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Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions γp → nπ~0π~+ off free protons and γd →(p)pπ~0π~? and γd → (n)nπ~0π~+ off...
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Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions γp → nπ~0π~+ off free protons and γd →(p)pπ~0π~? and γd → (n)nπ~0π~+ off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. Circularly polarized photons were produced from bremsstrahlung of longitudinally polarized electrons and tagged with the Glasgow-Mainz magnetic spectrometer. The charged pions, recoil protons, recoil neutrons, and decay photons from π~0 mesons were detected in the 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. Using a complete kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons, suggesting that the quasi-free neutron results are also a close approximation of the free-neutron asymmetries. A comparison of the results to the predictions of the Two-Pion-MAID reaction model shows that the reaction mechanisms are still not well understood, in particular at low incident photon energies in the second nucleon-resonance region.
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The exclusive electroproduction of π+ above the resonance region was studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory by scattering a 6 GeV continuous electron beam off a hydrogen target. The l...
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The exclusive electroproduction of π+ above the resonance region was studied using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Laboratory by scattering a 6 GeV continuous electron beam off a hydrogen target. The large acceptance and good resolution of CLAS, together with the high luminosity, allowed us to measure the cross section for the γ?p → nπ~+ process in 140 (Q~2, xB, t) bins: 0.16 < xB < 0.58, 1.6 GeV~2 < Q~2 < 4.5 GeV~2 and 0.1 GeV~2 < ?t < 5.3 GeV~2. For most bins, the statistical accuracy is on the order of a few percent. Differential cross sections are compared to four theoretical models, based either on hadronic or on partonic degrees of freedom. The four models can describe the gross features of the data reasonably well, but differ strongly in their ingredients. In particular, the model based on Generalized Parton Distributions (GPDs) contain the interesting potential to experimentally access transversity GPDs.
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Quasi-free photoproduction of η-mesons has been measured off nucleons bound in ~3He 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 Ma...
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Quasi-free photoproduction of η-mesons has been measured off nucleons bound in ~3He 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 4π covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The η-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.
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Coherent photoproduction of π0-mesons from threshold (Eth ≈ 136MeV) throughout the Δ- resonance region and of η-mesons close to the production threshold (Eth ≈ 570MeV for η) has been measured for 7Li nuclei. The experiment w...
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Coherent photoproduction of π0-mesons from threshold (Eth ≈ 136MeV) throughout the Δ- resonance region and of η-mesons close to the production threshold (Eth ≈ 570MeV for η) has been measured for 7Li 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 4π solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missingenergy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent η production had been only identified for the lightest nuclear systems (2H and 3He). For 3He 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 η production agree with a plane-wave approximation. Contrary to 3He, neither a threshold enhancement of the total cross section nor a deviation of the angular distributions from the expected form factor dependence were observed.
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The reaction γp → pπ0γh-as 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 phot...
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The reaction γp → pπ0γh-as 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 γand angular differential cross-sections for the π0 have been determined with high statistics in the energy range of the Δ+(1232)-resonance. Cross-sections and the ratio of the cross-section to the nonradiative process γp → pπ0 are compared to theoretical reaction models, having the anomalous magnetic moment κΔ+ as free parameter. As the shape of the experimental distributions is not reproduced in detail by the model calculations, currently no extraction of κΔ+ is feasible.
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The very first data from MAX-lab in Lund, Sweden on pion photoproduction at threshold energies are presented. The decrease of the total π + yield in γ + 12C, 27Al reactions below 200 MeV as well as the dσ/dΩ cross-section data...
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The very first data from MAX-lab in Lund, Sweden on pion photoproduction at threshold energies are presented. The decrease of the total π + yield in γ + 12C, 27Al reactions below 200 MeV as well as the dσ/dΩ cross-section data essentially follow the predictions of an intranuclear-cascade model with an attractive potential for the pion–nucleus interaction. However, d2σ/dΩ dT , cross-section data at 176 MeV show deviations which call for refinements of the model and possibly also for the inclusion of coherent pion-production mechanisms.
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The in-medium properties of the ω meson were investigated at the MAMI-C electron accelerator using the Crystal Ball/TAPS detector system to identify ω mesons through their ω → π~0γ decay channel. Two solid targets (C, Nb) an...
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The in-medium properties of the ω meson were investigated at the MAMI-C electron accelerator using the Crystal Ball/TAPS detector system to identify ω mesons through their ω → π~0γ decay channel. Two solid targets (C, Nb) and a LH2 target as a reference have been used. In the energy range 900 to 1300MeV the ω lineshapes for the different nuclei are compared and a broadening of the ω signal for the niobium target is observed in comparison to the liquid-hydrogen target. In a second analysis the ω momentum distributions for the solid targets are determined for 50MeV/c wide bins. For both analyses a comparison to transport calculations shows that the data are consistent with an in-medium modification by collisional broadening, as observed by transparency ratio measurements.
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We report on a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single π~0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron. The...
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We report on a measurement of the neutron detection efficiency in NaI crystals in the Crystal Ball detector obtained from a study of single π~0 photoproduction on deuterium using the tagged photon beam at the Mainz Microtron. The results were obtained up to a neutron energy of 400MeV. They are compared to previous measurements made more than 15 years ago at the pion beam at the BNL AGS.
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A new precise determination of the η meson mass is presented. It is based on a measurement of the threshold for the γp → pη reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger ...
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A new precise determination of the η meson mass is presented. It is based on a measurement of the threshold for the γp → pη reaction using the tagger focal-plane microscope detector at the MAMI-B facility in Mainz. The tagger microscope has a higher energy resolution than the standard tagging spectrometer and, hence, allowed an improvement in the accuracy compared to the previous η mass measurement at MAMI-B. Special emphasis was put on a very careful energy calibration of the electron beam and the tagging device, which reduced considerably the systematic uncertainty compared to the previous MAMI experiment. The result m_η = (547.865±0.031_(stat)±0.062_(syst))MeV agrees very well with the precise values of the NA48, KLOE, CLEO and COSY-ANKE Collaborations and deviates by 6σ from the smaller value obtained by the GEM Collaboration at COSY.
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A description is given of the upgraded photon tagging facility at the MAX IV Laboratory. Two magnetic spectrometers are used to momentum analyze post-bremsstrahlung electrons. The tagged photon range extends from 10 to 180 MeV wit...
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A description is given of the upgraded photon tagging facility at the MAX IV Laboratory. Two magnetic spectrometers are used to momentum analyze post-bremsstrahlung electrons. The tagged photon range extends from 10 to 180 MeV with an energy resolution of about 300 keV. The system has been operated at rates up to 4 × 10~6 photons s~(-1) MeV ~(-1) Different diagnostic tools are described as well as the experimental program.
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