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Register a new userQuasifree photoproduction of $eta$ mesons off protons and neutrons
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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.
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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.
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 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 $N^{star}rightarrow Nsigma$ 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.
A study of the partial-wave content of the $gamma pto eta^prime p$ reaction in the fourth resonance region is presented, which has been prompted by new measurements of polarization observables for that process. Using the Bonn-Gatchina partial-wave formalism, the incorporation of new data indicates that the $N(1895)1/2^-$, $N(1900)3/2^+$, $N(2100)1/2^+$, and $N(2120)3/2^-$ are the most significant contributors to the photoproduction process. New results for the branching ratios of the decays of these more prominent resonances to $Neta^prime$ final states are provided; such branches have not been indicated in the most recent edition of the Review of Particle Properties. Based on the analysis performed here, predictions for the helicity asymmetry $E$ for the $gamma pto eta^prime p$ reaction are presented.
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.
Photoproduction of neutral pions has been studied with the CBELSA/TAPS detector in the reaction $gamma pto ppi^0$ for photon energies between 0.85 and 2.50 GeV. The $pi^0$ mesons are observed in their dominant neutral decay mode: $pi^0togammagamma$. For the first time, the differential cross sections cover the very forward region, $theta_{rm c.m.}<60^circ$. A partial-wave analysis of these data within the Bonn-Gatchina framework observes the high-mass resonances $G_{17}$(2190), $D_{13}$(2080), and $D_{15}$(2070).
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