Missing-mass spectroscopy with the ${}^{6}$Li($π^{-}, K^{+}$)X reaction to search for ${}^{6}_Λ$H
We searched for the bound state of the neutron-rich $Λ$-hypernucleus ${}^{6}_Λ$H, using the ${}^{6}$Li($π^{-}, K^{+}$)X double charge-exchange reaction at a $π^{-}$ beam momentum of 1.2 GeV/c at J-PARC. A total of $1.4 \times 10^{12}$ $π^{-}$ was driven onto a ${}^{6}$Li target of 3.5-g/$\rm cm^2$ thickness. No event was observed below the bound threshold, i.e., the mass of ${}^{4}_Λ$H + 2n, in the missing-mass spectrum of the ${}^{6}$Li($π^{-}, K^{+}$)X reaction in the $2^{\circ}$ < $θ_{πK}$ < $20^{\circ}$ angular range. Furthermore, no event was found up to 2.8 MeV/$c^2$ above the bound threshold. We obtained the the double-differential cross section spectra of the ${}^{6}$Li($π^{-}, K^{+}$)X reaction in the angular range of $2^{\circ}$ < $θ_{πK}$ < $14^{\circ}$. An upper limit of 0.56 nb/sr (90% C.L.) was obtained for the production cross section of the ${}^{6}_Λ$H hypernucleus bound state. In addition, not only the bound state region, but also the $Λ$ continuum region and part of the $Σ^{-}$ quasi-free production region of the ${}^{6}$Li($π^{-}, K^{+}$)X reaction, were obtained with high statistics. The present missing-mass spectrum will facilitate the investigation of the $Σ^{-}$-nucleus optical potential for $Σ^{-}$-${}^{5}$He through spectrum shape analysis.