Contribution to the study of mesons and their interactions with nuclei, by threshold production in low nucleon number systems

The threshold excitation function of the pd → 3HeX reaction and the total η-meson production cross section near threshold in the dd → αη reaction have been measured using the SPES4 beam line of the Laboratoire National Saturne. In the pd → 3HeX threshold function, clear signals are observed at ω, η' and φ6 mesons threshold; some structures appear at central masses of the a0 and f0-mesons, as just above the two kaons production threshold. These structures could be the signature of new hadronic exotic states. The two-pion and two-kaon physical background has been estimated in a one-pion exchange model allowing the calculation of the differential cross section for the π-p → nX elementary process. The results of this calculation of the physical background of the threshold excitation function. The the dd → αη study, the η-meson was clearly observed in the spectrometer's final focal plane spectra, in good agreement with a Monte Carlo simulation. A total cross section of the order of the nano-barn has been measured. The results are compared to some other η-meson production processes with lighter nuclei than helium-4. Using different nuclei a comparative study of the η-nucleus interaction has shown that the latter is more binding in the helium-4 case. Based on these data, the charge symmetry breaking in the dd → απ0 reaction is studied. Using a π0-η mixing model, the extracted cross sections are compatible with the only measurement of this reaction cross section. (author)