arXiv subjects

U. Mosel

Publications and source records attributed to U. Mosel.

At least 127 records · Page 7Linked to original sources

arXiv2004

In-medium properties of the omega meson through omega photoproduction in nuclei

The feasibility to study the in-medium properties of the omega meson in photon induced reactions on nuclei is investigated by means of a semiclassical transport model. The calculations are dedicated to an experiment presently being analyzed by the TAPS/Crystal Barrel collaborations. A full coupled-channel treatment of final state interactions allows a quantitative analysis of the influence of absorption and elastic scattering of the produced hadrons and likewise the application of kinematic cuts on the observables. Within the scope of our calculations, the preliminary data by the TAPS/Crystal Barrel collaborations indeed seem to indicate a modification of the isoscalar spectral density in the strongly interacting environment.

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arXiv2003

In-medium Properties of Hadrons -- Observables

We first briefly review the theoretical basis for calculations of changes of hadronic properties in dense nuclear matter. These changes have usually been investigated by means of relativistic heavy-ion reactions. Here we discuss that observable consequences of such changes can also be seen in more elementary reactions on nuclei. Particular emphasis is put on a discussion of actual observables in photonuclear reactions; we discuss in detail $η$- and vector-meson production. We show that photoproduction of $η$'s can yield essential information on in-medium properties of the $S_{11}(1535)$ resonance while the $ϕ$ meson properties will probably not be accessible through the $K^+K^-$ decay channel. However, for $ω$ mesons the $π^0γ$ decay channel, due to its reduced final state interaction, looks more promising in this respect. Completely free of final state interactions is dilepton production in the few GeV range. We show that the sensitivity of this decay channel to changes of hadronic properties in medium in photonuclear reactions on nuclei is as large as in ultrarelativistic heavy ion collisions. Finally we discuss that hadron production in nuclei at 10 -- 20 GeV photon energies can give important information on the hadronization process.

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arXiv2003

Inclusive omega photoproduction off nuclei

We investigate inclusive omega photoproduction off complex nuclei, concentrating on the feasibility to examine a possible in-medium change of the omega meson properties by observing the pi^0 gamma invariant mass spectrum. The simulations are performed by means of a BUU transport model including a full coupled-channel treatment of the final state interactions. In-medium changes of the omega spectral density are found to yield a moderate modification of the observables as compared to the situation in free space. Also the effects of a momentum dependence of the strong omega potential are discussed.

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arXiv2003

Influence of the nucleon spectral function in photon and electron induced reactions on nuclei

We study the influence of the nucleon spectral function on eta photo- and electroproduction on nuclei. Besides kinematical effects due to groundstate correlations, also a modification of the S11(1535) decay width is taken into account, which is caused by the possible decay into nucleons with mass smaller than the pole mass in the medium. Hence, resonances with masses below the free N eta threshold can contribute to eta production.

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arXiv2003

Hadron formation in electron induced reactions at HERMES energies

We investigate meson electroproduction off complex nuclei in the kinematic regime of the HERMES experiment using a semi-classical transport model which is based on the Boltzmann-Uehling-Uhlenbeck (BUU) equation. We discuss coherence length and color transparency effects in exclusive rho^0 production as well as hadron formation and attenuation of charged pions, kaons, protons and anti-protons in deep inelastic lepton scattering off nuclei.

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arXiv2003

The N N -> N Delta cross section in nuclear matter

We present calculations of the N N -> N Delta cross section in nuclear matter within the one-pion exchange model taking into account pion collectivity, vertex renormalization by the contact nuclear interactions and Dirac effective masses of the baryons due to coupling with the scalar sigma field. Introducing the Dirac effective masses leads to an in-medium reduction of the cross section. The experimental data on pion multiplicities from the collisions of Ca+Ca, Ru+Ru and Au+Au at 0.4-1.5 A GeV are well described by BUU calculations with the in-medium cross section.

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arXiv2003

In-medium modifications of the S11(1535) resonance and eta photoproduction

We investigate the influence of possible in-medium modifications of the S11(1535) in eta photoproduction in nuclei. Besides Fermi motion, Pauli blocking and binding effects also collisional broadening is accounted for. The in-medium width is obtained from a realistic resonance-hole model. Results on eta photoproduction are obtained from a semi-classical BUU transport model and compared with data. We find that calculations including a momentum dependent nucleon and resonance potential are in agreement with the recent KEK data. In contrast, collisional broadening has only little influence.

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arXiv2003

Electroproduction of hadrons on nuclei at GeV energies

We investigate coherence length effects and hadron attenuation in lepton scattering off nuclei in the kinematic regime of the HERMES experiment. The elementary electron-nucleon interaction is described within the event generator PYTHIA while a full coupled-channel treatment of the final state interactions is included by means of a BUU transport model. The results of our calculations are in good agreement with the experimentally measured transparency ratio of incoherent rho0 electroproduction off Nitrogen and the multiplicity ratio R_M^h for charged hadrons, pions, kaons, protons and anti-protons in deep inelastic scattering off Nitrogen and Krypton targets.

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arXiv2003

Ground state correlations in Deep Inelastic Scattering and the Drell-Yan process

Non-perturbative corrections to the LO cross section formulae for DIS and the Drell-Yan process were calculated. The interaction of partons was taken into account via dressing the incoming quark lines with spectral functions. We found the effect of ground state correlation in DIS to be large in the region of small Bjorken x and low momentum transfer Q. For a quark width of the order of 200 MeV, the deviation from parton model cross section reaches as much as 50% for Q^2=10 GeV^2. On the other hand, for the values of Q well above the resonance region, the effect of ISI turned out to be small in DIS, but still substantial for the triple differential Drell-Yan cross section. Our calculations show that the shape of the high mass Drell-Yan pair transverse momentum distribution is fully determined by quark off-shellness. From comparison to the resent data on the Drell-Yan cross section from NuSea collaboration, we obtained for the quark width in the nucleon the value about 200 MeV.

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arXiv2003

Eta electroproduction on nuclei in the nucleon resonance region

We investigate eta electroproduction on nuclei for Q^2=2.4 and 3.6 GeV^2 in the framework of a coupled-channel BUU transport model. We analyze the importance of final state interactions and side feeding and compare with findings drawn from eta photoproduction. It is shown that in contrast to photoproduction the influence of etas stemming from secondary processes becomes important at high Q^2.

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arXiv2003

Role of baryonic resonances in the dilepton emission in nucleon-nucleon collisions

Within an effective Lagrangian model, we present calculations for cross sections of the dilepton production in proton-proton and proton-neutron collisions at laboratory kinetic energies in 1-5 GeV range. Production amplitudes include contributions from the nucleon-nucleon bremsstrahlung as well as from the mechanism of excitation, propagation, and radiative decay of Delta(1232) and N*(1520) intermediate baryonic resonances. It is found that the delta isobar terms dominate the cross sections in the entire considered beam energy range. Our calculations are able to explain the data of the DLS collaboration on the dilepton production in proton-proton collisions for beam energies below 1.3 GeV. However, for incident energies higher than this the inclusion of contributions from other dilepton sources like Dalitz decay of pi0 and eta mesons, and direct decay of rho and omega mesons is necessary to describe the data.

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arXiv2003

Incoherent rho^0 electroproduction off nuclei

In the present paper we investigate incoherent rho^0 electroproduction off complex nuclei. We derive a novel, simple expression for the incoherent electroproduction cross section in which one can clearly separate the final state interactions of the reaction products from the 'initial state interactions' of the photon that give rise to nuclear shadowing. In the special case of purely absorptive final state interactions we deduce from our expression the known Glauber result. A more realistic treatment of the final state interactions within a transport model is then used to compare our predictions with experimental data from the HERMES experiment.

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arXiv2003

Vector meson production and nucleon resonance analysis in the Giessen coupled-channel model

We present a nucleon resonance analysis by simultaneously considering all pion- and photon-induced experimental data on the final states $γN$, $πN$, $2πN$, $ηN$, $KΛ$, $KΣ$, and $ωN$ for energies from the nucleon mass up to $\sqrt s = 2$ GeV. In this analysis we find strong evidence for various higher lying resonances as, e.g., the $P_{13}(1900)$. The $ωN$ production mechanism is dominated by large $P_{11}(1710)$ and $P_{13}(1900)$ contributions.

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arXiv2003

Spin-5/2 resonance effects in a coupled channel nucleon resonance analysis

Spin-$\ffh$ resonance contributions in pion-nucleon scattering are investigated within the coupled channel $K$-matrix approach for c.m. energies up to $\sqrt s=$ 2 GeV. All previously studied $πN$, $2πN$, $ηN$, $K Λ$, $K Σ$, and $ωN$ final states are included. We find a significant improvement of $χ^2$ in almost all channels by inclusion of the spin-$\ffh$ states. The obtained coupling parameters are discussed.

hep-ph↗

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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arXiv2002

Photoproduction of phi mesons from nuclei

We investigate the consequences of possible medium modifications of the phi meson at finite nuclear matter density on the K+K- mass distribution in photonuclear reactions. The inclusive cross sections for K+K- pair production are calculated within a semi--classical BUU transport model, which combines the initial state interaction of the incoming photon with the final state interactions of the produced particles. The effects of final state interactions on the invariant mass distribution of the observed K+K- pairs are discussed in detail. In addition we calculate the Coulomb correction and possible effects of hadronic kaon potentials on the K+K- mass spectrum. Due to the large cross sections for reactions of the final state particles with the surrounding nuclear medium and the influence of the Coulomb potential we find no measurable sensitivity of the observables to the phi properties at finite baryon density.

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arXiv2002

Vector meson production and nucleon resonance analysis in a coupled-channel approach for energies m_N < sqrt(s) < 2 GeV I: pion-induced results and hadronic parameters

We present a nucleon resonance analysis by simultaneously considering all pion- and photon-induced experimental data on the final states gamma N, pi N, 2 pi N, eta N, K Lambda, K Sigma, and omega N for energies from the nucleon mass up to sqrt(s) = 2 GeV. In this analysis we find strong evidence for the resonances P_{31}(1750), P_{13}(1900), P_{33}(1920), and D_{13}(1950). The omega N production mechanism is dominated by large P_{11}(1710) and P_{13}(1900) contributions. In this first part, we present the results of the pion-induced reactions and the extracted resonance and background properties with emphasis on the difference between global and purely hadronic fits.

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arXiv2002

Vector meson production and nucleon resonance analysis in a coupled-channel approach for energies m_N < sqrt(s) < 2 GeV II: photon-induced results

We present a nucleon resonance analysis by simultaneously considering all pion- and photon-induced experimental data on the final states gamma N, pi N, 2 pi N, eta N, K Lambda, K Sigma, and omega N for energies from the nucleon mass up to sqrt(s) = 2 GeV. In this analysis we find strong evidence for the resonances P_{31}(1750), P_{13}(1900), P_{33}(1920), and D_{13}(1950). The omega N production mechanism is dominated by large P_{11}(1710) and P_{13}(1900) contributions. In this second part we present the results on the photoproduction reactions and the electromagnetic properties of the resonances. The inclusion of all important final states up to sqrt(s) = 2 GeV allows for estimates on the importance of the individual states for the GDH sum rule.

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