Virtal-photon capture in the three-nucleon system

In nuclei consisting out of more than three nucleons, the interaction between any two is influenced by the presence of the others. The study of proton-deuteron reactions is therefore of great interest to understand three-nucleon systems. Consequently, the two following capture reactions have been investigated in this work: • d+p -> + (real-photon capture) • d+p -> + * (virtual-photon capture) To perform the experiment, a deuteron beam (d) is aimed at a hydrogen target (p). A is then created and the rest of the energy is carried out by a photon () or a virtual photon (*). The virtual photon is an unstable photon with a mass, experimentally detected as an electron-positron pair (e-, e+). The experiment took place at the KVI, in Groningen, in 2003, with a 180 MeV deuteron beam and a liquid hydrogen target, using the detectors “Big Bite Spectrometer” to detect the helium and the “Plastic Ball” to detect photons and leptons. The setup allowed us to differentiate between both types of particles. Good statistic data were collected: and events for the real- and the virtual-photon capture reactions, respectively. In the following two years, they have been analyzed and compared to theoretical models. One observed that the cross sections of both reactions are relatively well reproduced by the theory. In the case of the virtual-photon capture, the cross section can be decomposed into four terms, the “response functions”, which depend on the polarization of the photon. In that case, discrepancies between the data and the models were observed. Nevertheless, our accurate data seem to be reliable tools to improve the models