YAMAZAKI Hirohito Tohoku University, Graduate School of Science, Assistant Professor, 大学院理学研究科, 助手 (90260413)
ISHIKAWA Takatsugu Tohoku University, Graduate School of Science, Assistant Professor, 大学院理学研究科, 助手 (40400220)
|Budget Amount *help
¥15,200,000 (Direct Cost: ¥15,200,000)
Fiscal Year 2006: ¥7,400,000 (Direct Cost: ¥7,400,000)
Fiscal Year 2005: ¥7,800,000 (Direct Cost: ¥7,800,000)
We have observed a narrow baryon resonance N^*(1670) in this research project. The mass and the width of the resonance are 1665-1675 MeV/c^2 and 10-40 MeV/c^2,respectively, according to the latest analysis up to now. The resonance bump appeared in the total cross section data obtained in an η meson photoproduction experiment using a solid deuterium target. The bump did not show up in the case with a solid hydrogen target under the same experimental condition. Thus, the new resonance couples to the neutron, but not to the proton through radiative transitions. We examined the spin 1/2 and 3/2 states with both parities, fitting these states to the data one by one. The fitting was made for the data of the total cross section and the cosθ coefficients of the angular distributions. A negative parity state such as S11 or D13 has turned out to reproduce the data better than positive parity states, although it is difficult to determine the spin and parity of the resonance definitely with the present data. However, there are several broad resonances, S11(1650),D15(1675),F15(1680),D13(1700),P11(1700), and P13(1720), the width of which is roughly 100-200 MeV/c^2. And any of these broad resonances is placed over the mass region around 1670 MeV/c^2. The narrow resonance with the same quantum numbers as one of these broad states cannot be constructed as a member of three quark states. Taking the coupling situation of the resonance to the nucleons into account, it is natural, therefore, to regard the narrow resonance as a member of the antidecuplet of pentaquark baryons. Here U-spin conservation allows the process γn→ N^*(1670), but prohibits the transition γp→N^<*^+>(1670). Because the U-spin of γ,n, and p is 0,1,and 1/2,respectively, while that of N^*(1670) is 1 and 3/2 for N^<*^+>(1670) in the antidecuplet.