Alfred Svarc

Near threshold η production in proton-proton collisions

The total cross sections for the near threshold η production in proton-proton collisions has been investigated with the assumption that the production mechanism is due to the emission of a meson x(π,η,σ) from one of the nucleons followed by a xN → ηN transition on the second nucleon. The xN → ηN amplitudes are generated from the unitary multi-channel multi-resonance model which has recently been constructed by fitting the data of πN elastic scattering and πN → ηN reaction. The initial and final pp distortions are calculated from a coupled-channel πNN model which describes the NN scattering data up to about 2 GeV. With the xNN vertex functions taken from the Bonn potential, the predicted total cross sections of threshold pp → ηpp reaction are in good agreement with the data. In contrast to previous works, we find that the η-exchange plays an important role.

The New Determination of the ηN S-wave Scattering Length From a Three-Channel, Multi-Resonance Amplitude Analysis

A three-channel, multi-resonance, unitary model developed in 1995 is used to determine the πN → ηN and ηN → ηN amplitudes by re-analyzing the available data on πN elastic scattering and the weighted data for the πN → ηN total and differential cross sections. The input πN elastic scattering amplitude in the S11 channel has been improved, following suggestions of G. Hohler. Our new result of ηN scattering length, aηN = (0.717±0.030)+i (0.263±0.025) fm, suggests that the ηd system is unbound or loosely bound.

The physical meaning of scattering matrix singularities in coupled-channel formalisms⋆

Abstract.The physical meaning of bare and dressed scattering matrix singularities has been investigated. Special attention has been attributed to the role of the well-known invariance of the scattering matrix with respect to the field transformation of the effective Lagrangian. Examples of evaluating bare and dressed quantities in various models are given.

Relevance of complex branch points for partial wave analysis

A central issue in hadron spectroscopy is to deduce--and interpret--resonance parameters, namely, pole positions and residues, from experimental data, for those are the quantities to be compared to lattice QCD or model calculations. However, not every structure in the observables derives from a resonance pole: the origin might as well be branch points, either located on the real axis (when a new channel composed of stable particles opens) or in the complex plane (when at least one of the intermediate particles is unstable). In this paper we demonstrate first the existence of such branch points in the complex plane and then show on the example of the {pi}N P{sub 11} partial wave that it is not possible to distinguish the structures induced by the latter from a true pole signal based on elastic data alone.

Pole positions and residues from pion photoproduction using the Laurent-Pietarinen expansion method

Weappliedanewapproachtodeterminethepolepositionsandresiduesfrompionphotoproductionmultipoles. The method is based on a Laurent expansion of the partial-wave T matrices, with a Pietarinen series representing the regular part of energy-dependent and single-energy photoproduction solutions. The method is applied to multipole fits generated by the MAID and George Washington University SAID (GWU-SAID) groups. We show that the number and properties of poles extracted from photoproduction data correspond very well to results from πN elastic data and values cited by the Particle Data Group (PDG). The photoproduction residues provide new information for the electromagnetic current at the pole position, which are independent of background parametrizations, which is not the case for the Breit-Wigner representation. Finally, we present the photodecay amplitudes from the current MAID and SAID solutions at the pole for all four-star nucleon resonances below W = 2 GeV.

Pole structure from energy-dependent and single-energy fits to GWU-SAID πN elastic scattering data

The pole structure of the current GW/SAID partial-wave analysis of elastic

eta N S-wave scattering length in a three-coupled-channel, multiresonance, unitary model.

\pi N

Resolution of the multichannel anomaly in the extraction of S-matrix resonance-pole parameters

scattering and

Generalization of the model-independent Laurent–Pietarinen single-channel pole-extraction formalism to multiple channels

\eta N

Poles, the only true resonant-state signals, are extracted from a worldwide collection of partial wave amplitudes using only one, well controlled pole-extraction method

production data is studied. Pole positions and residues are extracted from both the energy-dependent and single-energy fits, using two different methods. For the energy-dependent fits, both contour integration and a Laurent+Pietarinen approach are used. In the case of single-energy fits, the Laurent+Pietarinen approach is used. Errors are estimated and the two sets of results are compared to other recent and older fits to data.