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Register a new userThe isospin structure of photoproduction of pi-eta pairs from the nucleon in the threshold region
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Photoproduction of $pieta$-pairs from nucleons has been investigated from threshold up to incident photon energies of $approx$~1.4~GeV. The quasi-free reactions $gamma prightarrow ppi^0eta$, $gamma nrightarrow npi^0eta$, $gamma prightarrow npi^+eta$, and $gamma nrightarrow ppi^-eta$ were for the first time measured from nucleons bound in the deuteron. The corresponding reactions from a free-proton target were also studied to investigate final-state interaction effects (for neutral pions the free-proton results could be compared to previous measurements; the $gamma prightarrow npi^+eta$ reaction was measured for the first time). For the $pi^0eta$ final state coherent production via the $gamma drightarrow dpi^0eta$ reaction was also investigated. The experiments were performed at the tagged photon beam of the Mainz MAMI accelerator using an almost $4pi$ coverage electromagnetic calorimeter composed of the Crystal Ball and TAPS detectors. The total cross sections for the four different final states obey the relation $sigma(ppi^0eta)$ $approx$ $sigma(npi^0eta)$ $approx$ $2sigma(ppi^-eta)$ $approx$ $2sigma(npi^+eta)$ as expected for a dominant contribution from a $Delta^{star}rightarrowetaDelta(1232)rightarrowpieta N$ reaction chain, which is also supported by the shapes of the invariant-mass distributions of nucleon-meson and $pi$-$eta$ pairs. The experimental results are compared to the predictions from an isobar reaction model.
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Precise total cross-sections and invariant-mass distributions have been measured for photoproduction of pion pairs off the proton producing $ppi^0pi^0$ and $npi^+pi^0$ final states from the threshold region up to 800 MeV incident photon energy. Additionally, beam helicity asymmetries have been measured in the second resonance region (550 MeV - 820 MeV). The experiment was performed at 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 results are much more precise than any previous measurements and confirm the chiral perturbation theory predictions for the threshold behavior of these reactions. In the second resonance region, the invariant-mass distributions of meson-meson and meson-nucleon pairs are in reasonable agreement with model predictions, but none of the models reproduce the asymmetries for the mixed-charge channel.
A comparison of the close-to-threshold total cross section for the eta prime production in pp --> pp eta prime and pn --> pn eta prime reactions constitutes a tool to investigate the eta prime meson structure and the reaction mechanism in the channels of isospin I=0 and I=1 and may provide insight into the flavour-singlet (perhaps also into gluonium) content of the eta prime meson. In this contribution we present preliminary results of measurement of the quasi-free production of the eta prime meson in the proton-neutron collisions conducted using the COSY-11 facility.
Differential cross-sections for quasi-free $pi^0$ photoproduction from the proton and neutron bound in the deuteron have been measured for $E_gamma= 200 - 400$ MeV at $theta^{rm lab}_gamma = 136.2^circ$ usind the Glasgow photon tagger at MAMI, the Mainz 48 cm $varnothing$ $times$ 64 cm NaI(Tl) photon detector and the Gottingen SENECA recoil detector. For the proton measurements made with both liquid deuterium and liquid hydrogen targets allow direct comparison of free $pi^0$ photoproduction cross-sections as extracted from the bound proton data with experimental free cross sections which are found to be in reasonable agreement below 320 MeV. At higher energies the free cross sections extracted from quasifree data are significantly smaller than the experimental free cross sections and theoretical predictions based on multipole analysis. For the first time, free neutron cross sections have been extracted in the $Delta$-region. They are also in agreement with the predictions from multipole analysis up to 320 MeV and significantly smaller at higher photon energies.
Cross-sections and recoil polarizations for the reactions gamma + p --> K^+ + Lambda and gamma + p --> K^+ + Sigma^0 have been measured with high statistics and with good angular coverage for center-of-mass energies between 1.6 and 2.3 GeV. In the K^+Lambda channel we confirm a structure near W=1.9 GeV at backward kaon angles, but our data shows a more complex s- and u- channel resonance structure than previously seen. This structure is present at forward and backward angles but not central angles, and its position and width change with angle, indicating that more than one resonance is playing a role. Rising back-angle cross sections at higher energies and large positive polarization at backward angles are consistent with sizable s- or u-channel contributions. None of the model calculations we present can consistently explain these aspects of the data.
Beam asymmetry and differential cross section for the reaction gamma+p->eta+p were measured from production threshold to 1500 MeV photon laboratory energy. The two dominant neutral decay modes of the eta meson, eta->2g and eta->3pi0, were analyzed. The full set of measurements is in good agreement with previously published results. Our data were compared with three models. They all fit satisfactorily the results but their respective resonance contributions are quite different. The possible photoexcitation of a narrow state N(1670) was investigated and no evidence was found.
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