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 Physics , 2015, Abstract: Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of $\pi^0\pi^0$ pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerator facility in Mainz using the Glasgow photon tagging spectrometer and the Crystal Ball/TAPS detector. The accelerator delivered electron beams of 1508 and 1557~MeV, which produced bremsstrahlung in thin radiator foils. The tagged photon beam covered energies up to 1400~MeV. The data from the free proton target are in good agreement with previous measurements and were only used to test the analysis procedures. The results for differential cross sections (angular distributions and invariant-mass distributions) for free and quasi-free protons are almost identical in shape, but differ in absolute magnitude up to 15\%. Thus, moderate final-state interaction effects are present. The data for quasi-free neutrons are similar to the proton data in the second resonance region (final state invariant masses up to $\approx$1550~MeV), where both reactions are dominated by the $N(1520)3/2^-\rightarrow \Delta(1232)3/2^+\pi$ decay. At higher energies, angular and invariant-mass distributions are different. A simple analysis of the shapes of the invariant-mass distributions in the third resonance region is consistent with strong contributions of an $N^{\star}\rightarrow N\sigma$ decay for the proton, while the reaction is dominated by a sequential decay via a $\Delta\pi$ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel analysis.
 Physics , 2014, DOI: 10.1140/epja/i2014-14054-5 Abstract: Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of mixed-charge pion pairs in the reactions $\boldsymbol{\gamma}p\rightarrow n\pi^0\pi^+$ off free protons and $\boldsymbol{\gamma}d\rightarrow (p)p\pi^0\pi^-$ and $\boldsymbol{\gamma}d\rightarrow (n)n\pi^0\pi^+$ 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 $\pi^0$ mesons were detected in the 4$\pi$ 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.
 Physics , 2013, DOI: 10.1016/j.physletb.2013.03.021 Abstract: Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions $\vec{\gamma}p\rightarrow p\pi^0\pi^0$ and $\vec{\gamma}d\rightarrow (n)p\pi^0\pi^0$, $\vec{\gamma}d\rightarrow (p)n\pi^0\pi^0$ off free protons and 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 magnetic spectrometer. Decay photons from the $\pi^0$ mesons, recoil protons, and recoil neutrons were detected in the 4$\pi$ covering electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. After kinematic reconstruction of the final state, excellent agreement was found between the results for free and quasi-free protons. This demonstrates that the free-nucleon behavior of such observables can be extracted from measurements with quasi-free nucleons, which is the only possibility for the neutron. Contrary to expectations, the measured asymmetries are very similar for reactions off protons and neutrons. The results are compared to the predictions from the Two-Pion-MAID reaction model and (for the proton) also to the Bonn-Gatchina coupled channel analysis.
 Oberle Markus EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20123709026 Abstract: Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions γp → pπ0π0 and γd → (n)pπ0π0, γd → (p)nπ0π0 off free protons and off quasi-free nucleons bound in the deuteron for incident photon energies up to 1.4 GeV. A circularly polarized photon beam was produced off a longitudinally polarized electron beam using bremsstrahlung processes. The photons were tagged with the Glasgow magnetic spectrometer. Decay photons from the π0 mesons, recoil protons, and recoil neutrons were detected in the 4π covering detector system composed of the Crystal Ball and TAPS electromagnetic calorimeters. The free and quasi-free results are in almost perfect agreement. The measured asymmetries for reactions off protons and neutrons are very similar, in contrary to expectations. The results are compared to the predictions from the Two-Pion-MAID reaction model.
 L. Lesniak Physics , 1998, Abstract: S-wave photoproduction of pion and kaon pairs on hydrogen is studied. Coupled channel final state interactions between different pseudoscalar mesons are included. Effective mass and momentum transfer distributions as well as total cross sections are calculated. Effective mass distributions in the Born approximation are structureless but the final state interactions produce peaks and dips near the energies corresponding to the scalar resonances f0(980) and f0(1400). Calculations are performed in the effective mass region between the pion-pion threshold and 1.6 GeV in the range of the momentum transfer squared up to 2 (GeV/c)^2. The photon energy dependence of the total cross sections is studied between 2 and 15 GeV. Valuable information on the f0(980) resonance structure can be extracted from data. These calculations are closely related to the experimental program of the CEBAF accelerator in USA.
 Physics , 2004, DOI: 10.1016/j.physletb.2005.01.101 Abstract: Energetic neutrons produced in $ep$ collisions at HERA have been studied with the ZEUS detector in the photoproduction regime at a mean photon-proton center-of-mass energy of 220 GeV. The neutrons carry a large fraction $0.64 < x_L <0.925$ of the incoming proton energy, and the four-momentum-transfer squared at the proton-neutron vertex is small, $|t|<0.425$ GeV$^2$. The $x_L$ distribution of the neutrons is measured in bins of $t$. The $(1-x_L)$ distributions in the $t$ bins studied satisfy a power law $dN/dx_L \propto (1-x_L)^{a(t)}$, with the powers $a(t)$ following a linear function of $t$: \mbox{$a(t)=0.88\pm 0.09 ({\rm stat.})^{+0.34}_{-0.39}({\rm syst.})-(2.81\pm 0.42({\rm stat.})^{+1.13}_{-0.62}({\rm syst.}) {\rm Ge V}^{-2})t$.} This result is consistent with the expectations of pion-exchange models, in which the incoming proton fluctuates to a neutron-pion state, and the electron interacts with the pion.
 Dieterle Manuel EPJ Web of Conferences , 2012, DOI: 10.1051/epjconf/20123709013 Abstract: We report on the preliminary results of a high statistics measurement of π0- photoproduction off quasi-free protons and neutrons from the deuteron with the CB/TAPS setup at the MAMI electron accelerator facility in Mainz. Preliminary di_erential and total cross sections covering the full angular range and photon energies up to the second and third resonance region have been measured. The comparison of free and quasi-free proton results allows a detailed analysis of possible nuclear effects on the cross section.
 Physics , 2014, DOI: 10.1103/PhysRevC.90.015205 Abstract: Differential and total cross sections for the quasifree reactions $\gamma p\rightarrow\eta p$ and $\gamma n\rightarrow\eta n$ have been determined at the MAMI-C electron accelerator using a liquid deuterium target. Photons were produced via bremsstrahlung from the 1.5 GeV incident electron beam and energy-tagged with the Glasgow photon tagger. Decay photons of the neutral decay modes $\eta\rightarrow 2\gamma$ and $\eta\rightarrow 3\pi^0 \rightarrow 6\gamma$ and coincident recoil nucleons were detected in a combined setup of the Crystal Ball and the TAPS calorimeters. The $\eta$-production cross sections were measured in coincidence with recoil protons, recoil neutrons, and in an inclusive mode without a condition on recoil nucleons, which allowed a check of the internal consistency of the data. The effects from nuclear Fermi motion were removed by a kinematic reconstruction of the final-state invariant mass and possible nuclear effects on the quasifree cross section were investigated by a comparison of free and quasifree proton data. The results, which represent a significant improvement in statistical quality compared to previous measurements, agree with the known neutron-to-proton cross-section ratio in the peak of the $S_{11}(1535)$ resonance and confirm a peak in the neutron cross section, which is absent for the proton, at a center-of-mass energy $W = (1670\pm 5)$ MeV with an intrinsic width of $\Gamma\approx 30$ MeV.
 Physics , 2001, DOI: 10.1007/s100500170085 Abstract: We investigate near threshold neutral pion photoproduction off protons to fourth order in heavy baryon chiral perturbation theory in the light of the new data from MAMI. We show that the unitarity cusp at the secondary \pi^+ n threshold is in agreement with expectations from the final state theorem. We also analyze the fourth order corrections to the P-wave low-energy theorems and show that potentially large Delta-isobar contributions are cancelled by sizeable pion loop effects. This solidifies the parameter free third order predictions, which are in excellent agreement with the data.
 Manuel Dieterle Physics , 2011, Abstract: There is evidence that the photoproduction of single and double pions off bound nucleons inside a nucleus are not only affected by Fermi motion but also by other nuclear effects, such as final state interactions or meson rescattering. We will present preliminary results of a high statistics measurement of single and double pion photoproduction of quasi-free protons and neutrons off the deuteron carried out at the Mainzer Microtron.
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