120916993
Photoproduction of positive pions from polarized protons
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.
15366431
Upgrade of the Glasgow photon tagging spectrometer for Mainz MAMI-C
The Glasgow photon tagging spectrometer at Mainz has been upgraded so that it can be used with the 1500MeV electron beam now available from the Mainz microtron MAMI-C. The changes made and the resulting properties of the spectrometer are discussed.
123483800
The Gerasimov-Drell-Hearn sum rule at MAMI
Abstract.An extended experimental program to investigate the Gerasimov-Drell-Hearn (GDH) sum rule and related partial reaction cross sections on proton and neutron has been carried out by the GDH collaboration at the electron accelerators MAMI (Mainz) and ELSA (Bonn). The GDH sum rule connects the helicity-dependent photoabsorption cross section with the anomalous magnetic moment of the nucleon. The GDH collaboration has measured the total cross section of circularly polarised photons with longitudinally polarised protons at MAMI and ELSA to check this sum rule experimentally. In addition partial reaction channels like pion, double pion and eta production were determined. This provides new information on the helicity-dependent excitation spectrum of the nucleon. With the help of partial wave analyses it is possible to extract new, complementary information on the broad, overlapping resonances in this energy region. The double polarisation observable E measured in this experiment enhances the smaller multipoles via interference terms. The analysis of our data provides a new possibility to study the photon couplings to the nucleon resonances, especially above the Δ resonance where many properties of the observed states (e.g., coupling constants, branching ratios, helicity amplitudes) are only poorly known. In this paper we present several new results from our measurements on polarised proton and deuteron targets.
19511077
Narrow structure in the excitation function of η photoproduction off the neutron.
The photoproduction of η mesons off nucleons bound in 2H and 3He has been measured in coincidence with recoil protons and recoil neutrons for incident photon energies from threshold up to 1.4 GeV. The experiments were performed at the Mainz MAMI accelerator, using the Glasgow tagged photon facility. Decay photons from the η→2γ and η→3π0 decays and the recoil nucleons were detected with an almost 4π electromagnetic calorimeter combining the Crystal Ball and TAPS detectors. The data from both targets are of excellent statistical quality and show a narrow structure in the excitation function of γn→nη. The results from the two measurements are consistent, taking into account the expected effects from nuclear Fermi motion. The best estimates for position and intrinsic width of the structure are W=(1670±5)  MeV and Γ=(30±15)  MeV. For the first time precise results for the angular dependence of this structure have been extracted.
122968271
Proton radii determinations from the ratio of pi+ elastic scattering from 11B and 12C
Abstract A new method using low energy π + elastic scattering is employed to determine the proton matter distribution of a nucleus relative to an adjacent nucleus differing by one proton. The difference in the rms charge radii of 12 C and 11 B was determined to be 0.072 ± 0.021 fm with π + beam energies of 38.6 and 47.7 MeV.
122369948
Systematic analysis of charmonium photoproduction
The available data on J/Ψ photoproduction are analysed in terms of pomeron exchange, two-gluon exchange and photon–gluon fusion models. Allowing the pomeron–quark interaction to be flavour-dependent and introducing both soft and hard pomerons it is possible to reproduce the data at GeV and small |t|. The two-gluon exchange calculations indicate strong sensitivity to the gluon distribution function. The results obtained with the most modern MRST2001 and DL PDF reproduce the forward J/Ψ photoproduction cross section at GeV. The calculations with the photon–gluon fusion model and with MRST2001 and DL PDF are also in reasonable agreement with the data on the total J/Ψ photoproduction cross section. However none of the models describe the data at GeV or |t| ≥ 1 GeV2. We attribute the J/Ψ photoproduction at low energies and large |t| to a mechanism different from pomeron or two-gluon exchange. We consider that this might be the exchange of an axial vector trajectory that couples with the axial form factor of the nucleon.
116957075
Theoretical Description of Nucleon Structure Functions
We review some recent progress in relating the spin, flavor and medium dependence of nucleon structure functions to familiar quark models.
10596461
The sigma commutator from lattice QCD
As a direct source of information on chiral symmetry breaking within QCD, the sigma commutator is of considerable importance. Since hadron structure is a non-perturbative problem, numerical calculations on a space-time lattice are currently the only rigorous approach. With recent advances in the calculation of hadron masses within full QCD, it is of interest to see whether the sigma commutator can be calculated directly from the dependence of the nucleon mass on the input quark mass. We show that, provided the correct chiral behaviour of QCD is respected in the extrapolation to realistic quark masses, one can indeed obtain a fairly reliable determination of the sigma commutator using present lattice data. For two-flavour dynamical fermion QCD the sigma commutator lies between 45 and 55 MeV based on recent data from CP-PACS and UKQCD.
120858943
Charge symmetry breaking in neutron-proton scattering above pion production threshold
We examine the effect of the coupled N.6. inelastic channel on charge symmetry breaking in the np system. Our calculation includes the effects of mass splitting for the nucleon and delta multiplets in the channel coupling potentials. The results suggest that such effects become increasingly important at about 500 MeV laboratory energy.