Σ hypernuclear bound state observed in stopped K^- reaction on⁴He

Results are presented of inclusive measurements of π^± momentum spectra from K^- absorption at rest in liquid helium. We found a peak in the π^- spectrum at ΔM≡M ( _Σ ⁴ He)-M(⁴He)=267.7±0.3 (statistical) _-0.1 ^+1.1 (systematic) MeV (B _Σ ⁺=3.2 MeV). The width of the peak was deduced to be 4.6±0.5 (statistical) _-1.3 ^+1.6 (systematic) MeV. The (K^-, π⁺) spectrum does not exhibit a clear peak in the Σ^- bound region. Comparison of these two spectra suggests that the peak in the π^- spectrum is due to the formation of the S=0, I=1/2 ground state of _Σ ⁴ He.