arXiv subjects

U. Mosel

Publications and source records attributed to U. Mosel.

At least 199 records · Page 11Linked to original sources

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

Dynamical Properties of Flux Tubes in the Friedberg-Lee Model

A dynamical model of confinement based on a microscopic transport description of the Friedberg-Lee model is extended to explicit color degrees of freedom. The string tension is reproduced by an adiabatic string formation from the nucleon ground state. As a particular application, we address the question of how a charmonium state might be dissociated by the strong color electric fields when moving through a color electric flux tube and speculate on the importance of such an effect with respect to the issue of $J/ψ$--suppression observed in ultrarelativistic heavy ion collisions. Furthermore, we show the dynamical breakup of flux tubes via $q-\bar{q}$-particle production and the disintegration into mesons. There we encounter some problems within the Vlasov-type realization of describing the motion of the quarks which can be resolved by a molecular dynamical approach.

hep-ph↗

arXiv1997

Hadrons in Medium - Observables

The observable consequences of a lowering of the restmasses of vector mesons on dilepton spectra in the SIS energy regime (HADES) and on photoabsorption cross sections on nuclei are studied in a transporttheoretical framework. We also investigate the consequences of a mass-shift of kaons in dense matter and compare with recent SIS data. The need for a detailed understanding of the elementary processes is underlined.

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arXiv1997

J/ψ-dissociation by a color electric flux tube

We adress the question of how a $c-\bar{c}$-state (a $J/ψ$) can be dissociated by the strong color electric fields when moving through a color electric flux tube. The color electric flux tube and the dissociation of the heavy quarkonia state are both described within the Friedberg-Lee color dielectric model. We speculate on the importance of such an effect with respect to the observed $J/ψ$-suppression in ultrarelativistic heavy ion collisions.

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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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arXiv1997

Dynamical fragmentation of flux tubes in the Friedberg-Lee model

We present two novel dynamical features of flux tubes in the Friedberg-Lee model. First the fusion of two (anti-)parallel flux tubes, where we extract a string-string interaction potential which has a qualitative similarity to the nucleon-nucleon potential in the Friedberg-Lee model obtained by Koepf et al. Furthermore we show the dynamical breakup of flux tubes via $q\bar{q}-$particle production and the disintegration into mesons. We find, as a shortcoming of the present realization of the model, that the full dynamical transport approach presented in a previous publication fails to provide the disintegration mechanism in the semiclassical limit. Therefore, in addition, we present here a molecular dynamical approach for the motion of the quarks and show, as a first application, the space-time development of the quarks and their mean-fields for Lund-type string fragmentation processes.

hep-ph↗

arXiv1996

The effect of electro-magnetic formfactors on dilepton production off pp-collisions

We investigate the effect of the electric formfactor on dilepton production off pp-collisions. In the model we use, the formfactor is separated in a pion-cloud contribution and a Vector Meson Dominance part. We find that the dilepton spectrum is hardly affected by the pion-cloud part, but significantly by the VMD contribution. We point out that data with sufficient resolution would provide a crucial test of the validity of VMD for the nucleon formfactor in the timelike region.

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arXiv1996

RPA-Approach to the Excitations of the Nucleon, Part I: Theory

In this paper we develop a theoretical framework which allows us to study excitations of the nucleon. Assuming an effective two-body interaction as a model for low-energy QCD, we derive a relativistic TDHF equation for a many-body system of quarks. To render the Dirac-sea contribution tothe mean field finite, we introduce a symmetry conserving regularization scheme. In the small amplitude limit we derive an RPA equation. The structure of the $ph$ interaction and modifications due to the regularization scheme are discussed. We give a prescription to obtain a nucleon state with good angular momentum (J) and isospin (T) quantum numbers on mean-field level. To study excitations, we develop a tensor-RPA approach, which is an extension of the conventional RPA techniques to systems with a nonscalar ground state. This allows us to construct excited states with good (J/T) quantum numbers. We discuss a method to reduce the overcomplete $ph$-space and compute the tensor-RPA interaction matrix elements. Finally we extend our scheme to include ({3/2}^+, {3/2})-states.

hep-ph↗

arXiv1996

$NN \to NN π$ reaction near threshold in a covariant one-boson-exchange model

We calculate the cross sections for the $p(p,nπ^{+})p$ and $p(p,pπ^{0})p$ reactions for proton beam energies near threshold in a covariant one-boson-exchange model, which incorporates the exchange of $π$, $ρ$, $σ$ and $ω$ mesons and treats both nucleon and delta isobar as intermediate states. The final state interaction effects have also been included. The $ω$ meson exchange is found to contribute significantly at these energies, which, along with other meson exchanges, provides an excellent agreement with the data. The cross sections at beam energies $\leq$ 300 MeV are almost free from the contributions of the $Δ$ isobar excitation.

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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

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.

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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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arXiv1996

Heavy Meson Production in Proton-Nucleus Reactions with Empirical Spectral Functions

We study the production of $K^+, ρ, ω$ and $ϕ$ mesons in $p + ^{12}C$ reactions on the basis of empirical spectral functions. The high momentum, high removal energy part of the spectral function is found to be negligible in all cases close to the absolute threshold. Furthermore, the two-step process ($pN \rightarrow πN N; πN \rightarrow N + K^+, ρ, ω, ϕ$) dominates the cross section at threshold energies in line with earlier calculations based on the folding model.

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arXiv1996

Electromagnetic Couplings of Nucleon Resonances

An effective Lagrangian calculation of pion photoproduction including all nucleon resonances up to $\sqrt s = 1.7$ GeV is presented. We compare our results to recent calculations and show the influence of different width parametrizations and offshell cutoffs on the photoproduction multipoles. We determine the electromagnetic couplings of the resonances from a new fit to the multipole data.

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arXiv1996

Photoabsorption on nuclei

We calculate the total photoabsorption cross section on nuclei in the energy range from 300 MeV to 1 GeV within the framework of a semi-classical phase space 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 $N \, R \to N \, N$, needed for the calculation of collision broadening, are obtained by detailed balance from a fit to $N \, N \to N \, N \, π$ 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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arXiv1996

Probing Hadronic Polarizations with Dilepton Anisotropies

We investigate the production of $e^+e^-$ pairs from $p + Be$ and nucleus-nucleus collisions from 2 GeV/A to 200 GeV/A within a covariant transport approach and focus on the dilepton angular anisotropies as a function of the dilepton invariant mass. Whereas the low mass regime yields information about the Dalitz decays of the $Δ, η$ and $ω$, above M $\approx $0.5 GeV the angular anisotropies provide additional information about the $π^+π^- \to ρ^0 \to e^+e^-$ channel in the medium. The anisotropy coefficient is found to change its sign for M $>$ 0.5 GeV in case of nucleus-nucleus reactions when increasing the bombarding energy from 2 GeV/A to 200 GeV/A which sheds some light on the $ππ\to ρ$ dynamics.

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arXiv1996

A Dynamical Model of Color Confinement

A dynamical model of confinement based on a transport theoretical description of the Friedberg-Lee model is extended to explicit color degrees of freedom. The string tension is reproduced by an adiabatic string formation from the nucleon ground state. Color isovector oscillation modes of a $q\bar{q}$-system are investigated for a wide range of relative $q\bar{q}$-momenta and the dynamical impact of color confinement on the quark motion is shown.

hep-ph↗

arXiv1996

Realistic Interactions and Dilepton production off pp-collisions

We present a model for dilepton production of proton-proton collisions using a realist T-matrix that by incorporating Delta-isobar degrees of freedom fits the NN-scattering data up to 2 GeV. The results we find differ in details from earlier work that use less sophisticated interactions but the overall agreement with these calculations is good.

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