K. Oyama

A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of ${}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}^{6}\mathrm{He}$ emitted from a ${\ensuremath{\Xi}}^{\ensuremath{-}}$ hyperon nuclear capture at rest. The mass of ${}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}^{6}\mathrm{He}$ and the $\ensuremath{\Lambda}\ensuremath{-}\ensuremath{\Lambda}$ interaction energy $\ensuremath{\Delta}{B}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}$ have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of $\ensuremath{\Delta}{B}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}$ is $1.01\ifmmode\pm\else\textpm\fi{}{0.20}_{\ensuremath{-}0.11}^{+0.18}\phantom{\rule{0ex}{0ex}}\mathrm{MeV}$. This demonstrates that the $\ensuremath{\Lambda}\ensuremath{-}\ensuremath{\Lambda}$ interaction is weakly attractive.

A hybrid experiment with nuclear emulsion and scintillating-fiber detectors (KEK-E373) has been performed to search for double-strangeness systems. Among about 10${}^{3}$ events of ${\ensuremath{\Xi}}^{\ensuremath{-}}$ hyperons captured at rest by emulsion nuclei, we have observed four events which clearly show the topology of cascade weak decays of double-$\ensuremath{\Lambda}$ hypernuclei including the ``Nagara'' event. Regarding the Nagara event, values of the two-$\ensuremath{\Lambda}$ binding energy (${B}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}$) and the $\ensuremath{\Lambda}$-$\ensuremath{\Lambda}$ interaction energy ($\ensuremath{\Delta}{B}_{\ensuremath{\Lambda}\ensuremath{\Lambda}}$) of ${}_{\ensuremath{\Lambda}}^{}{}_{\ensuremath{\Lambda}}^{6}\mathrm{He}$ have been revised to 6.91 $\ifmmode\pm\else\textpm\fi{}$ 0.16 and 0.67 $\ifmmode\pm\else\textpm\fi{}$ 0.17 MeV, respectively, due to the recent change of the ${\ensuremath{\Xi}}^{\ensuremath{-}}$ mass value (Particle Data Group). For another three events, we have determined possible species of double-$\ensuremath{\Lambda}$ hypernuclei together with their binding energies.

We find a remarkable agreement of the excitation energy spectrum of the ${}^{208}\mathrm{Pb}{(d,}^{3}\mathrm{He})$ reaction measured at ${T}_{d}=600\mathrm{MeV}$ near the ${\ensuremath{\pi}}^{\ensuremath{-}}$ production threshold with its theoretical prediction. Their comparison leads us to assign the distinct narrow peak observed at about 5 MeV below the threshold to the formation of bound pionic states ${\ensuremath{\pi}}^{\ensuremath{-}}{\ensuremath{\bigotimes}}^{207}\mathrm{Pb}$ of the quasisubstitutional configurations ${(2p)}_{{\ensuremath{\pi}}^{\ensuremath{-}}}{(3p}_{3/2}{,3p}_{1/2}{)}_{n}^{\ensuremath{-}1}.$ A small bump observed on the tail of the peak is assigned to the pionic $1s$ state. The binding energies ${(B}_{\mathrm{nl}})$ and the widths $({\ensuremath{\Gamma}}_{\mathrm{nl}})$ of the pionic orbitals are deduced to be ${B}_{2p}=5.13\ifmmode\pm\else\textpm\fi{}0.02(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.12(\mathrm{syst})\mathrm{}\mathrm{MeV}$ and ${\ensuremath{\Gamma}}_{2p}=0.43\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{stat})\ifmmode\pm\else\textpm\fi{}0.06(\mathrm{syst})\mathrm{}\mathrm{MeV}$ by decomposing the experimental spectrum into the pionic $1s$ and $2p$ components. While ${B}_{2p}$ and ${\ensuremath{\Gamma}}_{2p}$ are determined with small ambiguity, ${B}_{1s}$ and ${\ensuremath{\Gamma}}_{1s}$ are strongly correlated with each other, and are affected by the relative $1s/2p$ cross section ratio assumed, since the $1s$ component is observed only as an unresolved bump. Thus, we have to allow large uncertainties $6.6\mathrm{MeV}<{B}_{1s}<6.9\mathrm{MeV}$ and $0.4\mathrm{MeV}<{\ensuremath{\Gamma}}_{1s}<1.2\mathrm{MeV}.$ The experimental binding energies and widths are compared with theoretically calculated values based on various optical potential parameter sets, and are jointly used to deduce the effective ${\ensuremath{\pi}}^{\ensuremath{-}}$ mass in the nuclear medium.