The double polarization observable E and the helicity dependent cross sections σ_{1/2} and σ_{3/2} were measured for η photoproduction from quasifree protons and neutrons. The circularly polarized ...tagged photon beam of the A2 experiment at the Mainz MAMI accelerator was used in combination with a longitudinally polarized deuterated butanol target. The almost 4π detector setup of the Crystal Ball and TAPS is ideally suited to detect the recoil nucleons and the decay photons from η→2γ and η→3π^{0}. The results show that the narrow structure previously observed in η photoproduction from the neutron is only apparent in σ_{1/2} and hence, most likely related to a spin-1/2 amplitude. Nucleon resonances that contribute to this partial wave in η production are only N 1/2^{-} (S_{11}) and N 1/2^{+} (P_{11}). Furthermore, the extracted Legendre coefficients of the angular distributions for σ_{1/2} are in good agreement with recent reaction model predictions assuming a narrow resonance in the P_{11} wave as the origin of this structure.
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.
The quasifree
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\begin{document}$$\overrightarrow{\gamma } d\rightarrow \pi ^0n(p)$$\end{document}
γ
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0
n
(
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photon beam asymmetry,
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\begin{document}$$\varSigma $$\end{document}
Σ
, has been measured at photon energies,
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\begin{document}$$E_\gamma $$\end{document}
E
γ
, from 390 to 610 MeV, corresponding to center of mass energy from 1.271 to 1.424 GeV, for the first time. The data were collected in the A2 hall of the MAMI electron beam facility with the Crystal Ball and TAPS calorimeters covering pion center-of-mass angles from 49
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∘
to 148
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. In this kinematic region, polarization observables are sensitive to contributions from the
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\begin{document}$$\varDelta (1232)$$\end{document}
Δ
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1232
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N
(1440) resonances. The extracted values of
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\begin{document}$$\varSigma $$\end{document}
Σ
have been compared to predictions based on partial-wave analyses (PWAs) of the existing pion photoproduction database. Our comparison includes the SAID, MAID and Bonn–Gatchina analyses; while a revised SAID fit, including the new
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measurements, has also been performed. In addition, isospin symmetry is examined as a way to predict
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\begin{document}$$\pi ^0n$$\end{document}
π
0
n
photoproduction observables, based on fits to published data in the channels
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\begin{document}$$\pi ^0p$$\end{document}
π
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\begin{document}$$\pi ^+n$$\end{document}
π
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\begin{document}$$\pi ^-p$$\end{document}
π
-
p
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The γ n → π 0 n differential cross section evaluated for 27 energy bins span the photon-energy range 290– 813 MeV (W = 1.195–1.553 GeV) and the pion c.m. polar production angles, ranging from 18° to ...162°, making use of model-dependent nuclear corrections to extract π0 production data on the neutron from measurements on the deuteron target. Additionally, the total photoabsorption cross section was measured. The tagged photon beam produced by the 883 MeV electron beam of the Mainz Microtron MAMI was used for the π0-meson production. Our accumulation of 3.6 × 106 γ n → π 0 n events allowed a detailed study of the reaction dynamics. Our data are in reasonable agreement with previous A2 measurements and extend them to lower energies. The data are compared with predictions of previous SAID, MAID, and BnGa partial-wave analyses and to the latest SAID fit MA19 that included our data. Selected photon-decay amplitudes N * → γ n at the resonance poles are determined for the first time.
We report a measurement of the spin polarization of the recoiling neutron in deuterium photodisintegration, utilizing a new large acceptance polarimeter within the Crystal Ball at MAMI. The measured ...photon energy range of 300-700 MeV provides the first measurement of recoil neutron polarization at photon energies where the quark substructure of the deuteron plays a role, thereby providing important new constraints on photodisintegration mechanisms. A very high neutron polarization in a narrow structure centered around E_{γ}∼570 MeV is observed, which is inconsistent with current theoretical predictions employing nucleon resonance degrees of freedom. A Legendre polynomial decomposition suggests this behavior could be related to the excitation of the d^{*}(2380) hexaquark.
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 beam-helicity asymmetry was measured, for the first time, in photoproduction of π0η pairs on carbon, aluminum, and lead, with the A2 experimental setup at MAMI. The results are compared to an ...earlier measurement on a free proton and to the corresponding theoretical calculations. The Mainz model is used to predict the beam-helicity asymmetry for the nuclear targets. The present results indicate that the photoproduction mechanism for π0η pairs on nuclei is similar to photoproduction on a free nucleon. This process is dominated by the D33 partial wave with the ηΔ(1232) intermediate state.
The double-polarization observable Eand the helicity-dependent cross sections σ1/2and σ3/2have been measured for the first time for single π0photoproduction from protons and neutrons bound in the ...deuteron at the electron accelerator facility MAMI in Mainz, Germany. The experiment used a circularly polarized photon beam and a longitudinally polarized deuterated butanol target. The reaction products, recoil nucleons and decay photons from the π0 meson were detected with the Crystal Ball and TAPS electromagnetic calorimeters. Effects from nuclear Fermi motion were removed by a kinematic reconstruction of the π0N final state. A comparison to data measured with a free proton target showed that the absolute scale of the cross sections is significantly modified by nuclear final-state interaction (FSI) effects. However, there is no significant effect on the asymmetry E since the σ1/2 and σ3/2 components appear to be influenced in a similar way. Thus, the best approximation of the two helicity-dependent cross sections for the free neutron is obtained by combining the asymmetry E measured with quasi-free neutrons and the unpolarized cross section corrected for FSI effects under the assumption that the FSI effects are similar for neutrons and protons.
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.
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