Observation of a ${}_{\mathit{\Sigma }}^4\mathrm{He}$ Bound State in the ${}_{}{}^4{}_{}{}^{}He(K^{-},{\pi }^{-})$ Reaction at $600MeV/\mathit{c}$

We have observed a clear peak below the ${\Sigma }^{+}$-production threshold in the ${}^4\mathrm{He}(K^{-},{\pi }^{-})$ reaction at $600\mathrm{MeV}/c$ and ${\theta }_{K\pi }{=4\mathrm{}}^{\circ }$. This is confirmation of the existence of the bound state of ${}_{\Sigma }^4\mathrm{He}$, which was reported in the ${}^4\mathrm{He}(\mathrm{stopped}{K}^{-},{\pi }^{-})$ reaction. As in the case of stopped kaons, no such peak was found in the ${}^4\mathrm{He}(K^{-},{\pi }^{+})$ spectrum. Quantitatively reliable parameters for this level have been established. The binding energy and the width of the bound state are $4.4\pm 0.3\left(\mathrm{stat}\right)\pm 1\left(\mathrm{syst}\right)$ MeV and $7.0\pm 0.7\left(\mathrm{stat}{)}_{-0.0\mathrm{}}^{+1.2\mathrm{}}\right(\mathrm{syst})$ MeV, respectively.