G. Anton

Near-threshold photoproduction of η-mesons from the deuteron

Abstract The first measurement of incoherent η-photoproduction from the deuteron in the threshold region is reported. The experiment was carried out at the MAMI accelerator with the TAPS spectrometer. Total and differential inclusive cross sections have been obtained between 627 and 790 MeV. It is found that the reaction is completely dominated by the incoherent part. An upper limit for coherent η-photoproduction on the deuteron is deduced, which is substantially lower than the result from an earlier measurement. The incoherent cross section is reproduced in a participant-spectator approach under the assumption of an energy-independent ratio between the neutron and proton cross sections. Best agreement is found for the ratio σ n σ p ≈ 2 3 . The implications for the isospin components of the electromagnetic excitation of the S11(1535) resonance are discussed.

Near threshold photoproduction of η-mesons from complex nuclei

Total and differential cross sections for photoproduction of eta mesons from C-12, Ca-40, Nb-93 and Pb-nat have been obtained up to 790 MeV incident photon energy at the Mainz Microtron (MAMI) with the TAPS spectrometer. The absorption cross section sigma(eta N)(abs) = (30 +/- 2.5 +/- 6) mb of eta mesons in nuclear matter and the absorption length lambda(eta) = (2.0 +/- 0.2 +/- 0.4) fm are extracted. No significant depletion of the S(1)1(1535) strength in the eta photoproduction on nuclei is observed.

Neutral pion photoproduction from the proton near threshold

Abstract Differential and total cross sections for the photoproduction of neutral pions from the proton have been measured for incident photon energies from 140–270 MeV, using the photon spectrometer TAPS at the tagged photon beam of the 855 MeV Mainz Microtron. The energy dependence of the s- and p-wave multipoles close to threshold was deduced from a multipole fit and a multipole analysis. The extracted s-wave amplitude E 0+ at threshold is found to be significantly smaller than the prediction of the classical low energy theorems, but is in reasonable agreement with the chiral perturbation theory.

Near threshold photoproduction of

Abstract The first measurement of incoherent η-photoproduction from the deuteron in the threshold region is reported. The experiment was carried out at the MAMI accelerator with the TAPS spectrometer. Total and differential inclusive cross sections have been obtained between 627 and 790 MeV. It is found that the reaction is completely dominated by the incoherent part. An upper limit for coherent η-photoproduction on the deuteron is deduced, which is substantially lower than the result from an earlier measurement. The incoherent cross section is reproduced in a participant-spectator approach under the assumption of an energy-independent ratio between the neutron and proton cross sections. Best agreement is found for the ratio σ n σ p ≈ 2 3 . The implications for the isospin components of the electromagnetic excitation of the S11(1535) resonance are discussed.

\eta

Abstract The differential cross section of the reaction γ p→ π + n has been measured with the PHOENICS detector at ELSA in Bonn. For the first time this cross section has been determined simultaneously over a large range of photon energies ( E γ = 220−900 MeV) and pion angles ( Θ c . m . pi = 35°−135°) with a tagged photon facility. The experimental set-up allowed a considerable kinematic overdetermination of the investigated reaction. Accordingly, the background contributions have been suppressed to below 1%. The measured differential cross section is in good agreement with existing data. The comparison with different model calculations is presented.

-mesons from the deuteron

The differential cross section of the reactionγ+p→π+ was measured at pion CM-angles of 20° and 30° for photon energies between 500 MeV and 1,400 MeV. The pions were detected in a magnetic spectrometer. By measuring each pion trajectory and by offline calculation of the initial pion parameters an energy resolution of about 2.5% FWHM was achieved. The results complete a set of data which were measured in recent years at the Bonn 2.5 GeV synchrotron. In comparison to photoproduction analyses two effects were revealed: The η cusp appears in the energy dependence of the cross section as a sharp drop atKγ=710 MeV. In the region of the third resonance the data show a greater enhancement than predicted by most of the analyses.

Photoproduction of positive pions from hydrogen with PHOENICS at ELSA

Abstract Target asymmetry data for single positive pion photoproduction from polarized protons have been obtained at the Bonn Electron Stretcher Accelerator ELSA. Data were taken in a wide kinematical range of E γ = 215–800 MeV and Θ CMS π = 35°–135° with the PHOENICS-detector. This detector permits a considerable kinematical overdetermination so that the nominal process could be clearly separated from the background. For the first time a combination of a frozen spin target with a tagged photon beam has been used. Butanol (C 4 H 9 OH) was used as target material with a maximum proton polarization of more than ±90%. Due to the simultaneous energy and angle coverage the systematical error in the order of ΔT sys = ±2.5% is common for all data points. The results at low photon energies show significant deviations from existing multipole analyses.

Positive pion photoproduction from hydrogen at photon energies between 500 MeV and 1,400 MeV in forward direction

Abstract Total cross sections, differential cross sections and asymmetries for the reaction p( p ,π 0 )pp from threshold to 1 GeV have been measured. The outgoing π0 was detected by a 4π neutral meson spectrometer. Our data are compared with the corresponding np→NNπ± reactions. The isoscalar partial cross sections are extracted from the relevant total cross sections. They are significant throughout the whole energy range. Below the Δ-region, energy-integrated differential cross sections show a large anisotropy difference between the neutral and charged pion channels, respectively. A partial-wave fit shows that the initial T = 0 state is dominated by the 3D1 wave in this energy region. Finally, predictions of phenomenological models give reasonable good account of the measured observables but cannot reproduce the sign of the asymmetries.

Photoproduction of positive pions from polarized protons

Near threshold photoproduction ofη-mesons from the proton has been measured at the MAMI accelerator with the TAPS spectrometer. The mass of theη-meson was deduced from the threshold energy forη-photoproduction. The result of mη=(547.12 ± 0.06 ± 0.25) MeV supports the low value of theη-mass reported from a dp →3Heη measurement at SATURNE in 1992. Theη-decay branching ratio Γ3π0/Γγγ was measured to be (0.832±0.005±0.012).

Cross sections and asymmetries for the p(p,π0)pp reaction from threshold to 1 GeV

Abstract Studying the reaction γ d → pp π − , a narrow resonance structure in the pp invariant mass distribution has been observed at 2014 ± 2 MeV/ c 2 with a statistical significance of 4.7 standard deviations. Using a bremsstrahlung photon beam ( k 0 = 520 MeV), the kinematics of the reaction were completely determined by detecting both outgoing protons in two large solid angle time-of-flight spectrometers. By accepting only proton momenta greater than ∼300 MeV/ c , the predominant contribution from quasi-free pion production was suppressed.