B. Kopf

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.

Measurement of the beam asymmetry in η photoproduction off the proton

The beam asymmetry, Σ, was measured at ELSA in the reaction γ p → η p using linearly polarised tagged photon beams, produced by coherent bremsstrahlung off a diamond. The crystal was oriented to provide polarised photons in the energy range Eγ = 800 to 1400 MeV with the maximum polarisation of Pγ = 49% obtained at 1305 MeV. Both dominant decay modes of the η into two photons and 3π were used to extract the beam asymmetry from the azimuthal modulation of the cross section. The measurements cover the angular range Θcm ≃ 50 – 150 degrees. Large asymmetries up to 80 % are observed, in agreement with a previous measurement. The eta-MAID model and the Bonn–Gatchina partial wave analysis describe the measurements, but the required partial waves differ significantly. PACS. 13.60.-r Photon and charged-lepton interactions with hadrons – 13.60.Le Meson production – 13.88.+e Polarization in interactions and scattering – 14.20.Gk Baryon resonances with S=0

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

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.

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

Single pi(0) photoproduction has been studied with the CB-ELSA experiment at Bonn using tagged photon energies between 0.3 and 3.0 GeV. The experimental setup covers a very large solid angle of approximately 98% of 4pi. Differential cross sections dsigma/dOmega have been measured. Complicated structures in the angular distributions indicate a variety of different resonances being produced in the s channel intermediate state gammap-->N(*)(Delta(*))-->ppi(0). A combined analysis including the data presented in this letter along with other data sets reveals contributions from known resonances and evidence for a new resonance N(2070)D15.

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

Abstract.Photoproduction of π0-mesons was studied with the Crystal-Barrel detector at ELSA for incident energies from 300MeV to 3GeV. Differential cross-sections dσ/dΩ, dσ/dt, and the total cross-section are presented. For Eγ < 3GeV, the angular distributions agree well with the SAID parametrization. At photon energies above 1.5GeV, a strong forward peaking indicates t-channel exchange to be the dominant process. The rapid variations of the cross-section with energy and angle indicate production of resonances. An interpretation of the data within the Bonn-Gatchina partial-wave analysis is briefly discussed.

Photoproduction of eta mesons off protons for 0.75 GeV<E(gamma)<3 GeV.

Total and differential cross sections for the reaction p(gamma, eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into p eta. At medium energies we find evidence for a new resonance N(2070)D15 with (mass, width) = (2068+-22, 295+-40) MeV. At photon energies above 1.5 GeV, a strong peak in forward direction develops, signalling the exchange of vector mesons in the t channel.

In-medium omega mass from the gamma+Nb -> pi(0)gamma+X reaction

Data on the photoproduction of omega mesons on nuclei have been reanalyzed in a search for in-medium modifications. The data were taken with the crystal barrel (CB)/two-arm photon spectrometer (TAPS) detector system at the ELectron Stretcher Anlage (ELSA) accelerator facility in Bonn. First results from the analysis of the data set were published by Trnka et al. [Phys. Rev. Lett. 94, 192303 (2005)], claiming a lowering of the. mass in the nuclear medium by 14% at normal nuclear matter density. The extracted. line shape was found to be sensitive to the background subtraction. For this reason a reanalysis of the same data set has been initiated, and a new method has been developed to reduce the background and to determine the shape and absolute magnitude of the background directly from the data. Details of the reanalysis and of the background determination are described. The omega signal on the Nb target, extracted in the reanalysis, does not show a deviation from the corresponding line shape on a LH2 target, measured as reference. The earlier claim of an in-medium mass shift is thus not confirmed. The sensitivity of the omega line shape to different in-medium modification scenarios is discussed.

Photoproduction of eta mesons off protons for 0.75 GeV

Total and differential cross sections for the reaction p(gamma, eta)p have been measured for photon energies in the range from 750 MeV to 3 GeV. The low-energy data are dominated by the S11 wave which has two poles in the energy region below 2 GeV. Eleven nucleon resonances are observed in their decay into p eta. At medium energies we find evidence for a new resonance N(2070)D15 with (mass, width) = (2068+-22, 295+-40) MeV. At photon energies above 1.5 GeV, a strong peak in forward direction develops, signalling the exchange of vector mesons in the t channel.

Transparency ratio in gamma A -> eta ' A ' and the in-medium eta ' width

The photoproduction of eta`-mesons off different nuclei has been measured with the CBELSA/TAPS detector system for incident photon energies between 1500-2200 MeV. The transparency ratio has been deduced and compared to theoretical calculations describing the propagation of eta`-mesons in nuclei. The comparison indicates a width of the eta`-meson of the order of Gamma = 15-25 MeV at rho = rho(0) for an average momentum p(eta`) = 1050 MeV/c, at which the eta`-meson is produced in the nuclear rest frame. The inelastic eta`N cross section is estimated to be 3-10 mb. Parameterizing the photoproduction cross section of eta`-mesons by sigma(A) = sigma(0)A(alpha), a value of alpha = 0.84 +/- 0.03 has been deduced

Photoproduction of omega mesons on nuclei near the production threshold

Abstract.The photoproduction of