Measurement of the beam-helicity asymmetry $I^{odot}$ in the photoproduction of $pi^0$-pairs off the proton and off the neutron

Beam-helicity asymmetries have been measured at the MAMI accelerator in Mainz for the photoproduction of neutral pion pairs in the reactions $vec{gamma}prightarrow ppi^0pi^0$ and $vec{gamma}drightarrow (n)ppi^0pi^0$, $vec{gamma}drightarrow (p)npi^0pi^0$ 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.

Coherent photoproduction of pi^0- and eta-mesons off 7Li

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.

Photoproduction of pi0pi0 and pi0pi+-pairs off the proton from threshold to the second resonance region

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.

Quasi-free photoproduction of eta-mesons off the deuteron

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.