Meson photoproduction on deuterium at GRAAL

During my PhD, I investigated various photoproduction reactions on a deuterium target, analysing data from the GRAAL experiment. GRAAL is a photoproduction experiment that took place in Grenoble (France) at the European Synchrotron Radiation Facility (ESRF). The GRAAL facility collected data on photoproduction reactions for 10 years, i.e. reactions in which a gamma photon interacts with a nucleon or a light nucleus, producing mesons and nucleons in the final state, and concluded its data collection in 2008. At GRAAL, photoproduction reactions of mesons were carried out using unpolarized liquid hydrogen or deuterium targets and a gamma photon beam produced by Compton backscattering of a laser beam (from visible to FUV line) on 6 GeV ultrarelativistic electrons circulating in the ESRF storage ring. Several channels were investigated at GRAAL, and the corresponding results have been published and are available in the literature. During my PhD, I worked on the recovery of GRAAL data from its original software structure, based on PAW and Fortran libraries, to the new one based on the latest ROOT and C++ libraries. The new shape of data allowed me to investigate several photoproduction channels. The GRAAL data are actually preserved at the University of Messina, but the final plan is to make them available on a public server for future investigations. In this work, a part of my work was devoted to the study of η′ photoproduction on quasifree neutrons in a liquid deuterium target, with a focus on the measurement of the beam asymmetry at threshold. The aim of this work is to perform a comparison between the beam asymmetry measurements for the channel γ n → η′ n in the energy interval of the incoming γ photon: 1.447 GeV- 1.475 GeV, and the beam asymmetry measurements for the channel γ p → η′ p, in the same energy interval performed in the past by GRAAL. The η′ photoproduction channel off the nucleon is interesting, because it acts as an isospin filter, allowing the excitation of only one-half isospin resonances. The photoproduction channel of η′ off the free protons was investigated by GRAAL, by providing beam asymmetry measurements. The beam asymmetries for the channel γ p → η′ p produced by GRAAL are characterised by a low uncertainty, thanks to the higher statistics available for the data on the hydrogen target, and the high degree of polarization of the beam, which is very close to 100 % near the η′ reaction threshold. In particular, this feature of the beam is due to Compton backscattering. The beam asymmetry measurements of GRAAL for the channel γ p → η′ p, near the threshold, show an unusual nodal structure, more intense as closer to the threshold. This structure is compatible with an interference between S and F, or P and D partial waves. At the same time, the CLAS measurements show an enhancement in the same energy intervals. The two theoretical groups MAID and Bonn-Gatchina proposed two models for describing the behaviour shown by the GRAAL and CLAS measurements. The two models need to include a resonance to reproduce the measurements from both experiments, which is an S11 resonance in the MAID model, and a D13 resonance in the Bonn-Gatchina model. The theoretical groups strongly require beam asymmetry measurements for the η′ photoproduction off the neutron, to provide a comparison with the measurements on the free proton channel in order to solve the ambiguity on the resonance contribution to be included to describe the data. In this work, the beam asymmetry measurements for the η′ photoproduction channel off the bound neutron inside the deuterium target in the energy interval of the incoming beam:1.447 GeV - 1.475 GeV, by analyzing the data collected by GRAAL utilizing a deuterium target and UV laser lines, have been performed. Our measurements are in agreement with those on the free proton channel. I also investigated the coherent η π0 and double π0 photoproduction off the deuteron, i.e. the channels: γ d → η π0d and γ → π0 π0d, estracting the yields of invariant mass spectra of final two-body systems and Dalitz plots. Concerning our analysis of the coherent η π0 photoproduction off the deuteron, the invariant mass distributions of the π0d, ηd and π0η systems in the two center-of-mass energy intervals: 2661 Mev ≤W≤ 2703 MeV and 2703 MeV ≤ 2799 MeV are quatitatively in agreement with the cross-section distributions measured and pubblished by FOREST and BGOOD collaborations. The BGOOD measurements are described by a model assuming a quasi-free excitation of N* and Δ resonances on a single nucleon, followed by pion rescattering on the spectator nucleon, and coalescence of the two nucleons into the deuteron. This model is also in agreement with the existence of a backward peak in the GRAAL distribution of the center-of-mass θ angle of the deuteron in the two multi-meson channels on the deuteron. Our analysis on the double π0 photoproduction channel off the deuteron at GRAAL reveals the existence of some resonances in the center-of-mass reference frame. The masses and widths of the two center-of-mass resonances observed at GRAAL are compatible with those of the two highest-mass resonances observed at FOREST and BGOOD collaborations. The analysis of BGOOD and FOREST reveals a third resonance, identified with the d*(2380) resonance, which is not detected at GRAAL because of the tagging system used.