Beam asymmetry and differential cross-section for the reaction γp → ηp were measured from production threshold to 1500 MeV photon laboratory energy. The two dominant neutral decay modes of the η-meson, η → 2γ and η → 3π 0 , 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. PACS. 13.60.Le Meson production-13.88.+e Polarization in interactions and scattering-25.20.Lj Pho-toproduction reactions
The possibility of anisotropies in the speed of light relative to the limiting speed of electrons is considered. The absence of sidereal variations in the energy of Compton-edge photons at the European Synchrotron Radiation Facility's GRAAL facility constrains such anisotropies representing the first nonthreshold collision-kinematics study of Lorentz violation. When interpreted within the minimal standard-model extension, this result yields the two-sided limit of 1.6×10(-14) at 95% confidence level on a combination of the parity-violating photon and electron coefficients (κ(o+))(YZ), (κ(o+))(ZX), c(TX), and c(TY). This new constraint provides an improvement over previous bounds by 1 order of magnitude.
Abstract. The Σ beam asymmetry in η photoproduction off the proton was measured at the GrAAL polarised photon beam with incoming photon energies of 1.461 and 1.480 GeV. For both energies the asymmetry as a function of the meson production angle shows a clear structure, more pronounced at the lowest one, with a change of sign around 90• . The observed behaviour is compatible with P -wave D-wave (or S-wave F -wave) interference, the closer to threshold the stronger. The results are compared to the existing state-of-the-art calculations that fail to account for the data.The experimental study of nucleon excited states is fundamental for the understanding of its internal structure. Important differences are still observed today be- . Recent reports on advances in the experimental studies of the excited nucleon state spectrum can be found in [9,10].Several states predicted by these models have not been observed (missing resonances). The nucleon excited states decay strongly with meson emission; therefore meson photoproduction experiments off the nucleon are an ideal way of searching for missing resonances and complement the information obtained with pion-nucleon scattering experiments.
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