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

By: Manabu Moritsu

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Synopsis: Thistheses reports on an experimental search for an exotic hadron, &ThΗ+(1540)pentaquark, which is a genuine exotic hadron with a five-quark system of uuddsbar. Theresults of this book support that the existence of &ThΗ+ wasstrongly constrained. The &ThΗ+ pentaquark was searched for via the Π- p ⇒ K- Xreaction using a beam momentum of 2. 01 GeV/c at the J-PARC hadron experimentalfacility, taking advantage of high-statistics and high-resolution compared withprevious experiments, some of which claimed the evidence of &ThΗ+. Inorder to realize a good missing-mass resolution of 2 MeV, the beam spectrometerand superconducting kaon spectrometer were constructed. No clear peak was observed in the missing mass spectrum of the Π- p ⇒ K- Xreaction, and the upper limit of the production cross section was found to beless than 0. 28 Μb/sr at the 90% confidence level in a mass region of1500-1560 MeV/c2. This upper limit is an order of magnitude smallerthan that of the previous KEK experiment. Compared with a theoreticalcalculation using the effective Lagrangian approach, the decay width of &ThΗ+ wasevaluated. The upper limits on the decay width were estimated to be 0. 36 and1. 9 MeV for the &ThΗ+ spin-parity of 1/2+ and 1/2-,respectively. These are quite small for a width of ordinary hadron resonances,and the existence of &ThΗ+ was strongly constrained and isdoubtful.