SPring-8, the large synchrotron radiation facility

   Physicists in the Laser Electron Photon experiment at SPring-8 (LEPS) collaboration (spokesperson: Prof. Takashi Nakano of Research Center for Nuclear Physics (RCNP), Osaka University), comprising 52 scientists from 19 Japanese and foreign institutions, including Osaka University, Japan Atomic Energy Research Institute (JAERI) and Japan Synchrotron Radiation Research Institute (JASRI), have discovered a new baryon state at SPring-8.
   The new baryon is made up of five quarks (penta-quark baryon). The LEPS group analyzed the reactions on neutrons irradiated by world's highest-energy laser electron photon beam at BL33LEP at SPring-8 and identified the new baryon with a mass that is 1.6 times greater than a neutron.

   A nucleus is made up of nucleons (protons and neutrons). A neucleon is made up of three quarks. The baryons, including nucleons, which have been observed so far are all three-quark states, and an individual quark has never been observed. This discovery is drawing attention as the first discovery in the world that can address the questions of whether there are baryons which are not made up of three quarks and why quarks are confined in the particles.
This result appears in Physical Review Letters 4 July 2003; Phys. Rev. Lett. 91, 012002 (2003); T. Nakano et al.; "Evidence for a Narrow S = +1 Baryon Resonance in Photoproduction from the Neutron".

   The LEPS group irradiated carbon nucleus with a laser electron photon beam of approximately 2 billion electron volts and simultaneously measured two K mesons with positive and negative charges. By analyzing the data carefully, the group has found that a K^- meson and a new particle are simultaneously generated when a neutron inside the nucleus absorbs a photon and that the new particle subsequently decayed into a neutron and K⁺ meson. The mass of the new particle, 1.54 billion electron volts, which was calculated from the energy of the photon beam and the K^- meson, is very close to the five-quark Theta⁺ particle predicted by a Russian theorist, Dmitri Diakonov, and his colleagues in 1997 (see Fig. 1).

   Recently, groups from Thomas Jefferson National Accelerator Facility (Jefferson Lab) in the United States and Institute for Theoretical and Experimental Physics (ITEP) in Russia reported their results confirming the discovery.