arXiv subjects

U. Mosel

Publications and source records attributed to U. Mosel.

At least 163 records · Page 9Linked to original sources

arXiv2000

Perspectives of $e^+e^-$ production in pp, pd and p Be reactions at SIS energies

We study dilepton production from pp, pd and p Be collisions from 1 - 5 GeV including the $π^0$, $η$, $ω$ and $Δ$ Dalitz decays, direct decays of vector mesons ($ρ$, $ω$) as well as subthreshold $ρ$ production via baryonic resonances (e.g. $D_{13}(1520), P_{11}(1710)$). Our calculations compare rather well with the pp and pd data from the DLS Collaboration, however, overestimate slightly the 'old' p Be data from that group. Futhermore, detailed predictions for differential dilepton spectra at SIS energies are made with a high mass resolution that can be controlled experimentally by the HADES Collaboration in near future.

nucl-th↗

arXiv2000

Squeeze-out of nuclear matter in peripheral heavy-ion collisions and momentum-dependent effective interactions

We perform a systematic study of in-plane and out-of-plane proton and neutron flow from nucleus-nucleus collisions within the BUU transport approach employing different parameter sets for the mean-field potentials that can be characterized by the nuclear incompressibility and the stiffness of the momentum-dependent potential. We find that a simultaneous description of the experimental data from the BEVALAC and the SIS on both the nucleon squeeze-out $v_2$ and the in-plane flow $F$ at beam energies $E_{lab} = 0.15 ÷2$ AGeV requires a mean field with strong momentum dependence, i.e. an effective Landau mass $m^* \simeq 0.68 m_0$ at normal nuclear matter density $ρ_0=0.17$ fm$^{-3}$, where $m_0=0.938$ GeV is the bare nucleon mass. Some experimental data on the squeeze-out require an even stiffer momentum dependence ($m^* \approx 0.62 m_0$). All systems investigated are found to be compatible with $m^*/m_0 = 0.65 \pm 0.03$.

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arXiv2000

Low energy kaon photoproduction from nuclei

We study $K^+$-meson production in $γ{A}$ interaction at energies below the reaction threshold in free space. The Thomas-Fermi and spectral function approaches are used for the calculations of the production process. It is found that the measurement of the differential spectra may allow to reconstruct the production mechanism and to investigate the dispersion relations entering the production vertex. It is shown that the contribution from secondary pion induced reactions to the total kaon photoproduction is negligible for $E_γ{<}$1.2 GeV so that strangeness production at low energies is sensitive to the nuclear spectral function.

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arXiv2000

Nucleon Resonances in Meson Nucleon Scattering with Strangeness Production

An effective Lagrangian model in a coupled channels framework is applied to extract nucleon resonance parameters. In the K-matrix approximation, we simultaneously analyze all the available data for the transitions from pi N to five possible meson-baryon final states, pi N, pipi N, eta N, K Lambda, and KSigma, in the energy range from pi N threshold up to W = 2 GeV. In this work, we focus our efforts on the K Sigma channel. In particular, we include a set of Delta resonances around 1900 MeV: the S31(1900), P31(1910), and P33(1920) states. In most cases the parameters of nucleon resonances determined by the fits are consistent with the commonly accepted values in the literature. We find that in the low-energy region, the S11(1650), P11(1710), and P13(1720) states are important for the p(pi-,K0)Sigma0 and p(pi-,K+)Sigma- processes while the P33(1600) and D33(1700) states are significant for the pure I=3/2 reaction p(pi+,K+)Sigma+. In the high energy region, we find a D13 state at 1945 MeV which is important for the p(pi-,K0)Sigma0 reaction while the Delta resonances around 1900 MeV show up significantly in the p(pi-,K+)Sigma- and p(pi+,K+)Sigma+ processes.

nucl-th↗

arXiv2000

Missing Nucleon Resonances in Kaon Production with Pions and Photons

Resonances with strong couplings to strangeness channels are investigated in a coupled-channels framework of kaon production with pions and photons. We find evidence for an additional D13 resonance around 1900 MeV that has a significant decay width into the K Lambda final state. Such a state has been predicted in a relativistic quark model at 1960 MeV with significant gamma N and K Lambda branching ratios.

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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.

nucl-th↗

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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arXiv2000

Low energy onset of nuclear shadowing in photoabsorption

The early onset of nuclear shadowing in photoabsorption at low photon energies has recently been interpreted as a possible signature of a decrease of the rho meson mass in nuclei. We show that one can understand this early onset within simple Glauber theory if one takes the negative real part of the rho N scattering amplitudes into account, corresponding to a higher effective mass of the rho meson in nuclear medium.

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arXiv2000

Hadronic in-Medium Effects with Elementary Probes

The sensitivity of dilepton production in elementary reactions of photons and pions with nuclei on in-medium changes of hadronic properties is studied. It is shown that this sensitivity is comparable to that encountered in ultrarelativistic heavy-ion collisions. It is also shown how a significant broadening of the mesons affects other photonuclear processes.

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arXiv2000

Hadronization in the Chromodielectric Model

We study the dynamics of quarks and gluons in the Chromodielectric Model (CDM), which contains a dynamical description of confinement. Results are presented for hadronization out of an equilibrated quark--gluon plasma. It is shown that hadronization is complete and hadrons are classified within the scheme of CDM.

hep-ph↗

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

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.

nucl-th↗

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.

nucl-th↗