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U. Mosel

Publications and source records attributed to U. Mosel.

At least 145 records · Page 8Linked to original sources
arXiv2002

Hadron formation in high energy photonuclear reactions

We present a new method to account for coherence length effects in a semi-classical transport model. This allows us to describe photo- and electroproduction at large nuclei (A>12) and high energies using a realistic coupled channel description of the final state interactions that goes beyond simple Glauber theory. We show that the purely absorptive treatment of the final state interactions can lead to wrong estimates of color transparency and formation time effects in particle production. As an example, we discuss exclusive rho^0 photoproduction on Pb at a photon energy of 7 GeV as well as K^+ production in the photon energy range 1-7 GeV.

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arXiv2002

Off-shell pions in Boltzmann-Uehling-Uhlenbeck transport theory

Due to large pi N --> Delta cross section a pion acquires a large width in nuclear matter. Therefore, a Delta-resonance decaying in the nuclear medium can produce far off-shell pions, which will gradually lose their virtuality while propagating to the vacuum. To describe such off-shell dynamics, we implemented a pion spectral function for finite nuclear matter density -- calculated on the basis of the Delta-hole model -- in the BUU transport approach. The pion off-shell dynamics leads to an enhancement of the low-p_t pion yields in the collisions of Au+Au at 1 A GeV.

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arXiv2002

Pi N -> Omega N in a Coupled-Channel Approach

We describe the pi N -> omega N cross section from threshold to a center of mass energy of 2 GeV in a unitary coupled-channel model and analyze it in terms of rescattering and resonance excitations. The amplitude is mainly composed of D13, P13, and P11 contributions, where the D13 dominates over the complete considered energy range. We also outline the generalization of the standard partial-wave formalism necessary for the decomposition of the omega N final state.

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arXiv2002

Formation time of hadrons in photon induced reactions on nuclei

We present a way to account for coherence length effects in a semi-classical transport model. This allows us to describe photo- and electroproduction at large nuclei (A>12) and high energies using a realistic coupled channel description of the final state interactions that goes far beyond simple Glauber theory. We show that the purely absorptive treatment of the final state interactions as usually done in simple Glauber theory might lead to wrong estimates of color transparency and formation time effects in particle production. As an example we discuss exclusive rho^0 photoproduction on Pb at a photon energy of 7 GeV as well as K^+ and K^- production on C and Pb in the photon energy range 1-7 GeV.

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arXiv2002

What is the Correct pi^- p -> omega n Cross Section at Threshold?

The pi^- p -> omega n threshold cross section measured in the seventies resisted up to now a consistent theoretical description in line with experiment, mainly caused by too large Born contributions. This led to a discussion in the literature about the correctness of the extraction of these data points. We show that the extraction method used in these references is indeed correct and that there is no reason to doubt the correctness of these data.

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arXiv2001

Nuclear Matter Spectral Functions by Transport Theory

Quantum transport theory is used to calculate the nucleon spectral function in infinite nuclear matter. A self-consistent description is obtained by utilizing the relations between collision rates and correlation functions. Static and dynamical self-energies are taken into account in the single particle propagators. The real parts of the non-static self-energy contributions are calculated by dispersion theory thus conserving the analyticity of momentum distributions. The transport theoretical spectral functions, momentum distributions, occupation probabilities and responce functions are in close agreement with results of variational and other many-body theoretical calculations. The results indicate that the nucleon spectral functions are determined only by the average short-range correlation strength.

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arXiv2001

Photon and Electron induced Particle Production on Nuclei

We discuss several particle production reactions on nuclei within a BUU transport model. After a short presentation of the model we show our results for the transparency ratio for the nucleon knock-out reaction in different nuclei. The influence of the nucleon spectral function on subthreshold particle production is discussed.

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arXiv2001

Quenching of resonance production in heavy-ion collisions at 1-2 A GeV

We study the pion production in heavy-ion collisions at SIS energies within the coupled resonance BUU model. The standard calculation overpredicts the pion multiplicity in the central Au+Au collision at 1 A GeV by about a factor of 2, which is interpreted as a signature for the quenching of the resonance production/absorption in nucleon-nucleon collisions at high nuclear density. Results of the calculations taking into account a specific form of quenching are presented for pion spectra from light and heavy systems as well as for the pion in-plane and out-of-plane flow.

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arXiv2001

In-medium broadening of nucleon resonances

We analyze the effects of an in-medium broadening of nucleon resonances on the exclusive photoproduction of mesons on nuclei as well as on the total photoabsorption cross sections in a transport calculation. We show that the resonance widths observed in semi-inclusive photoproduction on nuclei are insensitive to an in-medium broadening of nucleon resonances. This is due to a simple effect: the sizeable width of the nuclear surface and Fermi motion.

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arXiv2001

Nuclear shadowing at low photon energies

We calculate the shadowing effect in nuclear photoabsorption at low photon energies (1-3 GeV) within a multiple scattering approach. We avoid some of the high energy approximations that are usually made in simple Glauber theory like the narrow width and the eikonal approximation. We find that the main contribution to nuclear shadowing at low energies stems from $ρ^0$ mesons with masses well below their pole mass. We also show that the possibility of scattering in non forward directions allows for a new contribution to shadowing at low energies: the production of neutral pions as intermediate hadronic states enhances the shadowing effect in the onset region. For light nuclei and small photon energies they give rise to about 30% of the total shadowing effect.

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arXiv2001

The Giessen Model - Vector Meson Production on the Nucleon in a Coupled Channel Approach

In this paper we present the extension of our unitary gauge invariant effective Lagrangian model to vector meson nucleon final states. This model already included the final states $γN$, $πN$, $2πN$, $ηN$, $K Λ$, and $K Σ$ and was developed for a simultaneous analysis of all avaible experimental data for photon- and pion-induced reactions on the nucleon. We discuss how the extension to vector meson nucleon final states is realized, outline a generalization of the standard partial wave formalism, that is applicable to any meson-/photon-baryon reaction, and show first results for $ωN$ production.

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arXiv2001

Vector Meson Decay of Baryon Resonances

We investigate the coupling of vector mesons with nucleons to nucleon resonances in an isospin-selective VMD approach and explore the in-medium properties of vector mesons.

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arXiv2001

Anti-Kaon Induced Reactions on the Nucleon

Using a previously established effective Lagrangian model we describe anti-kaon induced reactions on the nucleon. The dominantly contributing channels in the cm-energy region from threshold up to 1.72 GeV are included (K N, πΣ, πΛ). We solve the Bethe-Salpeter equation in an unitary $K$-matrix approximation.

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arXiv2001

The $ρ$ Spectral Function in a Relativistic Resonance Model

We calculate the spectral function $A_ρ$ of the $ρ$ meson in nuclear matter. The calculation is performed in the {\it low density} approximation, where the in-medium self energy $Σ_{med}$ is completely determined by the vacuum $ρN$ forward scattering amplitude. This amplitude is derived from a relativistic resonance model. In comparison to previous non-relativistic calculations we find a much weaker momentum dependence of $Σ_{med}$, especially in the transverse channel. Special attention is paid to uncertainties in the model. Thus, we compare the impact of different coupling schemes for the $RNρ$ interaction on the results and discuss various resonance parameter sets.

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arXiv2000

Role of N*(1650) in the near threshold pp --> p Lambda K+ and pp --> p Sigma0 K+ reactions

We investigate the pp --> p Lambda K+ and pp --> p Sigma0 K+ reactions at beam energies near their thresholds within an effective Lagrangian model, where the strangeness production proceeds via the excitation of N*(1650), N*(1710), and N*(1720) baryonic resonances. It is found that the $N^*$(1650) resonance dominates both these reactions at near threshold energies. The contributions from this resonance together with the final state interaction among the outgoing particles are able to explain the observed beam energy dependence of the ratio of the cross sections of the two reactions in the near threshold region.

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arXiv2000

Coupling of Baryon Resonances to the $N ω$ channel

We estimate the resonance coupling strength $f_{RNω}$ and $f_{RNρ}$ from a Vector Meson Dominance (VMD) analysis. The isoscalar and isovector part of the photon coupling are obtained separately from helicity amplitudes. The reliability of this approach is tested by comparing VMD predictions for $f_{RNρ}$ with values obtained from fitting the hadronic decay widths into $N ρ$. A reasonable agreement is found, but VMD tends to underestimate the coupling constants. In order to confirm consistency with experimental data, we calculate the cross-sections for photon-and pion induced reactions within a {\it Breit-Wigner} model. Finally, we study how the properties of $ω$ mesons in nuclear matter are affected from the excitation of resonance-hole loops. For an $ω$ at rest, we find a broadening of about 40 MeV, while at higher momenta the effect of resonance excitations is reduced.

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