Coherent photoproduction of pi(0)-mesons from threshold (E-th approximate to 136 MeV) throughout the Delta-resonance region and of eta-mesons close to the production threshold (E-th approximate to 570 MeV for eta) has been measured for Li-7 nuclei. The experiment was performed using the tagged-photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4 pi solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missing-energy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent eta production had been only identified for the lightest nuclear systems (H-2 and He-3). For He-3 a large enhancement of the cross section above plane-wave approximations had been reported, indicating the formation of a quasi-bound state. The present Li data for eta production agree with a plane-wave approximation. Contrary to He-3, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form factor dependence were observed.

Total and differential cross-sections for the reaction gamma p -> pi(0)eta p have been measured with the Crystal Ball/TAPS detector using the tagged photon facility at the MAMI C accelerator in Mainz. In the energy range E(gamma) = 0.95-1.4 GeV the reaction is dominated by the excitation and sequential decay of the Delta(1700)D(33)-resonance. Angular distributions measured with high statistics allow us to determine the ratio of hadronic decay widths Gamma(eta Delta)/Gamma pi S(11) and the ratio of the helicity amplitudes A(3/2)/A(1/2) for this resonance.

The Dalitz decay eta -> gamma e(+)e(-) has been measured using the combined Crystal Ball and TAPS photon detector setup at the electron accelerator MAMI-C. Compared to the most recent transition form-factor measurement in the e(+)e(-) channel, statistics have been improved by one order of magnitude. The e(+)e(-)invariant mass distribution shows a deviation from the QED prediction for a point-like particle, which can be described by a form-factor. Using the usual monopole transition form-factor parameterization, F(m(2)) = (1 - m(2)/Lambda(2))(-1), a value of Lambda(-2) = (1.92 +/- 0.35(stat) +/- 0.13(syst)) GeV(-2) has been determined. This value is in good agreement with a recent measurement of the eta Dalitz decay in the mu(+)mu(-) channel and with recent form-factor calculations. An improved value of the branching ratio BR(eta -> gamma e(+)e(-)) = (6.6 +/- 0.4(stat) +/- 0.4(syst)) . 10(-3) has been determined. (C) 2011 Elsevier B.V. All rights reserved.

A technique is presented for precision measurements of the area densities, rho T, of approximately 5% radiation length carbon and (208)Pb targets used in an experiment at Jefferson Laboratory to measure the neutral pion radiative width. The precision obtained in the area density for the carbon target is +/- 0.050%, and that obtained for the lead target through an X-ray attenuation technique is +/- 0.43%. (C) 2009 Elsevier B.V. All rights reserved.

The reaction gamma p -> p pi(0)gamma' has been measured with the Crystal Ball/TAPS detectors using the energy-tagged photon beam at the electron accelerator facility MAMI-B. Energy and angular differential cross-sections for the emitted photon gamma' and angular differential cross-sections for the pi(0) have been determined with high statistics in the energy range of the Delta(+)(1232)-resonance. Cross-sections and the ratio of the cross-section to the nonradiative process gamma p -> p pi(0) are compared to theoretical reaction models, having the anomalous magnetic moment kappa(Delta+) as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of kappa(Delta+) is feasible.

Quasi-free photoproduction of eta-mesons has been measured off nucleons bound in He-3 nuclei for incident photon energies from the threshold region up to 1.4 GeV. The experiment was performed at the tagged photon facility of the Mainz MAMI accelerator with an almost 4 pi covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The eta-mesons were detected in coincidence with the recoil nucleons. This allowed a comparison of the production cross section off quasi-free protons and quasi-free neutrons and a full kinematic reconstruction of the final state, eliminating effects from nuclear Fermi motion. In the S-11(1535) resonance peak, the data agree with the neutron/proton cross section ratio extracted from measurements with deuteron targets. More importantly, the prominent structure observed in photoproduction off quasi-free neutrons bound in the deuteron is also clearly observed. Its parameters (width, strength) are consistent with the expectations from the deuteron results. On an absolute scale the cross sections for both quasi-free protons and neutrons are suppressed with respect to the deuteron target pointing to significant nuclear final-state interaction effects.

We report on the measurement of inclusive electron scattering off a carbon target performed with CLAS at Jefferson Laboratory. A combination of three different beam energies 1.161, 2.261 and 4.461 GeV allowed us to reach an invariant mass of the final-state hadronic system W approximate to 2.4 GeV with four-momentum transfers Q(2) ranging from 0.2 to 5 (GeV/c)(2). These data, together with previous measurements of the inclusive electron scattering off proton and deuteron, which cover a similar continuous two-dimensional region of Q(2) and Bjorken variable x, permit the study of nuclear modifications of the nucleon structure. By using these, as well as other world data, we evaluated the F(2) structure function and its moments. Using an OPE-based twist expansion, we studied the Q(2)-evolution of the moments, obtaining a separation of the leading-twist and the total higher-twist terms. The carbon-to-deuteron ratio of the leading-twist contributions to the F(2) moments exhibits the well-known EMC effect, compatible with that discovered previously in x-space. The total higher-twist term in the carbon nucleus appears, although with large systematic uncertainties, to be smaller with respect to the deuteron case for n < 7, suggesting partial parton deconfinement in nuclear matter. We speculate that the spatial extension of the nucleon is changed when it is immersed in the nuclear medium. (C) 2010 Elsevier B.V. All rights reserved.

The circular photon asymmetry for pi(0)eta photoproduction on the proton was measured for the first time at the tagged photon facility of the MAMI C accelerator using the Crystal Ball/TAPS photon spectrometer. The experimental results are interpreted within a phenomenological isobar model that confirms the dominant role of the Delta(1700)D(33) resonance. The measured asymmetry allows us to identify small contributions from positive-parity resonances via interference terms with the dominant D(33) amplitude. (C) 2010 Elsevier B.V. All rights reserved.

Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing p pi(0)pi(0) and n pi(+)pi(0) final states from the threshold region up to 800 MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550 MeV-820 MeV). The experiment was performed at the tagged photon beam of the Mainz MAMI accelerator with the Crystal Ball and TAPS detectors combined to give an almost 4 pi solid-angle electromagnetic calorimeter. The results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel.