Study of reaction mechanism of Theta+photo-production

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The Theta+ particle is a 5-quark particle (pentaquark) for which the evidence was first reported by LEPS/Spring-8 collaboration by analyzing an experimental data taken with a Laser-Electron-Photon beam on a nuclear target.This study aims to confirm the existence of the Theta+ by using a high intensity beam with higher statistics. And if the its existence was confirmed, the structure of the pentaquark such as spin and parity would be investigated. In this study, first, the beam intensity was doubled by replacing an Ar laser by two solid-state lasers with the output power of 8W. The LEP beam intensity reached at 2 M Hz by inserting two laser beams into the electron ring simultaneously. The laser optics has been modified to avoid the beam loss due to distortion of the optics elements by heat. The tagger system has been also modified so that dead-time due to the high rate becomes small. The efficiency and the resolution of the tagger was studied and turned out to be good enough for the Theta+ study. In 2006-2007, the major physics run has been carried out and the data with 3 more statistics than the previous run was obtained. The detailed analysis of the previous run was also carried out, and the Theta+ signal with a 5 standard sigma significance was observed by correcting Fermi-motion effect in the missing mass calculation. A blind analysis will be carried out for the new data set to confirm the signal.Hyperon and meson photo-production reactions were also studied by analyzing the same data. Results were published on journals.