Analysis of data from Jefferson Lab’s CEBAF Large Acceptance Spectrometer supports the pentaquark discovery recently announced by the SPring-8 physics lab in Japan.
Physics Lab in Japan reports evidence for the Pentaquark; Jefferson Lab data supports discovery
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.aip.org/mgr/png/2003/193.htm).
The Laser-Electron Photon Facility (LEPS) at the SPring-8 machine (http://www.rcnp.osaka-u.ac.jp/Divisions/np1-b/index.html ) is reporting exactly this sort of excess at a mass of 1540 MeV with an uncertainty of 10 MeV. The statistical certainty that this peak is not just a fluctuation in the natural number of background events, and that the excess number of events is indicative of a real particle, is quoted as being 4.6 standard deviations above the background. This, according to most particle physicists, is highly suggestive of discovery. (Nakano et al., Physical Review Letters, upcoming article, probably 11 July 2003; text at www.aip.org/physnews/select; contact Takashi Nakano, firstname.lastname@example.org)
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