Search for the Pentaquark ¿+ via the ¿¿p ¿ K¿X Reaction at J-PARC

This theses reports on an experimental search for an exotic hadron, ¿⁺(1540) pentaquark, which is a genuine exotic hadron with a five-quark system of uudds^bar. The results of this book support that the existence of ¿⁺ was strongly constrained. The ¿⁺ pentaquark was searched for via the ¿^- p ¿ K^- X reaction using a beam momentum of 2.01 GeV/c at the J-PARC hadron experimental facility, taking advantage of high-statistics and high-resolution compared with previous experiments, some of which claimed the evidence of ¿⁺. In order to realize a good missing-mass resolution of 2 MeV, the beam spectrometer and superconducting kaon spectrometer were constructed. No clear peak was observed in the missing mass spectrum of the ¿^- p ¿ K^- X reaction, and the upper limit of the production cross section was found to be less than 0.28 ¿b/sr at the 90% confidence level in a mass region of 1500¿1560 MeV/c². This upper limit is an order of magnitude smaller than that of the previous KEK experiment. Compared with a theoretical calculation using the effective Lagrangian approach, the decay width of ¿⁺ was evaluated. The upper limits on the decay width were estimated to be 0.36 and 1.9 MeV for the ¿⁺ spin-parity of 1/2⁺ and 1/2^-, respectively. These are quite small for a width of ordinary hadron resonances, and the existence of ¿⁺ was strongly constrained and is doubtful.