A FIVE-QUARK STATE HAS BEEN DISCOVERED, first reported by a group of physicists working at the SPring-8 physics lab in Japan. All confirmed particles known previously have been either combinations of three quarks (baryons, such as protons or neutrons) or two quarks (mesons such as pions or kaons). Although not forbidden by the standard model of particle physics, other configurations of quarks had not been found till now. The "pentaquark" particle, with a mass just above 1.5 GeV, was discovered in the following way. At the Spring-8 facility a laser beam is scattered from a beam of 8-GeV electrons circulating in a synchrotron racetrack. These scattered photons constitute a beam of powerful gamma rays, which were scattered from a fixed target consisting of carbon-12 atoms. The reaction being sought was one in which a gamma and a neutron inside a carbon nucleus collided, leaving a neutron, a K+ meson, and a K- meson in the final state. Efficient detectors downstream of the collision area looked for the evidence of the existence of various combinations of particles, including a short-lived state in which the K+ and the neutron had coalesced. In this case the amalgamated particle, or resonance, would have consisted of the three quarks from the neutron (two "down" quarks and one "up" quark) and the two quarks from the K+ (an up quark and a strange antiquark). The evidence for this collection of five quarks would be an excess of events (a peak) on a plot of "missing" masses deduced from K- particles seen in the experiment (http://www.
The Laser-Electron Photon Facility (LEPS) at the SPring-8 machine (http://www.
Confirmation of this discovery comes quickly. A team of physicists in the US, led by Ken Hicks of Ohio University (firstname.lastname@example.org, 740-593-1981) working in the CLAS collaboration at the Dept. of Energy's Thomas Jefferson National Accelerator Facility, has also found evidence for the pentaquark. A photon beam (each photon being created by smashing the Jefferson Lab electron beam into a target and then measuring the energy of the scattered electron in order to determine the energy of the outgoing gamma) was directed onto a nuclear target. The photon collides with a deuteron target and the neutron-kaon (nK+) final state is studied in the CLAS detector (http://www.
The discovery of a 5-quark state should be of compelling interest to particle physicists, and this might be only the first of a family of such states. Not only that but a new classification of matter, like a new limb in the family tree of strongly interacting particles: first there were baryons and mesons, now there are also pentaquarks. According to Ken Hicks, a member of both SPring-8 and Jefferson Lab experiments, this pentaquark can be considered as a baryon. Unlike all other known baryons, though, the pentaquark would have a strangeness value of S=+1, meaning that the baryon contains an anti-strange quark. Past searches for this state have all been inconclusive. Hicks attributes the new discovery to better beams, more efficient detectors, and more potent computing analysis power.
(From Physics News Update, 2 July 2003)
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