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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 photon 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.
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Precise data for quasi-free photoproduction of $eta$ mesons off the deuteron have been measured at the Bonn ELSA accelerator with the combined Crystal Barrel/TAPS detector for incident photon energies up to 2.5 GeV. The $eta$-mesons have been detected in coincidence with recoil protons and neutrons. Possible nuclear effects like Fermi motion and re-scattering can be studied via a comparison of the quasi-free reaction off the bound proton to $eta$-production off the free proton. No significant effects beyond the folding of the free cross section with the momentum distribution of the bound protons have been found. These Fermi motion effects can be removed by an analysis using the invariant mass of the $eta$-nucleon pairs reconstructed from the final state four-momenta of the particles. The total cross section for quasi-free $eta$-photoproduction off the neutron reveals even without correction for Fermi motion a pronounced bump-like structure around 1 GeV of incident photon energy, which is not observed for the proton. This structure is even narrower in the invariant mass spectrum of the $eta$-neutron pairs. Position and width of the peak in the invariant mass spectrum are $Wapprox 1665$ MeV and FWHM $Gammaapprox 25$ MeV. The data are compared to the results of different models.
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 has been measured off nucleons bound in $^3$He 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 $4pi$ covering electromagnetic calorimeter, combining the TAPS and Crystal Ball detectors. The $eta$-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.
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.
A formalism is developed for the partial wave analysis of data on meson photoproduction off deuterons and applied to photoproduction of $eta$ and $pi^0$ mesons. Different interpretations of a dip-bump structure of the $eta$ photoproduction cross section in the 1670 MeV region are presented and discussed. Helicity amplitudes for two low-mass $S_{11}$ states are determined.
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