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At the Institut für Kernphysik in Mainz, Germany, the microtron MAMI has been upgraded to 1.5 GeV electron beam energy and can now be used to study strange hadronic systems. The magnetic spectrometer KAOS from GSI was dismantled and re-installed in the spectrometer facility operated by the A1 collaboration. The spectrometer's primary purpose is to study strangeness electro-production. Its compact design and its capability to detect negative and positive charged particles simultaneously under forward scattering angles complements the existing spectrometers. In 2008, an important milestone has been reached by the successful measurement of kaon production off a liquid hydrogen target. The identification of Λ and Σ0 hyperons in the missing mass spectra has demonstrated the capability of the extended facility to perform strangeness electro-production spectroscopy. To operate KAOS as a double-arm spectrometer under zero degree scattering angle major modifications are needed. A new vacuum chamber was constructed and installed, a new coordinate detector with many thousands of channels was built and tested, a new trigger system for the high rate environment was developed, and a magnetic chicane for the primary electron beam is under construction. [ABSTRACT FROM AUTHOR]

The helicity dependence of the total cross-section and the invariant-mass distributions in the ( pπ±) and (π+π-) final states for the γ p → pπ+π- reaction have been measured for the first time at incident photon energies from 400 to 800MeV. The measurement was performed with the large-acceptance detector DAPHNE at the tagged photon beam facility of the MAMI accelerator in Mainz. Although this channel is found to be predominantly excited by the intermediate production of a Δπ state, both the measured unpolarized and the helicity-dependent observables are generally not well described by the existing theoretical models. [ABSTRACT FROM AUTHOR]

The reaction p( e, e'p)π0 has been studied at Q 2 = 0.2 (GeV/ c)2 in the region of W = 1232MeV. From measurements left and right of $ \vec{q}\,$ , cross-section asymmetries ρLT have been obtained in forward kinematics ρLT( $ \theta_{{\pi^0}}^{{cm}}$ = 20°) = (- 11.68±2.36stat±2.36sys) and backward kinematics ρLT( $ \theta_{{\pi^0}}^{{cm}}$ = 160°) = (12.18±0.27stat±0.82sys) π0. Multipole ratios $ \Re$ { S 1+ * M 1+}/| M 1+|2 and $ \Re$ { S 0+ * M 1+}/| M 1+|2 were determined in the framework of the MAID2003 model. The results are in agreement with older data. The unusally strong negative $ \Re$ { S 0+ * M 1+}/| M 1+|2 required to bring also the result of Kalleicher et al. in accordance with the rest of the data is almost excluded. [ABSTRACT FROM AUTHOR]

The G asymmetry of the γ p↦ Nπ reaction has been measured for the first time for E γ = 340±14 MeV. This observable, for which very scarce published data exist, plays an important role to disentangle the contributions of the various nucleon resonances. The experiment, performed at the Mainz microtron MAMI, used a 4π-detector system, a linearly polarized, tagged photon beam, and a longitudinally polarized proton target. [ABSTRACT FROM AUTHOR]

The electron target asymmetries A|| and A⊥ with target spin parallel and perpendicular to the momentum transfer 𝗾 were measured for both the two- and three-body breakup of 3He in the 3(...) ((...), e'p)-reaction. Polarized electrons were scattered off polarized 3He in the quasielastic regime in parallel kinematics with the scattered electron and the knocked-out proton detected using the Three-Spectrometer Facility at MAMI. The results are compared to Faddeev calculations which take into account Final-State Interactions as well as Meson Exchange Currents. The experiment confirms the prediction of a large effect of Final-State Interactions in the asymmetry of the three-body breakup and of an almost negligible one for the two-body breakup. [ABSTRACT FROM AUTHOR]

The electric form factor of the neutron, GE,n, has been measured at the Mainz Microtron by recoil polarimetry in the quasielasticD(¯e,e’¯n)preaction. Three data points have been extracted at squared four-momentum transfersQ2 = 0.3, 0.6 and 0.8 (GeV/c)2. Corrections for nuclear binding effects have been applied. [ABSTRACT FROM AUTHOR]

Differential cross-sections for quasi-free π0 photoproduction from the proton and neutron bound in the deuteron have been measured for Eγ 200-400 MeV at θγlab = 136.2° using the Glasgow photon tagger at MAMI, the Mainz 48 cm ø × 64 cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. For the proton measurements made with both liquid-deuterium and liquid-hydrogen targets allow direct comparison of "free" π0 photoproduction cross-sections as extracted from the bound-proton data with experimental free cross-sections which are found to be in reasonable agreement below 320 MeV. At higher energies the "free" cross-sections extracted from quasi-free data are significantly smaller than the experimental free cross-sections and theoretical predictions based on multipole analysis. For the first time, "free" neutron cross-section have been extracted in the Δ-region. They are also in agreement with the predictions from multipole analysis up to 320 MeV and significantly smaller at higher photon energies. [ABSTRACT FROM AUTHOR]

Differential cross-sections for quasi-free Compton scattering from the proton and neutron bound in the deuteron have been measured using the Glasgow/Mainz photon tagging spectrometer at the Mainz MAMI accelerator together with the Mainz [48]cm ; SPMOslash; × [64]cm NaI(Tl) photon detector and the Göttingen SENECA recoil detector. The data cover photon energies ranging from [200]MeV to [400]MeV at θLABγ = 136.2°. Liquid deuterium and hydrogen targets allowed direct comparison of free and quasi-free scattering from the proton. The neutron detection efficiency of the SENECA detector was measured via the reaction p(γ,π+ n). The “free” proton Compton scattering cross-sections extracted from the bound proton data are in reasonable agreement with those for the free proton which gives confidence in the method to extract the differential cross-section for free scattering from quasi-free data. Differential cross-sections on the free neutron have been extracted and the difference of the electromagnetic polarizabilities of the neutron has been determined to be αn - βn = 9.8±3.6(stat)+2.1-1.1(syst)±2.2(model) in units of[10-4] fm3. In combination with the polarizability sum αn + βn = 15.2±0.5 deduced from photoabsorption data, the neutron electric and magnetic polarizabilities, αn = 12.5±1.8(stat)+1.1-0.6(syst)±1.1(model) and βn = 2.7±1.8(stat)+0.6-1.1(syst)±1.1(model) are obtained. The backward spin polarizability of the neutron was determined to be γ(n)π = (58.6±4.0)×10-4 fm4. [ABSTRACT FROM AUTHOR]

Compton scattering by the proton has been measured over a wide range covering photon energies 250MeV Eγ 800MeV and photon scattering angles 30° labγ 150°, using the tagged-photon facility at MAMI (Mainz) and the large-acceptance arrangement LARA. The previously existing data base on proton Compton scattering is greatly enlarged by more than 700 new data points. The new data are interpreted in terms of dispersion theory based on the SAID-SM99K parametrization of photo-meson amplitudes. It is found that two-pion exchange in the t-channel is needed for a description of the data in the second resonance region. The data are well represented if this channel is modeled by a single pole with the mass parameter mσ≈ 600 MeV. The asymptotic part of the spin-dependent amplitude is found to be well represented by π0 exchange in the t-channel. No indications of additional effects were found. Using the mass parameter mσ of the two-pion exchange determined from the second resonance region and using the new global average for the difference of the electric and magnetic polarizabilities of the proton, α - β = 10.5±0.9 stat+ syst±0.7 model ×10-4fm3, as obtained from a recent experiment on proton Compton scattering below pion photoproduction threshold, a backward spin polarizability of γπ = -37.1±0.6 stat+ syst±3.0 model ×10-4fm4 has been determined from data of the first resonance region below 455 MeV. This value is in a good agreement with predictions of dispersion relations and chiral perturbation theory. From a subset of data between 280 and 360 MeV, the resonance pion photoproduction amplitudes were evaluated leading to a E2/ M1 multipole ratio of the p↦Δ radiative transition of EMR(340 MeV) = (- 1.7±0.4stat+syst±0.2model)%. It was found that this number is dependent on the parameterization of photo-meson amplitudes. With the MAID2K parameterization, an E2/ M1 multipole ratio of EMR(340 MeV) = (- 2.0±0.4stat+syst±0.2model)% is obtained. [ABSTRACT FROM AUTHOR]

Differential cross-sections for Compton scattering from the proton have been measured at the MAMI tagged photon facility using the TAPS setup. The data cover an angular range of θlab γ = 59°-155° and photon energies ranging from 55 MeV to 165 MeV. Our results are in good agreement with those from previous experiments, but yield higher precision. Using dispersion relations the proton polarizabilities have been determined to be = [11.9±0.5stat.±1.3syst.±0.3mod.] . 10-4 fm3 and = [1.2±0.7stat.±0.3syst.±0.4mod.)] . 10-4 fm3. These results confirm the Baldin sum rule which was re-evaluated to be + = [13.8±0.4] . 10-4 fm3. We can also conclude that there is no significant additional asymptotic contribution to the backward spin polarizability γπ beyond the t-channel π0-exchange. [ABSTRACT FROM AUTHOR]

The reaction 12C(e,e'Δ0)11C →12C(e,e'pπ−)11C was investigated at the Mainz Microtron MAMI in a triple coincidence measurement using the three spectrometer setup of the A1 Collaboration. The good missing mass resolution of σm= 0.27 MeV/c2 allowed to select the events belonging to the ground state of 11C. Cutting on these events the excitation energy spectra of 12CΔ0 show evidence for two peaks of about 4 MeV width (FWHM) at 282 MeV and 296 MeV with a significance of about 4.5 standard deviations. The peaks are interpreted in a simple weak coupling model as bound Δ0 states in 12CΔ0. [ABSTRACT FROM AUTHOR]

Strained layer GaAs[sub .95]P[sub .05] photo cathodes are presented, which emit electron beams spinpolarized to a degree of P = 75% typically. Quantum yields around QE = 0.4% are observed routinely. The figure of merit P² × QE = 2.3 × 10[sup -3] is comparable to that of the best strained layer cathodes reported in literature. The optimum wavelength of irradiating light around 830 nm is in convenient reach of Ti:sapphire lasers or diode lasers respectively. The cathodes are produced using MOCVD-techniques. A GaAs[sub .55]P[sub .45]GaAs[sub .85]P[sub .15] superlattice structure prevents the migration of dislocations from the substrate and bottom layers to the strained overlayer. The surface is protected by an arsenic layer so that no chemical cleaning is necessary before installation into vacuum. The source of polarized electrons attached to the Mainz race track microtron MAMI works with such cathodes now. More than 1000 hours beamtime have been performed successfully. [ABSTRACT FROM AUTHOR]

Measurements of electroproduced pions with an energy of 10MeV are presented. The results of a calculation based on the PWIA are compared with the presented data as well as with data at higher pion energies. [ABSTRACT FROM AUTHOR]

The polarization transfer from longitudinally polarized electrons to protons in the elastic scattering p( e, e' p) has been measured around Q2 = 0.4 (GeV/ c)2 with the three-spectrometer facility at the Mainz microtron MAMI. From this polarization transfer the ratio GEp/( GMp/μp) has been determined. The ratio is found to be slightly less than unity in agreement with recent results from other laboratories and from the Rosenbluth separation of cross-sections measured with unpolarized electrons. [ABSTRACT FROM AUTHOR]