S. Janssen

New results on the Roper resonance and the P-11 partial wave

Abstract Properties of the Roper resonance, the first scalar excitation of the nucleon, are determined. Pole positions and residues of the P 11 partial wave are studied in a combined analysis of pion- and photo-induced reactions. We find the Roper pole at { ( 1371 ± 7 ) − i ( 92 ± 10 ) } MeV and an elasticity of 0.61 ± 0.03 . The largest decay coupling is found for the Nσ ( σ = ( π π ) - S -wave). The analysis is based on new data on γ p → p π 0 π 0 for photons in the energy range from the two-pion threshold to 820 MeV from TAPS at Mainz and from 0.4 to 1.3 GeV from Crystal Barrel at Bonn and includes further data from other experiments. The partial wave analysis excludes the possibility that the Roper resonance is split into two states with different partial decay widths.

Photoproduction of πmesons from nuclei

Abstract.Photoproduction of neutral pions from nuclei (carbon, calcium, niobium, lead) has been studied for incident-photon energies from 200 MeV to 800 MeV with the TAPS detector using the Glasgow photon tagging spectrometer at the Mainz MAMI accelerator. Data were obtained for the inclusive photoproduction of neutral pions and the partial channels of quasifree single-π0, double-π0, and π0π± photoproduction. They have been analyzed in terms of the in-medium behavior of nucleon resonances and the pion-nucleus interaction. They are compared to earlier measurements from the deuteron and to the predictions of a Boltzmann-Uehling-Uhlenbeck (BUU) transport model for photon-induced pion production from nuclei.

Double pi 0 photoproduction off the proton at threshold

The reaction gammap --> pi(0)pi(0)p has been measured using the TAPS BaF2 calorimeter at the tagged photon facility of the Mainz Microtron accelerator. Chiral perturbation theory (ChPT) predicts that close to threshold this channel is significantly enhanced compared to double pion final states with charged pions. In contrast to other reaction channels, the lower order tree terms are strongly suppressed in 2pi(0) photoproduction. The consequence is the dominance of pion loops in the 2pi(0) channel close to threshold-a result that opens new prospects for the test of ChPT and in particular its inherent loop terms. The present measurement is the first which is sensitive enough for a conclusive comparison with the ChPT calculation and is in agreement with its prediction. The data also show good agreement with a calculation in the unitary chiral approach

A genetic algorithm analysis of N∗ resonances in p(γ,K+)Λ reactions

The problem of extracting information on new and known

The role of hyperon resonances in p(γ, K+)Λ processes

N^{*}

Electric and magnetic form factors of strange baryons

resonances by fitting isobar models to photonuclear data is addressed. A new fitting strategy, incorporating a genetic algorithm, is outlined. As an example, the method is applied to a typical tree-level analysis of published

Double pion photoproduction off 40Ca

p(\gamma,K^{+})\Lambda

Spin-dependent correlations and the semi-exclusive O-16(e, e ' p) reaction.

data. It is shown that, within the limitations of this tree-level analysis, a resonance in addition to the known set is required to obtain a reasonable fit. An additional

Strangeness production on the nucleon

P_{11}

The reaction γp → π0γ'p and the magnetic dipole moment of the Δ+(1232) resonance

resonance, with a mass of about 1.9 GeV, gives the best agreement with the published data, but additional