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M. Effenberger

Publications and source records attributed to M. Effenberger.

At least 19 recordsLinked to original sources
arXiv2002

Resonance lifetime in Boltzmann-Uehling-Uhlenbeck theory: observable consequences

Within the transport BUU theory we study the influence of the choice for the $Δ$ resonance lifetime on pion-nucleus reactions and on heavy-ion collisions at 1 A GeV. A quite small effect of these modifications on the pion absorption on nuclei is found provided that the absorption probability of the $Δ$ resonance is modified consistently. Observable effects in the case of heavy-ion collisions are demonstrated.

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arXiv2000

Thermal and chemical equilibration of hadronic matter

We study thermal and chemical equilibration in 'infinite' hadron matter as well as in finite size relativistic nucleus-nucleus collisions using a BUU cascade transport model with resonance and string degrees-of-freedom. The 'infinite' hadron matter is simulated within a cubic box employing periodic boundary conditions. The various equilibration times depend on baryon density and energy density and are much shorter for particles consisting of light quarks then for particles including strangeness. For kaons and antikaons the chemical equilibration time is found to be larger than $\simeq$ 40 fm/c for all baryon and energy densities considered. The inclusion of continuum excitations, i.e. hadron 'strings', leads to a limiting temperature of $T_s\simeq$ 150 MeV.

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arXiv2000

Transport Theoretical Approach to the Nucleon Spectral Function in Nuclear Matter

The nucleon spectral function in infinite nuclear matter is calculated in a quantum transport theoretical approach. Exploiting the known relation between collision rates and correlation functions the spectral function is derived self-consistently. By re-inserting the spectral functions into the collision integrals the description of hard processes from the high-momentum components of wave functions and interactions is improved iteratively until convergence is achieved. The momentum and energy distributions and the nuclear matter occupation probabilities are in very good agreement with the results obtained from many-body theory.

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arXiv2000

Photoproduction of mesons in nuclei at GeV energies

In a transport model that combines initial state interactions of the photon with final state interactions of the produced particles we present a calculation of inclusive photoproduction of mesons in nuclei in the energy range from 1 to 7 GeV. We give predictions for the photoproduction cross sections of pions, etas, kaons, antikaons, and $π^+π^-$ invariant mass spectra in ^{12}C and ^{208}Pb. The effects of nuclear shadowing and final state interaction of the produced particles are discussed in detail.

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arXiv2000

Aspects of thermal and chemical equilibration of hadronic matter

We study thermal and chemical equilibration in 'infinite' hadron matter as well as in finite size relativistic nucleus-nucleus collisions using a BUU cascade transport model that contains resonance and string degrees-of-freedom. The 'infinite' hadron matter is simulated within a cubic box with periodic boundary conditions. The various equilibration times depend on baryon density and energy density and are much shorter for particles consisting of light quarks then for particles including strangeness. For kaons and antikaons the chemical equilibration time is found to be larger than $\simeq$ 40 fm/c for all baryon and energy densities considered. The inclusion of continuum excitations, i.e. hadron 'strings', leads to a limiting temperature of $T_s\simeq$ 150 MeV. We, furthermore, study the expansion of a hadronic fireball after equilibration. The slope parameters of the particles after expansion increase with their mass; the pions leave the fireball much faster then nucleons and accelerate subsequently heavier hadrons by rescattering ('pion wind'). If the system before expansion is close to the limiting temperature $T_s$, the slope parameters for all particles after expansion practically do not depend on (initial) energy and baryon density. Finally, the equilibration in relativistic nucleus-nucleus collision is considered. Since the reaction time here is much shorter than the equilibration time for strangeness, a chemical equilibrium of strange particles in heavy-ion collisions is not supported by our transport calculations. However, the various particle spectra can approximately be described within the blast model.

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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

Medium Modified Cross Sections, Temperature and Finite Momentum Effects for Antikaon Production in Heavy-Ion Collisions

The medium modifications of antikaons in dense matter are studied in a coupled channel calculation for scenarios more closely related to the environment encountered in heavy-ion collisions. We find that the optical potential of the antikaons turns repulsive or is drastically reduced for finite momenta or finite temperature. Hence, the antikaon mass does not decrease substantially in heavy-ion collisions to provide an explanation for the observed antikaon production rates at threshold. We demonstrate that the in-medium production cross section of antikaons via pions and Sigma hyperons is remarkably enhanced up to an order of magnitude. The effect of in medium correction on the resulting kaon spectra is studied within a transport model. We find that both in medium correction lead to about the same enhancement of the spectrum. However, once the temperature dependence is taken into account no enhancement is found.

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arXiv1999

e^+e^- Pair Production from $γ$ A Reactions

We present a calculation of e^+e^- production in $γA$ reactions at MAMI and TJNAF energies within a semi-classical BUU transport model. Dilepton invariant mass spectra for $γ$C, $γ$Ca and $γ$Pb are calculated at 0.8, 1.5 and 2.2 GeV. We focus on observable effects of medium modifications of the $ρ$ and $ω$ mesons. The in-medium widths of these mesons are taken into account in a dynamical, consistent way. We discuss the transport theoretical treatment of broad resonances.

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arXiv1999

Electron and Photon induced Reactions on Nuclei in the Nucleon Resonance Region

We calculate total photon-nucleus and electron-nucleus cross sections $γA\to X$, $e A\to e^\prime X$ and several cross sections on pion and eta production in nuclei in the energy regime between the first and the third resonance region for photon virtualities $Q^2\le 1.0 GeV^2$ within a semi-classical BUU transport model. In both cases we discuss the varying influence of several medium modifications on the cross sections for different values of Q^2.

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arXiv1999

Off-shell effects on particle production

We investigate the observable effects of off-shell propagation of nucleons in heavy-ion collisions at SIS energies. Within a semi-classical BUU transport model we find a strong enhancement of subthreshold particle production when off-shell nucleons are propagated.

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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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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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arXiv1997

Photoabsorption in nuclei

We calculate the total photoabsorption cross section and cross sections for inclusive pion and eta photoproduction in nuclei in the energy range from 300 MeV to 1 GeV within the framework of a semi-classical BUU transport model. Besides medium modifications like Fermi motion and Pauli blocking we focus on the collision broadening of the involved resonances. The resonance contributions to the elementary cross section are fixed by fits to partial wave amplitudes of pion photoproduction. The cross sections for $NR \to NN$, needed for the calculation of collision broadening, are obtained by detailed balance from a fit to $NN \to NNπ$ cross sections. We show that a reasonable collision broadening is not able to explain the experimentally observed disappearance of the $D_{13}$(1520)-resonance in the photoabsorption cross section on nuclei.

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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

Photoproduction of pions and etas in nuclei

We calculate the cross sections for inclusive one-pion, two-pion and eta photoproduction in nuclei in the photon energy range from 300 MeV to 900 MeV within the framework of a semi-classical BUU transport model. Our results are compared with existing experimental data and discussed with respect to a calculation of the total photoabsorption cross section.

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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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