Pentaquark explorations using hadron beams. At KEK and J-PARC

Pentaquark (Φ+) is an exotic particle which consists of at least four quarks and an s-bar antiquark. Its width is smaller than a few MeV/c2. We have explored Φ+ at KEK and J-PARC using π- and K+ beams. It had been expected by the meson beam experiments to attain higher production cross section than LEPS at SPring-8. Another expectation was to use high resolution spectrometers to enhance the sensibility and to collect direct information of the width. At KEK-PS experiments using π- beam to produce Φ+, a bump structure was detected in the missing mass spectrum but it was not strong enough statistically. The upper limit of the forward differential cross section was estimated to be 2.9 μb/sr which is anomalously small for a hadron reaction. Another KEK-PS experiment using reverse reaction did not give any peak corresponding to the Φ+, resulting in only the estimation of the upper limit of 3.5 μb/sr. In the case of K+ beam having s-bar antiquark, result was also small. Accordingly it is most natural to explain the smallness of the production cross section by assuming that there exists little chance of t-channel process to exchange K* and consequently Φ+ width is very narrow. J-PARC experiment further to confirm in KEK-PS results using meson beams did not show any definite existence of Φ+, and the upper limit of the differential cross section in the forward was estimated to be 0.3 μb/sr. If Φ+ exists its width must be smaller than 1 MeV/c2 judging from the above results. (S. Funahashi)