A nucleon (proton or neutron) has a radius of about 10-15 m. This finite size of nucleons indicates that there are smaller constituents, which are called quarks and gluons. On the basis of current scientific knowledge, nucleons consist of up and down quarks (ud quarks) that are envisioned as point objects with a mass of 5 - 10 keV. If this is so, what is responsible for the remaining large nucleon mass of about 1 GeV? Are there numerous gluons or ud quark pairs which carry the remaining portion? Do strange quarks with a mass of 150 MeV/c2 exist in a nucleon? These are our basic questions.
Our group is studying the quark structure of nucleons via the observation of phi mesons in photonuclear reactions generated by an inverse Compton gamma-ray beam at SPring-8. Figure 4-1(a) shows the mass spectrum observed when a polyethylene target is irradiated with inverse Compton gamma-rays of energy Egamma= 1.6 - 2.4 GeV. We have observed events due to the generation of protons, deuterons, tritons, and of K-, pi-, pi+, and K+ mesons. The yields of pi- and pi+ mesons are almost the same. The pi+ and pi- mesons are generated in the reactions gamma+p  pi++n and gamma+n pi-+p, respectively. We have also observed K mesons, including a strange quark. K+ mesons are generated in the reactions gamma+p K++Lambda and gamma+n K++Sigma0. In the case of phi meson production, first the reaction gamma+p phi+p occurs, then the phi meson spontaneously decays into K- and K+ mesons, producing up and anti-up quarks from vacuum. We have observed these phi mesons by making coincidence measurements of K+ and K- mesons (Fig. 4-1(b)). The phi mesons are generated by pair creation of strange and anti-strange quarks in nucleons by means of high energy gamma-rays.
The energies of inverse Compton gamma-rays at SPring-8 are the highest in the world and have the special properties of linear and circular polarization. We have now arrived at the stage where we will be able to study strange-quark dynamics as well as the role of up and down quarks in nucleons. |
