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تسجيل مستخدم جديدCoherent photoproduction of pi^0- and eta-mesons off 7Li
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Coherent photoproduction of $pi^0$-mesons from threshold ($E_{th} approx$ 136 MeV) throughout the $Delta$-resonance region and of $eta$-mesons close to the production threshold ($E_{th} approx$ 570 MeV for $eta$) has been measured for $^7$Li 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 4$pi$ solid-angle electromagnetic calorimeter. The reactions were identified by a combined invariant-mass and missing-energy analysis. A comparison of the pion data to plane-wave impulse modelling tests the nuclear mass form factor. So far coherent $eta$-production had been only identified for the lightest nuclear systems ($^2$H and $^3$He). For $^3$He 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 $eta$-production agree with a plane-wave approximation. Contrary to $^3$He, 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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Coherent photoproduction of $eta$-mesons off $^3$He, i.e. the reaction $gamma ^3{He}rightarrow eta ^3{He}$, has been investigated in the near-threshold region. The experiment was performed at the Glasgow tagged photon facility of the Mainz MAMI accel
erator with the combined Crystal Ball - TAPS detector. Angular distributions and the total cross section were measured using the $etarightarrow gammagamma$ and $etarightarrow 3pi^0rightarrow 6gamma$ decay channels. The observed extremely sharp rise of the cross section at threshold and the behavior of the angular distributions are evidence for a strong $eta {^3{He}}$ final state interaction, pointing to the existence of a resonant state. The search for further evidence of this state in the excitation function of $pi^0$-proton back-to-back emission in the $gamma ^3{He}rightarrow pi^0 pX$ reaction revealed a very complicated structure of the background and could not support previous conclusions.
Total and differential cross sections for $eta$ and $eta ^prime$ photoproduction off the proton have been determined with the CBELSA/TAPS detector for photon energies between 0.85 and 2.55 GeV. The $eta$ mesons are detected in their two neutral decay
modes, $etatogammagamma$ and $etato 3pi^0to 6gamma$, and for the first time, cover the full angular range in $rm cos theta_{cm}$ of the $eta$ meson. These new $eta$ photoproduction data are consistent with the earlier CB-ELSA results. The $eta ^prime$ mesons are observed in their neutral decay to $pi^0pi^0etato 6gamma$ and also extend the coverage in angular range.
Quasi-free photoproduction of $eta $ mesons off nucleons bound in the deuteron has been measured with the combined Crystal Barrel - TAPS detector. The experiment was done at a tagged photon beam of the ELSA electron accelerator in Bonn for incident p
hoton energies from the production threshold up to 2.5 GeV. The $eta $-mesons have been detected in coincidence with recoil protons and recoil neutrons. The quasi-free proton data are in good agreement with the results for free protons, indicating that nuclear effects have no significant impact. The coincidence with recoil neutrons provides the first data for the $gamma n rightarrow neta $ reaction. In addition, also first estimates for coherent $eta $-production off the deuteron have been obtained. In agreement with model predictions, the total cross section for this channel is found to be very small, at most at the level of a few nb. The data are compared to model calculations taking into account contributions from nucleon resonances and $t$-channel exchanges.
Differential and total cross sections for the quasifree reactions $gamma prightarroweta p$ and $gamma nrightarroweta n$ 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 $etarightarrow 2gamma$ and $etarightarrow 3pi^0 rightarrow 6gamma$ 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 $W = (1670pm 5)$ MeV with an intrinsic width of $Gammaapprox 30$ MeV.
Total cross sections, angular distributions, and invariant-mass distributions have been measured for the photoproduction of $pi^0pi^0$ pairs off free protons and off nucleons bound in the deuteron. The experiments were performed at the MAMI accelerat
or 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 $N^{star}rightarrow Nsigma$ decay for the proton, while the reaction is dominated by a sequential decay via a $Deltapi$ intermediate state for the neutron. The data are compared to predictions from the Two-Pion-MAID model and the Bonn-Gatchina coupled channel analysis.
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