Study On The Photoproduction Of The Scalar Meson F₀（980）

The study of the low-lying scalar mesons is a very hot issue in middle and high energy nuclear physics. For the four scalar mesons [σ(500),k(800),a0(980) and f0(980)]whose mass are below 1 GeV, the mass spectrum ordering of them disfavors the naive q ˉq meson picture and that might have some special structures, like(loosely bound)mesonic molecular states, Jaffe’s tetraquark states, and so on. The problem comes from the fact that the situation becomes complex in lower energy region and the approaches based on the QCD theory, such as lattice QCD, QCD sum rules and DysonSchwinger equations, encounter different degree of difficulties when I study the properties of the light scalar mesons, which makes the nature and structure of light scalar meson still a mystery, thus there came kinds of interpretation models, such as the traditional quark model, glueballs, tetraquarks, hadronic molecules and hybrids.In this thesis, I choose the f0(980) meson as my research object, because it is so unique and shows some unusual dynamic properties for its mass degenerates with the isospin triplet a0(980) and its mass is close to the K ˉK threshold. In the first section,I briefly review the history of the particle physics, the process of proposed the quark model and unique properties of the hadronic states. In the second section, the low mass states have been considered as quark-antiquark mesons, multiquarks, and hadron molecules, I introduce all these hypotheses. In the third section, I use the effective Lagrangian approach to calculate the differential cross sections of the photoproduction reaction γ p â†' p f0(980), and compare our results with the experimental data obtained by the CLAS Collaboration in JLab. Energy of the photon beam is relatively low, and in this case I find that comparing to the Regge propagator, the normal Feynman propagator has certain advantages. In addition, I predict the Ï€+Ï€-invariant mass distribution of the γ p â†' p f0(980) â†' pÏ€+Ï€-reaction, which can be tested by the future experiments. At the meanwhile, I also simply introduce the theoretical framework of this process, which is calculated based on the vector meson dominance model, although I don’t display the specific calculation results, I believe this method is more scientificand rigorous, and it will be in better accordance with the experimental data. In the last section, I will make a brief summary of my work during my undergraduate period, and look forward to the research on the light mass scalar mesons.