Spectroscopy of hypernuclei with stopped kaons - recent result of KEK experiment

Pion momentum spectra from K^- absorption at rest on various nuclear targets were measured by means of a magnetic spectrometer with wide momentum range (100 ∼ 300 MeV/c). The ground state of Σ^- hypernucleus, if well bound with a narrow width, is expected to be populated in ¹²C(stopped K^-, π⁺) inclusive (untagged) spectrum, but such a peak was not observed. The spectrum is compared with the DWIA calculations by Morimatsu and Yazaki, indicating that the depth of the nuclear potential of Σ^- is shallower than 12 MeV, and that its imaginary part is larger than 5 MeV if the potential depth is about 10 MeV. In the ¹²C(stopped K^-, π^-) spectrum, the ground state (( p3 2)_n^-1 ( s1 2)Λ) and the excited states (( p3 2)_n^-1(p)Λ) of _Λ¹²C were observed, and their formation probabilities were roughly in agreement with the results of DWIA calculations. In the π^- spectra on ¹²C, ⁹Be, and ⁷Li targets a distinct peak was observed at 132.1±0.7 MeV/c. It is ascribed to π^- from the mesic decay of _Λ⁴H; _Λ⁴H → ⁴He π^-. The formation probability of _Λ⁴H on C, Be, or Li target is much larger than those of the discrete states of _Λ¹²C via the direct (stopped K^-, π^-) reaction.