Fig. 7-4 A scheme to detect Theta+ particles consisting five quarks. When a high energy gamma-ray is absorbed by a neutron, a K- meson and a five-quark particle (Theta+) are produced. The production of a Theta+ particle is confirmed by identifying two K mesons.

Fig. 7-5 Diagram of five-quark baryons. The Theta+ particle located at the top of the pyramid has one anti-strange quark, two u-quarks, and two d-quarks. The numbers in parentheses are the particle masses in units of MeV/c2.

A neutron consists of one up quark (u-quark) and two down quarks (d-quarks). A meson consists of one quark and one anti-quark. The pie meson theoretically predicted to explain the strong nuclear force by Dr. Yukawa is a typical meson. This meson is well known nowadays as the basic meson which binds protons and neutrons in nuclei. This prescription may look very strange. However, it is a basic concept for understanding the elementary ingredients of matter in our universe. Nuclei, which are the fundamental source of nuclear power, are a world composed of neutrons and protons (called "nucleons" ) with a size of 10-12 cm. The basic nucleons consist of three quarks. Up to now, scientists have observed only particles with two or three quarks. No observations have been reported of single quarks. In addition, however, there is a longstanding question: Why have scientists not observed particles composed of four, five, or six quarks? Quark theories do not forbid the presence of multi-quark particles. We have initiated hadron physics experiments with high-energy gamma-ray beams produced by inverse Compton scattering at the SPring-8 electron storage ring. A carbon target is bombarded by a gamma-ray beam with a maximum energy of 2.4 GeV, and positive and negative, charged K mesons are measured in coincidence. After making an invariant mass spectrum, we have confirmed the presence of particles at 1.540 GeV. This particle is suggested to correspond to the five-quark Theta+ particle predicted theoretically to occur at 1.530 GeV. Fig. 7-4 shows a scheme to detect Theta+ particles consisting five quarks. First, a neutron absorbs a high energy gamma-ray. Second, a negative K- meson and a Theta+ particle are produced. Finally, the Theta+ particle decays by emitting a K+ meson and a neutron. We have confirmed that the mass of this particle is 1.54 GeV with a width of less than 25 MeV. This particle is now being discussed as a very strong candidate for the "Penta-quark Theta+ particle," consisting of one anti-strange quark (s-quark), two u-quarks, and two d-quarks. Thus, it is a new type of hadron particle. Several experimental groups have reported similar resonances near 1.54 GeV from independent observations at research institutes in the United States and Europe. As shown in Fig. 7-5, this newly discovered particle very likely corresponds to one of the penta-quark particles first discussed by Jaffe and subsequently predicted by Diakonov, thus making a big impression on scientists around the world. After the publication of our report, more than 200 theoretical papers have been presented to discuss the nature of the penta-quark particle. The central issues of the discussion are, "What is the quark structure of the penta-quark particle?" and "What are the spin and parity of the particle, ½+ or ½- ? ." On the other hand, there are negative experiments which report no evidence for the presence of such a penta-quark particle. In experiments using beam energies higher than 500 GeV, scientists have not been able to observe this particle. The reason for this is not yet clear. Since the existence of the penta-quark particle will change our understanding of the hierarchical structure of the universe, hot discussions are ongoing among experimentalists and theorists concerning "Penta-Quark Physics."

Reference T. Nakano et al., Evidence for a Narrow S=+1 Baryon Resonance in Photoproduction form the Neutron, Phys. Rev. Lett., 91, 012002 (2003).

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