussed with respect to high energy proton spectra, high energy photon production as well as kaon yields in comparison to the available data from GANIL.

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arXiv1999

Probing the antikaon potential by K$^-$A elastic scattering

We extract the antikaon potential from differential data on elastic $K^-A$ scattering within Glauber theory at an antikaon momentum of 800 MeV/c. The $K^-N$ cross section at densities up to 0.1 $fm^3$ is found to be close to the value given in free space $K^-p$ and $K^-n$ interactions (averaged over the number of protons and neutrons) whereas the ratio $α$ of the real to imaginary part of the forward scattering amplitude $f(0)$ differs from the ratio $α$ in free space indicating a dropping of $Re f(0)$ at finite nuclear density. The extrapolated $K^-$ potential at a momentum of 0.8 GeV/c amounts to $\approx $--28 MeV at density $ρ_0$ which is substantially less than the values extracted from kaonic atoms at zero momentum or heavy-ion data at 300 MeV/c${\leq}p_K{\leq}$600 MeV/c. We, furthermore, investigate the perspectives of scattering experiments on $^{12}C$ at low ($\approx$300 MeV/c) and high ($\approx$1.2 GeV/c) momenta.

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arXiv1999

On the search for a narrow penta-quark Z$^+$ baryon in NN interactions

The possibility for an observation of a narrow penta-quark $Z^+$ baryon in $NN$ reactions is discussed. It is shown that the $pp{\to}nΣ^+K^+$ reaction at excess energies around 100 MeV above threshold provides optimal conditions for $Z^+$ baryon detection by an analysis of the $nK^+$ invariant mass spectrum, if the $Z^+$ mass is located around 1.5 GeV involving a rather narrow width.

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arXiv1999

The antikaon potential in nuclear matter at finite momentum

We study the antikaon potential at finite momentum for proton and neutron nuclear matter. Our approach is based on the momentum dependence of the real part of the total $K^-N$ scattering amplitude, which is fixed in free space by experimental data, using the dispersion relation approach. We decompose the contributions to the scattering amplitude from different processes and consider the $Λ$(1405) and $Σ$(1385) to be dissolved in nuclear matter at density $ρ_0$. Within this approach the $K^-$ potential is found to be attractive up to high kaon momenta. Our result is in line with the data on the antikaon potential evaluated from the analysis of kaonic atoms (low momenta), heavy-ion (interdmediate momenta) and proton-nucleus (high momenta) collisions.

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arXiv1999

$ωN$ final state interactions and $ω$-meson production from heavy-ion collisions

We calculate the elastic and inelastic $ωN\to ωN, \to πN, \to ρN, \to ρπN, \to ππN, \to σN$ reactions within a boson exchange approximation where the $ωρπ$ coupling constant and form factor are fixed by the reaction $πN \to ωN$ in comparison to the experimental data. We find rather large $ωN$ cross sections at low relative momenta of the $ω$-meson which leads to a substantial broadening of the $ω$-meson width in nuclear matter. The implications of the $ωN$ final state interactions are studied for $ω$ production in $^{12}C{+}^{12}C, ^{40}Ca{+}^{40}Ca$ and $^{58}Ni{+}^{58}Ni$ reactions at about 2 A GeV within the HSD transport approach; the drastic changes of the transverse mass spectra relative to a general m_T-scaling (for $π^0$ and $η$ mesons) might be controlled experimentally by the TAPS Collaboration.

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arXiv1999

Excitation function of strangeness in A+A reactions from SIS to RHIC energies

The properties of $K^+$ and $K^-$ mesons are studied in nuclear reactions from SIS to RHIC energies within the covariant transport approach HSD in comparison to the experimental data whenever available. Whereas kaon abundancies and spectra indicate little repulsive or vanishing selfenergies in the medium, antikaons 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 again well described by the HSD approach based on quark, diquark, string and hadronic degress of freedom. At RHIC energies hadronic rescatterings are found to enhance the strangeness yield from the partonic phase quite substantially.

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arXiv1999

e^+e^- production from pp reactions at BEVALAC energies

We have performed a detailed study of dilepton production from pp collisions including the subthreshold $ρ$ production via baryonic resonances (N(1520), N(1700)) in addition to the conventional dilepton sources as $π^0, η, ω$ and $Δ$ Dalitz decays and direct decays of vector mesons ($ρ, ω$). The role of baryonic resonances in $ρ$ production from nucleon-nucleon collisions is studied in comparison to the DLS data which are well described.

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arXiv1999

e+ e- pairs from pi- A reactions

We calculate dilepton production for the reactions $π^-$C and $π^-$Pb at 1.3 GeV within a semi-classical BUU transport model and compare our results to a previously published calculation. We show that a modified treatment of the $ρ$-meson production and propagation gives substantially different results. We, furthermore, discuss uncertainties related to the electromagnetic decay of the $ρ$-meson and the elementary $π^- N \to e^+ e^- X$ channels.

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arXiv1999

Final State Interactions in Near Threshold Meson Production from pp Collisions

We analyse the experimental data on near threshold $π$, $η$, $ω$, $η'$ and $K^+$ production from $pp$ collisions and show that all information gained so far is compatible with approximately constant production matrix elements when including the rescattering between the baryons in the final states. Different methods to include the final state interactions are discussed and their range of validity is indicated. We, furthermore, show that Dalitz plots for the proton-meson invariant mass spectra at different energies should be suited to distinguish between final state interactions and resonant production amplitudes.

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arXiv1999

On the Delta I = 1/2 rule in the Lambda N ----> N N reaction

It is shown that the mass dependence of the $Λ$-lifetime in heavy hypernuclei is sensitive to the ratio of neutron-induced to proton-induced non-mesonic decay rates R_n/R_p. A comparison of the experimental mass dependence of the lifetimes with the calculated ones for different values of R_n/R_p leads to the conclusion that this ratio is larger than 2 on the confidence level of 0.75. This suggests that the phenomenological $Δ$I=1/2 rule might be violated for the nonmesonic decay of the $Λ$-hyperon.

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arXiv1999

Damping of Collective Nuclear Motion and Thermodynamic Properties of Nuclei beyond Mean Field

The dynamical description of correlated nuclear motion is based on a set of coupled equations of motion for the one-body density matrix $ρ(11';t)$ and the two-body correlation function $c_2(12,1'2';t)$, which is obtained from the density-matrix hierarchy beyond conventional mean-field approaches by truncating 3-body correlations. The resulting equations nonperturbatively describe particle-particle collisions (short-range correlations) as well as particle-hole interactions (long-range correlations). Within a basis of time-dependent Hartree-Fock states these equations of motion are solved for collective vibrations of $^{40}Ca$ at several finite thermal excitation energies corresponding to temperatures $T=0-6$ MeV. Transport coefficients for friction and diffusion are extracted from the explicit solutions in comparison to the solutions of the associated TDHF, VUU, Vlasov or damped quantum oscillator equations of motion. We find that the actual magnitude of the transport coefficients is strongly influenced by partlicle-hole correlations at low temperature which generate large fluctuations in the nuclear shape degrees of freedom. Thermodynamically, the specific heat and the entropy of the system as a function of temperature does not differ much from the mean-field limit except for a bump in the specific heat around $T\simeq 4$ MeV which we attribute to the melting of shell effects in the correlated system.

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