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S. Krewald

Publications and source records attributed to S. Krewald.

72 records · Page 4Linked to original sources
arXiv2003

Lifetime of Kaonium

The kaon--antikaon system is studied in both the atomic and the strongly interacting sector. We discuss the influence of the structures of the $f_0(980)$ and the $a_0(980)$ mesons on the lifetime of kaonium. The strong interactions are generated by vector meson exchange within the framework of the standard $SU(3)_V\times SU(3)_A$ invariant effective Lagrangian. In the atomic sector, the energy levels and decay widths of kaonium are determined by an eigenvalue equation of the Kudryavtsev--Popov type, with the strong interaction effects entering through the complex scattering length for $K\bar K$ scattering and annihilation. The presence of two scalar mesons, $f_0(980)$ and $a_0(980)$, leads to a ground state energy for the kaonium atom that is shifted above the point Coulomb value by a few hundred eV. The effect on the lifetime for the kaonium decay into two pions is much more dramatic. This lifetime is reduced by two orders of magnitude from $0.8\times 10^{-16}$ sec for annihilation in a pure Coulomb field down to $3.1\times 10^{-18}$ sec when the strong interactions are included. The analysis of the two photon decay width of the $f_0(980)$ suggests a generalization of the molecular picture which reduces the lifetime of kaonium still further to $1.1\times 10^{-18}\textrm{sec}$.

hep-ph↗
arXiv2003

Indication of anomalous exchange in exclusive charmonium photoproduction

We find that available data on $J/Ψ$ photoproduction at $|t|{\ge}$1 GeV are energy independent over the range $22{\le}\sqrt{s}{\le}170$ GeV and show a very soft $t$ dependence. Through a systematic analysis of the data we detect a new trajectory, which manifests itself at threshold and is energy independent. The trajectory couples to the axial form factor of the nucleon and can reproduce the $J/Ψ$ exclusive photoproduction data at large -$t$.

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arXiv2003

Photoproduction of eta-prime-mesons from the Proton

The presently available data for the reaction $γp{\to}η^\prime{p}$ are analyzed in terms of a model in which the dominant production mechanism is the exchange of the vector mesons $ω$ and $ρ$. To describe the data at photon energies close to the production threshold we introduce a resonance contribution due to the well established $S_{11}(1535)$ resonance. Finally we study the contributions due to nucleon exchange to the $η^\prime$ photoproduction and find, that those contributions can be seen at large angles in the differential cross section.

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arXiv2003

Charmonium photoproduction

The available data on $J/Ψ$ photoproduction are analyzed in terms of pomeron exchange, two gluon exchange and photon-gluon fusion models. Allowing the pomeron-quark interaction to be flavour dependent and introducing the soft and hard pomerons it is possible to reproduce the data at $\sqrt{s}{>}$10 GeV and small $|t|$. The two gluon exchange calculations indicate strong sensitivity to the gluon distribution function. The results obtained with the most modern MRST2001 and DL PDF reproduce the forward $J/Ψ$ photoproduction cross section at $\sqrt{s}{>}$10 GeV. The calculations with the photon-gluon fusion model and with MRST2001 and DL PDF are also in reasonable agreement with the data on the total $J/Ψ$ photoproduction cross section. We allocate the $J/Ψ$ photoproduction at low energies and large $|t|$ to the mechanism different from pomeron or two gluon exchanges. We consider that this might be axial vector trajectory exchange that couples to the axial form factor of the nucleon.

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arXiv2003

Systematic Regge theory analysis of omega photoproduction

Systematic analysis of available data for $ω$-meson photoproduction is given in frame of Regge theory. At photon energies above 20 GeV the $γ{+}p{\to}ω{+}p$ reaction is entirely dominated by Pomeron exchange. However, it was found that Pomeron exchange model can not reproduce the $γ{+}p{\to}ρ{+}p$ and $γ{+}p{\to}ω{+}p$ data at high energies simultaneously with the same set of parameters. The comparison between $ρ$ and $ω$ data indicates a large room for meson exchange contribution to $ω$-meson photoproduction at low energies. It was found that at low energies the dominant contribution comes from $π$ and $f_2$-meson exchanges. There is smooth transition between the meson exchange model at low energies and Regge theory at high energies.

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arXiv2002

Omega photoproduction

Photoproduction of omega-meson is analyzed within meson exchange model and Regge model and compared to rho-meson photoproduction. An interplay between two models and uncertainties in data reproduction are discussed.

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arXiv2002

Photoproduction of the Eta Meson from the Deuteron Near Threshold

Very recent data for the reaction gamma{d} --> eta{np} are analyzed within a model that includes contributions from the impulse approximation and next order corrections due to the np and eta{N} interactions in the final state. Comparison between the calculations and data indicate sizable contributions from the np and eta{N} final state interactions close to threshold.

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arXiv2002

Incoherent Eta Photoproduction from the Deuteron near Threshold

Very recent data for the reaction gamma+d ->eta np, namely total cross sections, angular and momentum spectra, are analyzed within a model that includes contributions from the impulse approximation and next order corrections due to the np and eta-N interactions in the final state. Comparison between the calculations and the new data indicate sizable contributions from the np and eta-N final state interactions. Some systematic discrepancies between the calculations and the data are also found.

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arXiv2002

Omega photoproduction beyond the resonance region at small u

Within the meson exchange model we study $ω$ meson photoproduction at energies above the s channel resonance region. Different model prescriptions for the $ω{NN}$ vertex function are investigated imposing gauge invariance as well as crossing symmetry. The calculations reproduce the energy dependence of the differential $ω$ photoproduction cross sections at moderate $|u|$ for $E_γ{\le}4.7$ GeV, which previously were discussed as an indication of the hard interaction between the photon and quarks of the nucleon.

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arXiv2001

Eta-N Final State Interaction in incoherent Photoproduction of Eta-mesons from the Deuteron near Threshold

An analysis of incoherent photoproduction of $η$ mesons off the deuteron for photon energies from threshold to 800 MeV is presented. The dominant contribution, the $γ$N-$η$N amplitude, is described within an isobar model. Effects of the final state interactions in the $NN$ as well as the $ηN$ systems are included employing models derived within the meson-exchange approach. It is found that their consideration is important. Specifically, due to an interference effect the influence of the $ηN$ final state interaction is enhanced in the reaction $γd \to np η$ close to threshold.

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arXiv1999

What is the structure of the Roper resonance?

We investigate the structure of the nucleon resonance N^*(1440) (Roper) within a coupled-channel meson exchange model for pion-nucleon scattering. The coupling to pipiN states is realized effectively by the coupling to the sigmaN, piDelta and rhoN channels. The interaction within and between these channels is derived from an effective Lagrangian based on a chirally symmetric Lagrangian, which is supplemented by well known terms for the coupling of the Delta isobar, the omega meson and the 'sigma', which is the name given here to the strong correlation of two pions in the scalar-isoscalar channel. In this model the Roper resonance can be described by meson-baryon dynamics alone; no genuine N^*(1440) (3 quark) resonance is needed in order to fit piN phase shifts and inelasticities.

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arXiv1999

The piNN vertex function in a meson-theoretical model

The piNN vertex function is calculated within a dispersion theoretical approach, including both pi-rho and pi-sigma intermediate states, where the sigma meson is an abbreviation for a correlated pion pair in a relative S-wave with isospin I=0. A strong coupling between the pi-sigma and the pi-rho states is found. This leads to a softening of the piNN form factor. In a monopole parameterization, a cut-off Lambda=800 MeV is obtained as compared to Lambda=1000 MeV using pi-rho intermediate states only.

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arXiv1999

What does `rho-exchange' in piN scattering mean?

We present an alternative method for calculating amplitudes for correlated pi pi exchange in the ``sigma'' and rho channel in piN scattering. Starting from a fixed mass meson exchange potential, we introduce the width of the exchanged particles by integrating over a mass spectral function. The spectral functions are constructed from the pseudoempirical N\bar{N} -> pi pi data. Using this approach we develop a prescription for resolving ambiguities of the correlated pi pi exchange in the rho channel that occur when different dispersion theoretical formulations of rho exchange are used to construct piN potentials.

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arXiv1997

Strangeness in the nucleon and the ratio of proton-to-neutron neutrino-induced quasi-elastic yield

The electroweak form factors of the nucleon as obtained within a three flavor pseudoscalar vector meson soliton model are employed to predict the ratio of the proton and neutron yields from $^{12}C$, which are induced by quasi-elastic neutrino reactions. These predictions are found to vary only moderately in the parameter space allowed by the model. The antineutrino flux of the up-coming experiment determining this ratio was previously overestimated. The corresponding correction is shown to have only a small effect on the predicted ratio. However, it is found that the experimental result for the ratio crucially depends on an accurate measurement of the energy of the knocked out nucleon.

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arXiv1996

Bulk properties of light deformed nuclei derived from a medium-modified meson-exchange interaction

Deformed Hartree-Fock-Bogoliubov calculations for finite nuclei are carried out. As residual interaction, a Brueckner G-matrix derived from a meson-exchange potential is taken. Phenomenological medium modifications of the meson masses are introduced. The binding energies, radii, and deformation parameters of the Carbon, Oxygen, Neon, and Magnesium isotope chains are found to be in good agreement with the experimental data.

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arXiv1996

Final State Charge Exchange Interactions in the $^{12}C(e,e'p)$ Reaction

The $^{12}C(e,e'p)$ reaction is analyzed in a model which explicitly includes final state interactions due to the coupling of the proton and neutron emission channels. We find that the effects of the final state interactions due to charge exchange reactions are important to get a good description of the symmetry properties of the recently measured Mainz spectral functions. We discuss the possible role the off-shell effects may play for the correct interpretation of spectral functions at large positive missing momenta.

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arXiv1995

Faddeev approach to the octet and decuplet baryons

A relativistic Faddeev model for the baryon octet is extended to treat the baryon decuplet. We find that after determining the model parameters in the mesonic sector the masses of both nucleon and delta deviate by less than 5\% from the experimental data and show only a very weak dependence on the constituent quark mass.

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