The Dalitz decays η → e+e-γ and ω → π0e+e- have been measured in the γp → ηp and γp → ωp reactions, respectively, with the A2 tagged-photon facility at the Mainz Microtron. The value obtained for the ...slope parameter of the electromagnetic transition form factor of η, Λ$-2\atop{η}$ = (1.97 ± 0.11tot) GeV-2, is in good agreement with previous measurements of the η → e+e-γ and η → μ+μ-γ decays. The uncertainty obtained in the value of Λ$-2\atop{η}$ is lower than in previous results based on the η → e+e-γ decay. The value obtained for the ω slope parameter, Λ$-2\atop{ωπ^0}$ = (1.99 ± 0.21tot) GeV-2, is somewhat lower than previous measurements based on ω → π0μ+μ-, but the results for the ω transition form factor are in better agreement with theoretical calculations, compared to earlier experiments.
The Compton double-polarization observable Σ 2z has been measured for the first time in the Δ (1232) resonance region using a circularly polarized photon beam incident on a longitudinally polarized ...proton target at the Mainz Microtron. This paper reports these results, together with the model-dependent extraction of four proton spin polarizabilities from fits to additional asymmetry data using either a dispersion relation calculation or a baryon chiral perturbation theory calculations with the weighted average of these two fits resulting in γE1E1=–2.87±0.52, γM1M1=2.70±0.43, γE1M2=–0.85±0.72 , and γM1E2=2.04±0.43 , in units of 10–4 fm4 .
The Dalitz decay π0 → e+e-γ has been measured in the γp → π0p reaction with the A2 tagged-photon facility at the Mainz Microtron, MAMI. The value obtained for the slope parameter of the π0 ...electromagnetic transition form factor, aπ= 0.030 ± 0.010tot, is in agreement with existing measurements of this decay and with recent theoretical calculations. The uncertainty obtained in the value of aπ is lower than in previous results based on the π0 → e+e-γ decay.
Precise helicity-dependent cross sections and the double-polarization observable E were measured for η photoproduction from quasifree protons and neutrons bound in the deuteron. The η → 2γ and η → ...3π0 → 6γ decay modes were used to optimize the statistical quality of the data and to estimate systematic uncertainties. The measurement used the A2 detector setup at the tagged photon beam of the electron accelerator MAMI in Mainz. A longitudinally polarized deuterated butanol target was used in combination with a circularly polarized photon beam from bremsstrahlung of a longitudinally polarized electron beam. The reaction products were detected with the electromagnetic calorimeters Crystal Ball and TAPS, which covered 98% of the full solid angle. The results show that the narrow structure observed earlier in the unpolarized excitation function of η photoproduction off the neutron appears only in reactions with antiparallel photon and nucleon spin (σ1/2). It is absent for reactions with parallel spin orientation (σ3/2) and thus very probably related to partial waves with total spin 1/2. The behavior of the angular distributions of the helicity-dependent cross sections was analyzed by fitting them with Legendre polynomials. The results are in good agreement with a model from the Bonn-Gatchina group, which uses an interference of P11 and S11 partial waves to explain the narrow structure.
Here, the data available from the A2 Collaboration at MAMI were analyzed to select the γp → π0ηp reaction on an event-by-event basis, which allows for partial-wave analyses of three-body final states ...to obtain more reliable results, compared to fits to measured distributions. These data provide the world’s best statistical accuracy in the energy range from threshold to Eγ = 1.45 GeV, allowing a finer energy binning in the measurement of all observables needed for understanding the reaction dynamics. The results obtained for the measured observables are compared to existing models, and the impact from the new data is checked by the fit with the revised Mainz model.
Background: Photoproduction of mesons off quasi-free nucleons bound in the deuteron allows to study the elec- tromagnetic excitation spectrum of the neutron and the isospin structure of the ...excitation of nucleon resonances. The database for such reactions is much more sparse than for free proton targets. Purpose: Study experimentally single π0 photoproduction off quasi-free nucleons from the deuteron. Investigate nuclear effects by a comparison of the results for free protons and quasi-free protons. Use the quasi-free neutron data (corrected for nuclear effects) to test the predictions of reaction models and partial wave analysis (PWA) for γn → nπ0 derived from the analysis of the other isospin channels. Methods: High statistics angular distributions and total cross sections for the photoproduction of π0 mesons off the deuteron with coincident detection of recoil nucleons have been measured for the first time. The experiment was performed at the tagged photon beam of the Mainz Microtron (MAMI) accelerator for photon energies between 0.45 GeV and 1.4 GeV, using an almost 4π electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. A complete kinematic reconstruction of the final state removed the effects of Fermi motion. Results: Significant effects from final state interactions (FSI) were observed for participant protons in comparison to free proton targets (between 30% and almost 40%). The data in coincidence with recoil neutrons were corrected for such effects under the assumption that they are identical for participant protons and neutrons. Reaction model predictions and PWA for γn → nπ0, based on fits to data for the other isospin channels, disagreed between themselves and no model provided a good description of the new data. Conclusions: The results demonstrate clearly the importance of a measurement of the fully neutral final state for the isospin decomposition of the cross section. Model refits, for example from the Bonn-Gatchina analysis, show that the new and the previous data for the other three isospin channels can be simultaneously described when the contributions of several partial waves are modified. Finally, the results are also relevant for the suppression of the higher resonance bumps in total photoabsorption on nuclei, which are not well understood.
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