| Co-Investigator(Kenkyū-buntansha) |
TAKAHASHI Toshiyuki Tohoku Univ., Dept. of Physics, Research Associate, 大学院・理学研究科, 助手 (50281960)
FUJII Yu Tohoku Univ., Dept. of Physics, Research Associate, 大学院・理学研究科, 助手 (30302079)
MEDA Kazushige Tohoku Univ., Dept. of Physics, Associate Professor, 大学院・理学研究科, 助教授 (20125652)
OUTA Haruhiko KEK, Inst. for Particle and Nuclear Study, Research Associate, 素粒子原子核研究所, 助手 (60221818)
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| Research Abstract |
The purpose of this research is to investigate detailed level structure of various light Λ hypernuclei by precision γ spectroscopy with our germanium detector array, Hyperball. In particular, we aim at extracting strengths of all the spin-dependent interactions (spin-spin, spin-orbit and tensor interactions) between a Λ and a nucleon from hypernuclear level structure. This purpose has been well accomplished as follows.
1. We finished data analysis of the experiment KEK E419 carried out in 1998, where we succeeded in observing four γ transitions in ^7_ΛLi and we measured B(E2). We established the strength of the ΛN spin-spin force and confirmed the hypernuclear shrinking effect.
2. Since 1998, we have performed a series of experiments (BNL E930) at Brookhaven National Laboratory using (K^-,π^-) reaction with the AGS D6 beamline and Hyperball. For the ^9Be target data, we finished data analysis and succeeded in separately observing closely-located two γ-ray peaks at 3 MeV, which indicated a hypernuclear fine structure (3/2^+, 5/2^+) in ^9_ΛBe having only 31 keV level spacing. This spacing gives a very small spin-orbit force of Λ, which cannot be explained by meson-exchange models of baryon-baryon interactions but is consistent with quark models.
3. We improved various detectors of Hyperball and the D6 spectrometer in 2000-2001, and then performed next E930 runs with ^<16>O and ^<10>B targets in the fall in 2001. Although the data analysis is preliminary, we have observed ^<16>_ΛOγrays (1^-→1^-,0^-) at 6.55 MeV. From the level spacing of the (1^-,0^-) doublet, the strength of the ΛN tensor force will be extracted. We also observed three γ transitions in ^<15>_ΛN produced from ^<16>_ΛO excited states by proton emission. From ^<10>B target data, we have not observed the ^<10>_ΛBγ rays but observed several γ rays from various hyperfragments from highly excited states of ^<16>_ΛO.
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