Search

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.

The bound-state level structures of the {sub {lambda}}{sup 16}O and {sub {lambda}}{sup 15}N hypernuclei were studied by {gamma}-ray spectroscopy using a germanium detector array (Hyperball) via the {sup 16}O (K{sup -},{pi}{sup -}{gamma}) reaction. A level scheme for {sub {lambda}}{sup 16}O was determined from the observation of three {gamma}-ray transitions from the doublet of states (2{sup -},1{sup -}) at {approx}6.7 MeV to the ground-state doublet (1{sup -},0{sup -}). The {sub {lambda}}{sup 15}N hypernuclei were produced via proton emission from unbound states in {sub {lambda}}{sup 16}O. Three {gamma} rays were observed, and the lifetime of the 1/2{sup +};1 state in {sub {lambda}}{sup 15}N was measured by the Doppler shift attenuation method. By comparing the experimental results with shell-model calculations, the spin dependence of the {lambda}N interaction is discussed. In particular, the measured {sub {lambda}}{sup 16}O ground-state doublet spacing of 26.4{+-}1.6{+-}0.5 keV determines a small but nonzero strength of the {lambda}N tensor interaction.

The H-dibaryon resonance was sought by a {lambda}{lambda} invariant mass spectrum that was obtained by the {sup 12}C (K{sup -},K{sup +}{lambda}{lambda}X) reactions. We observed a bump near the {lambda}{lambda} threshold, as reported by the previous experiment (KEK E224), with better statistics. Data were compared with results of a cascade model calculation including {lambda}{lambda} final state interactions consistent with the newly measured binding energy of {sub {lambda}}{sub {lambda}}{sup 6}He. No significant enhancements above levels of the model predictions were observed. The resulting upper limit for the production cross section of the H with a mass range between the {lambda}{lambda} and {xi}N threshold is found to be 2.1 {+-} 0.6 (stat.) {+-} 0.1 (syst.) {mu}b/sr at a 90% confidence level.

In a \ensuremath{\gamma}-ray spectroscopy experiment with the $({K}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}})$ reaction on a $^{10}\mathrm{B}$ target, we have observed the $7/{2}^{+}\ensuremath{\rightarrow}5/{2}^{+}$ spin-flip $M1$ transition in ${}_{\ensuremath{\Lambda}}^{7}\text{Li}$ via \ensuremath{\gamma}-\ensuremath{\gamma} coincidence with the $5/{2}^{+}\ensuremath{\rightarrow}1/{2}^{+}E2$ transition. This is the first successful $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\gamma}$ coincidence measurement for hypernuclei. The measured $M1 \ensuremath{\gamma}$-ray energy is $470.8\ifmmode\pm\else\textpm\fi{}1.9$(stat)$\ifmmode\pm\else\textpm\fi{}0.6$(syst) keV. The spacing of the $7/{2}^{+},5/{2}^{+}$ doublet is consistently explained using the strengths of the spin-spin, \ensuremath{\Lambda}-spin-orbit, and tensor interactions obtained from previous \ensuremath{\gamma}-spectroscopy experiments.