Search for the neutron-rich hypernucleus _{Λ}^{9}He

Search for the neutron-rich hypernucleus ${}_{\ensuremath{\Lambda}}^{9}\mathrm{He}$ is reported by the FINUDA experiment at DA$\ensuremath{\Phi}$NE, INFN-LNF, studying $({\ensuremath{\pi}}^{+},{\ensuremath{\pi}}^{\ensuremath{-}})$ pairs in coincidence from the ${K}_{\mathrm{stop}}^{\ensuremath{-}}+{}^{9}\mathrm{Be}\ensuremath{\rightarrow}{}_{\ensuremath{\Lambda}}^{9}\mathrm{He}+{\ensuremath{\pi}}^{+}$ production reaction followed by ${}_{\ensuremath{\Lambda}}^{9}\mathrm{He}\ensuremath{\rightarrow}{}^{9}\mathrm{Li}+{\ensuremath{\pi}}^{\ensuremath{-}}$ weak decay. An upper limit of the production rate of ${}_{\ensuremath{\Lambda}}^{9}\mathrm{He}$ undergoing this two-body ${\ensuremath{\pi}}^{\ensuremath{-}}$ decay is determined to be $(2.3\ifmmode\pm\else\textpm\fi{}1.9)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}/{K}_{\mathrm{stop}}^{\ensuremath{-}}$ at the 90$%$ confidence level.