[en] 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_γ<800 MeV . The experiment used a 4π 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 γ₀ determined from the data are 226±5(stat)±12(syst) μb and -187±8(stat)±10(syst) x 10^-6 fm⁴ , respectively, for 200< E_γ<800 MeV

[en] Differential cross sections and beam asymmetries for coherent π⁰ photoproduction from ⁴He in the Δ energy-range have been measured with high statistical and systematic precision using both decay photons for identifying the process. The experiment was performed at the MAinz MIcrotron using the TAPS photon spectrometer and the Glasgow/Mainz tagged photon facility. The differential cross sections are in excellent agreement with predictions based on the DWIA if an appropriate parametrization of the Δ-nuclear interaction is applied. The beam asymmetries are interpreted in terms of degrees of linear polarization of collimated coherent bremsstrahlung. The expected increase of the degree of linear polarization with decreasing collimation angle is confirmed. Agreement with calculations is obtained on a few-percent level of precision in the maxima of the coherent peaks

[en] Coherent and incoherent π⁰ photoproduction off the deuteron has been measured for incident quasi-monoenergetic photons in the energy range from π⁰ threshold up to 300 MeV using the TAPS setup together with the Glasgow tagger at MAMI. Integral and differential cross sections over the full angular range have been obtained. Coherent and incoherent production are found to exhibit very different angle and energy dependences. The sum of both processes, the total π⁰ production off the deuteron is quantitatively extremely well described by a pure quasifree model, whereas the data for coherent and incoherent channels necessitate strong FSI effects for their description. Finally upper limits for the production of dibaryon resonances in this reaction have been obtained

[en] Here we correct a factor of 2 missed in one of figures and the corresponding sentence in the text. It does not change any conclusions.

[en] The new frozen spin polarized target for experiments at the polarized beam of the real photon facility A2 of the MAMI accelerator is described. The A2-collaboration at the Mainz Microtron MAMI is measuring photon absorption cross section using circularly and linearly polarized photons up to the energy of 1.5 GeV. The photons are produced in the’ Bremsstrahlungs’ process. In the years 2005/2006 the Crystal Ball detector with its unique capability to cope with multi photon final states was set up in Mainz. Since 2010 the experimental apparatus has been completed by a polarized target. The horizontal dilution refrigerator of the Frozen-Spin Target has been constructed and is operated in close cooperation with the Joint Institute for Nuclear Research in Dubna, Russia. The system offers the opportunity to provide longitudinally and transversely polarized protons and deuteron. In this paper the operation experience of this new Frozen-Spin Target and first results from the runs in 2010 and 2011 are presented.

[en] Differential cross sections for Compton scattering by the proton have been measured in the energy interval between 200 and 500 MeV at scattering angles of θcms = 75 circle and θcms = 90 circle using the CATS, the CATS/TRAJAN, and the COPP setups with the Glasgow Tagger at MAMI (Mainz). The data are compared with predictions from dispersion theory using photo-meson amplitudes from the recent VPI solution SM95. The experiment and the theoretical procedure are described in detail. It is found that the experiment and predictions are in agreement as far as the energy dependence of the differential cross sections in the Δ-range is concerned. However, there is evidence that a scaling down of the resonance part of the M3/21+ photo-meson amplitude by (2.8±0.9)% is required in comparison with the VPI analysis. The deduced value of the M3/21+-photoproduction amplitude at the resonance energy of 320 MeV is: vertical stroke M1+3/2 vertical stroke =(39.6±0.4) x 10-3 mπ+-1. (orig.)

[en] We present the first detailed measurement of incoherent photoproduction of neutral pions to a discrete state of a residual nucleus. The ¹²C(γ,π⁰)¹²C_4.4MeV* reaction has been studied with the Glasgow photon tagger at MAMI employing a new technique which uses the large solid angle Crystal Ball detector both as a π⁰ spectrometer and to detect decay photons from the excited residual nucleus. The technique has potential applications to a broad range of future nuclear measurements with the Crystal Ball and similar detector systems elsewhere. Such data are sensitive to the propagation of the Δ in the nuclear medium and will give the first information on matter transition form factors from measurements with an electromagnetic probe. The incoherent cross sections are compared to two theoretical predictions including a Δ-hole model

[en] Differential cross sections for Compton scattering from the free proton at Θ_γ'^lab=130.7 deg. in the energy region from 200 MeV to 410 MeV and for quasi-free Compton scattering from the proton bound in the deuteron at Θ_γ'^lab=148.8 deg. in the energy region from 200 MeV to 290 MeV have been measured. The free proton data are in agreement with dispersion-theory predictions based on standard parameters. The difference of the proton polarizabilities has been extracted from the quasi-free data. Our result, αbar-βbar=[9.1±1.7(stat+syst)±1.2(mod)]x10^-4 fm³, is in reasonable agreement with the world average of the free proton data if the backward spin polarizability γ_π is taken to be -37.6x10^-4 fm⁴ as predicted by dispersion theory in agreement with many theoretical calculations. This implies that quasi-free Compton scattering may also be used to determine the electromagnetic polarizabilities of the neutron. No indication has been found of a recently suggested new contribution to γ_π

[en] Measuring Compton scattered photons and recoil neutrons in coincidence, quasifree Compton scattering by the neutron has been investigated at MAMI (Mainz) at θ^lab_γ=136 deg. in an energy range from 200 to 400 MeV. From the data a polarizability difference of α_n-β_n=9.8±3.6( stat)^+2.1_-1.1(syst )±2.2(model) in units of 10^-4 fm³ has been determined. In combination with the polarizability sum α_n+β_n=15.2±0.5 deduced from photoabsorption data, the first precise results for the neutron electric and magnetic polarizabilities, α_n=12.5±1.8(stat)^+1.1_-0.6(syst)± 1.1(model) and β_n=2.7±1.8(stat)^+0.6_-1.1(syst)± 1.1(model) , are obtained

[en] The dynamics of the η→3π⁰ decay have been studied with the Crystal Ball multiphoton spectrometer and the TAPS calorimeter. Bremsstrahlung photons produced by the 1.5-GeV electron beam of the Mainz microtron MAMI-C and tagged by the Glasgow photon spectrometer were used for η-meson production. The analysis of 3x10⁶ γp→ηp→3π⁰p→6γp events yields the value α=-0.032±0.003 for the η→3π⁰ slope parameter, which agrees with the majority of recent experimental results and has the smallest uncertainty. The π⁰π⁰ invariant-mass spectrum was investigated for the occurrence of a cusplike structure in the vicinity of the π⁺π^- threshold. The observed effect is small and does not affect our measured value for the slope parameter