B. Schoch

First measurement of the Gerasimov-Drell-Hearn integral for Hydrogen from 200 to 800 MeV

A direct measurement of the helicity dependence of the total photoabsorption cross section on the proton was carried out at MAMI (Mainz) in the energy range 200<E_gamma<800 MeV. The experiment used a 4

Quasifree photoproduction of eta mesons off the neutron

\pi

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

detection system, a circularly polarized tagged photon beam and a frozen spin target. The contributions to the Gerasimov-Drell-Hearn sum rule and to the forward spin polarizability

Quasifree Photoproduction ofηMesons off the Neutron

\gamma_0

Modification of the {omega}-Meson Lifetime in Nuclear Matter

determined from the data are 226 \pm 5 (stat)\pm 12(sys) \mu b and -187 \pm 8 (stat)\pm 10(sys)10^{-6} fm^4, respectively, for 200<E_\gamma<800 MeV.

Measurement of the p (. gamma. ,. pi. sup 0 ) cross section at threshold

Quasifree photoproduction of eta mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta production off the neutron shows a pronounced bumplike structure at W=1.68 GeV (E{gamma} approximately 1 GeV), which is absent for the proton.

Quasi-free photoproduction of η-mesons off the deuteron

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.

Neutral-pion photoproduction off protons in the energy range 0.3 GeV<E(gamma)<3 GeV.

Quasifree photoproduction of eta mesons off nucleons bound in the deuteron has been measured with the CBELSA/TAPS detector for incident photon energies up to 2.5 GeV at the Bonn ELSA accelerator. The eta mesons have been detected in coincidence with recoil protons and recoil neutrons, which allows a detailed comparison of the quasifree n(gamma,eta)n and p(gamma,eta)p reactions. The excitation function for eta production off the neutron shows a pronounced bumplike structure at W=1.68 GeV (E{gamma} approximately 1 GeV), which is absent for the proton.

Photoproduction of πmesons off protons from the Δ(1232) region to Eγ = 3 GeV

Information on hadron properties in the nuclear medium has been derived from the photoproduction of omega mesons on the nuclei C, Ca, Nb, and Pb using the Crystal Barrel/TAPS detector at the ELSA tagged photon facility in Bonn. The dependence of the omega-meson cross section on the nuclear mass number has been compared with three different types of models: a Glauber analysis, a Boltzmann-Uehling-Uhlenbeck analysis of the Giessen theory group, and a calculation by the Valencia theory group. In all three cases, the inelastic omega width is found to be 130-150 MeV/c(2) at normal nuclear matter density for an average 3-momentum of 1.1 GeV/c. In the rest frame of the omega meson, this inelastic omega width corresponds to a reduction of the omega lifetime by a factor approximately 30. For the first time, the momentum dependent omegaN cross section has been extracted from the experiment and is in the range of 70 mb.

Detection of hard photons with BaF2 scintillators

Differential and absolute cross sections for the reaction {ital p}({gamma},{pi}{sup 0}) have been measured with energy-defined photons in the threshold region ({ital E}{sub {gamma}}=131.4--157.2 MeV). The {ital E}{sub 0+} amplitude has been extracted out of the data. The values found are in disagreement with predictions of low-energy theorems.