arXiv subjects

S. Krewald

Publications and source records attributed to S. Krewald.

At least 37 records · Page 2Linked to original sources

arXiv2010

The reaction pi+ p --> K+ Sigma+ in a unitary coupled-channels model

Elastic pi N scattering and the reaction pi+ p --> K+ Sigma+ are described simultaneously in a unitary coupled-channels approach which respects analyticity. SU(3) flavor symmetry is used to relate the t- and u-channel exchanges that drive the meson-baryon interaction in the different channels. Angular distributions, polarizations, and spin-rotation parameters are compared with available experimental data. The pole structure of the amplitudes is extracted from the analytic continuation.

nucl-th↗

arXiv2010

Analysis of recent eta photoproduction data

Recent data on eta-meson photoproduction off a proton target in the energy range 2 < sqrt{s} < 3 GeV are analyzed with regard to their overall consistency. Results from the ELSA and CLAS measurements are compared with predictions of a Regge model whose reaction amplitude was fixed via a global fit to pre-2000 measurements of differential cross sections and polarization observables for gamma p -> eta p at higher energies. We find that all recent experimental results on differential cross sections for eta-meson photoproduction are in good agreement with each other, except for the CLAS data from 2009. However, the latter can be made consistent with the other data at the expense of introducing an energy-dependent renormalization factor. We point out that there indications in the data for a possible excitation of baryon resonances with masses around 2.1 and 2.4 GeV.

nucl-th↗

arXiv2010

Pion photoproduction in a dynamical coupled-channel model

Pion photoproduction reactions are investigated in a dynamical coupled-channel approach based on the Juelich pi-N model, which presently includes the hadronic pi-N and eta-N stable channels as well as the pi-Delta, sigma-N and rho-N effective channels. This model has been quite successful in the description of pi-N to pi-N scattering for center-of-mass energies up to 1.9 GeV. The full pion photoproduction amplitude is constructed to satisfy the generalized Ward-Takahashi identity and hence, it is fully gauge invariant. The calculated differential cross sections and photon spin asymmetries up to 1.65 GeV center-of-mass energy for the reactions gamma p to pi+ n, gamma p to pi0 p and gamma n to pi- p are in good agreement with the experimental data.

nucl-th↗

arXiv2010

Saturation of nuclear matter in effective field theory

The two- and three-nucleon interaction derived in chiral effective field theory at next-to-next-to-leading order is used to obtain the binding energy of nuclear matter. Saturation is found at a binding energy per particle E/A = -16.2 \pm 0.3 MeV and a Fermi momentum k_F = 1.30 \pm 0.03 fm^{-1}, where the uncertainty is due to the cut-off dependence of the two-nucleon interaction. The sensitivity of these values to the three-nucleon force is also studied.

nucl-th↗

arXiv2010

Proton-proton scattering above 3 GeV/c

A large set of data on proton-proton differential cross sections, analyzing powers and the double polarization parameter A_NN is analyzed employing the Regge formalism. We find that the data available at proton beam momenta from 3 GeV/c to 50 GeV/c exhibit features that are very well in line with the general characteristics of Regge phenomenology and can be described with a model that includes the rho, omega, f_2, and a_2 trajectories and single Pomeron exchange. Additional data, specifically for spin-dependent observables at forward angles, would be very helpful for testing and refining our Regge model.

hep-ph↗

arXiv2010

Calculations of the isoscalar giant monopole resonance within the self-consistent RPA and its extensions

The strength distributions of the isoscalar giant monopole resonance have been calculated in $^{16}$O, $^{40}$Ca, $^{90}$Zr, $^{112-124}$Sn, $^{144}$Sm, and $^{208}$Pb nuclei within the self-consistent random phase approximation and its extensions which include pairing correlations and quasiparticle-phonon coupling. The results are compared with the available experimental data. The problem of the nuclear matter incompressibility is discussed.

nucl-th↗

arXiv2010

Primakoff effect in eta-photoproduction off protons

We analyse data on forward eta-meson photoproduction off a proton target and extract the eta to gamma gamma decay width utilizing the Primakoff effect. The hadronic amplitude that enters into our analysis is strongly constrained because it is fixed from a global fit to available gamma p to p eta data for differential cross sections and polarizations. We compare our results with present information on the two-photon eta-decay from the literature. We provide predictions for future PrimEx experiments at Jefferson Laboratory in order to motivate further studies.

hep-ph↗

arXiv2009

Backward pion-nucleon scattering

A global analysis of the world data on differential cross sections and polarization asymmetries of backward pion-nucleon scattering for invariant collision energies above 3 GeV is performed in a Regge model. Including the $N_α$, $N_γ$, $Δ_δ$ and $Δ_β$ trajectories, we reproduce both angular distributions and polarization data for small values of the Mandelstam variable $u$, in contrast to previous analyses. The model amplitude is used to obtain evidence for baryon resonances with mass below 3 GeV. Our analysis suggests a $G_{39}$ resonance with a mass of 2.83 GeV as member of the $Δ_β$ trajectory from the corresponding Chew-Frautschi plot.

nucl-th↗

arXiv2009

Charmed Exotics

These are the mini-proceedings of the CHARMEX workshop. The meeting focused on recent developments in charm spectroscopy, especially on the possible role of the states that do not fit into the quark model classification, the so-called exotic states. The goal of this write-up is to provide the community with a short summary of the individual talks as well as a comprehensive, up-to-date list of relevant references.

hep-ph↗

arXiv2009

The role of the background in the extraction of resonance contributions from meson-baryon scattering

The separation of background and resonance contributions in pion-nucleon scattering is an often discussed issue. We investigate to what extent the background can be separated from the pole contribution. For illustration we use results from an analytic model for the meson-baryon interaction derived from meson exchange. We focus on the two distinct cases of an elastic and a highly inelastic resonance, namely the Delta(1232) and the Delta*(1700). Our results are also relevant for studies of dynamically generated resonances and attempts to extract bare quantities from hadronic models to be compared to quark model results.

nucl-th↗

arXiv2009

Analytic properties of the scattering amplitude and resonances parameters in a meson exchange model

The analytic properties of scattering amplitudes provide important information. Besides the cuts, the poles and zeros on the different Riemann sheets determine the global behavior of the amplitude on the physical axis. Pole positions and residues allow for a parameterization of resonances in a well-defined way, free of assumptions for the background and energy dependence of the resonance part. This is a necessary condition to relate resonance contributions in different reactions. In the present study, we determine the pole structure of pion-nucleon scattering in an analytic model based on meson exchange. For this, the sheet structure of the amplitude is determined. To show the precision of the resonance extraction and discuss phenomena such as resonance interference, we discuss the S11 amplitude in greater detail.

nucl-th↗

arXiv2009

Neutral pion photoproduction at high energies

A Regge model with absorptive corrections is employed in a global analysis of the world data on the reactions gamma p -> pi0 p and gamma n -> pi0 n for photon energies from 3 to 18 GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross sections and single- and double polarization observables at -t < 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3 GeV. A detailed comparison with recent data from the CLAS and CB-ELSA Collaborations in that energy region is presented. Furthermore, the prospects for determining the pi0 radiative decay width via the Primakoff effect from the reaction gamma p -> pi0 p are explored.

hep-ph↗

arXiv2009

Backward pion photoproduction

We present a systematic analysis of backward pion photoproduction for the reactions gamma p -> pi0 p and gamma p -> pi+ n. Regge phenomenology is applied at invariant collision energies above 3 GeV in order to fix the reaction amplitude. A comparision with older data on pi0- and pi+ photoproduction at theta = 180 degrees indicates that the high-energy limit as given by the Regge calculation could be reached possibly at energies of around sqrt{s} = 3 GeV. In the energy region of sqrt{s} < 2.5 GeV, covered by the new measurements of gamma p -> pi0 p differential cross sections at large angles at ELSA, JLab, and LEPS, we see no clear signal for a convergence towards the Regge results. The baryon trajectories obtained in our analysis are in good agreement with those given by the spectrum of excited baryons.

hep-ph↗

arXiv2008

Description of the Giant Monopole Resonance in the Even-A $^{112-124}$Sn Isotopes within the Microscopic Model Including Quasiparticle-Phonon Coupling

We have calculated the strength distributions of the giant monopole resonance in the even-A tin isotopes (A = 112-124) which were recently measured in inelastic $α$-scattering. The calculations were performed within two microscopic models: the quasiparticle random phase approximation (QRPA) and the quasiparticle time blocking approximation which is an extension of the QRPA including quasiparticle-phonon coupling. We used a self-consistent calculational scheme based on the HF+BCS approximation. The single-particle continuum was exactly included on the RPA level. The self-consistent mean field and the effective interaction were derived from the Skyrme energy functional. In the calculations, two Skyrme force parametrizations were used. The T5 parametrization with comparatively low value of the incompressibility of infinite nuclear matter ($K_{\infty}$ = 202 MeV) gives theoretical results in good agreement with the experimental data including the resonance widths.

nucl-th↗

arXiv2008

Pion-nucleon charge-exchange amplitudes above 2 GeV

The amplitudes for the pion-nucleon charge exchange reaction of the Karlsruhe-Helsinki and the George-Washington-University partial wave analyses are compared with those of a Regge-cut model with the aim to explore the possibility to provide high energy constraints for theoretical baryon resonance analyses in the energy region above 2 GeV.

hep-ph↗

arXiv2008

Self-consistent calculations within the Green's function method including particle-phonon coupling and the single-particle continuum

The Green's function method in the \emph{Quasiparticle Time Blocking Approximation} is applied to nuclear excitations in $^{132}$Sn and $^{208}$Pb. The calculations are performed self-consistently using a Skyrme interaction. The method combines the conventional RPA with an exact single-particle continuum treatment and considers in a consistent way the particle-phonon coupling. We reproduce not only the experimental values of low- and high-lying collective states but we also obtain fair agreement with the data of non-collective low-lying states that are strongly influenced by the particle-phonon coupling.

nucl-th↗

arXiv2007

Regge approach to charged-pion photoproduction at invariant energies above 2 GeV

A Regge model with absorptive corrections is employed in a global analysis of the world data on positive and negative pion photoproduction for photon energies from 3 to 8 GeV. In this region resonance contributions are expected to be negligible so that the available experimental information on differential cross sections and single polarization observables at -t \leq 2 GeV^2 allows us to determine the non-resonant part of the reaction amplitude reliably. The model amplitude is then used to predict observables for photon energies below 3 GeV. Differences between our predictions and data in this energy region are systematically examined as possible signals for the presence of excited baryons. We find that the data available for the polarized photon asymmetry show promising resonance signatures at invariant energies around 2 GeV. With regard to differential cross sections the analysis of negative pion photoproduction data, obtained recently at JLab, indicates likewise the presence of resonance structures around 2 GeV

nucl-th↗

arXiv2007

Self-consistent calculations within the Extended Theory of Finite Fermi Systems

The Extended Theory of Finite Fermi Systems(ETFFS) describes nuclear excitations considering phonons and pairing degrees of freedom, using experimental single particle energies and the effective Landau-Migdal interaction. Here we use the Skyrme interactions in order to extend the range of applicability of the ETFFS to experimentally not yet investigated short-lived isotopes. We find that Skyrme interactions which reproduce at the mean field level both ground state properties and nuclear excitations are able to describe the spreading widths of the giant resonances in the new approach, but produce shifts of the centroid energies. A renormalization of the Skyrme interactions is required for approaches going beyond the mean field level.

nucl-th↗