Search

We determine ΛΛ scattering parameters from a ΛΛ invariant mass spectrum that was obtained by 12 C (K-, K+ΛΛ) reaction at the KEK Proton Synchrotron. In the framework of Watson's procedure, the obtained scattering length [Formula: see text] and effective range [Formula: see text] are most consistent with the values predicted by using the Nijmegen soft core models (NSC97's). However, the predicted values by using the Nijmegen hard-core ND ( G -matrix) and the extended soft-core (ESC00) models are out of two standard deviations from the determined scattering parameters.

The present status of hypernuclear gamma-ray spectroscopy with Hyperball is summarized. We observed two gamma transitions of 16-Lambda-O(1- -> 1-,0-) and obtained the strength of the Lambda-N tensor force. In 10B(K-,pi- gamma) data, we did not observe the spin-flip M1 transition of 10-Lambda-B(2- -> 1-), but gamma rays from hyperfragments such as 7-Lambda-Li(7/2+ -> 5/2+) and 9-Lambda-Be(3/2+ ->1/2+) were observed. In 11B(pi+,K+ gamma) data, we observed six gamma transitions of 11_Lambda-B. We also attempted an inclusive gamma-ray measurement with stopped K- beam.

Using a large-acceptance germanium detector array (Hyperball), we have observed a spin-flip M1 gamma transition between the ground-state spin doublet of (7)(Lambda)Li (3/2(+)-->1/2(+)). The observed energy of 691.7+/-0.6(stat)+/-1.0(syst) keV provides crucial information on the strength of the spin-spin interaction between a Lambda and a nucleon. This is the first observation of well-identified hypernuclear gamma transitions using germanium detectors.

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.

We have observed two $\ensuremath{\gamma}$-ray transitions in $_{\ensuremath{\Lambda}}^{16}\mathrm{O}$ from the 6.6 MeV excited ${1}_{2}^{\ensuremath{-}}$ state to both ground-state spin-doublet members (${1}_{1}^{\ensuremath{-}},{0}^{\ensuremath{-}}$) by the (${K}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}}\ensuremath{\gamma}$) reaction. We have obtained the ground-state doublet spacing to be $26.4\ifmmode\pm\else\textpm\fi{}1.6(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\ifmmode\pm\else\textpm\fi{}0.5(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\text{ }\text{ }\mathrm{k}\mathrm{e}\mathrm{V}$ and the excitation energy of the ${1}_{2}^{\ensuremath{-}}$ state to be $6561.7\ifmmode\pm\else\textpm\fi{}1.1(\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t})\ifmmode\pm\else\textpm\fi{}1.7(\mathrm{s}\mathrm{y}\mathrm{s}\mathrm{t})\text{ }\text{ }\mathrm{k}\mathrm{e}\mathrm{V}$. The ground-state doublet spacing provides a small but nonzero strength of the tensor interaction between a $\ensuremath{\Lambda}$ and a nucleon. This is the first experimental result on the $\ensuremath{\Lambda}N$ tensor interaction.

With a germanium detector array (Hyperball), we observed two $\ensuremath{\gamma}$-ray peaks corresponding to the two transitions ( ${5/2}^{+}\ensuremath{\rightarrow}{1/2}^{+}$ and ${3/2}^{+}\ensuremath{\rightarrow}{1/2}^{+}$) in the ${}_{\ensuremath{\Lambda}}^{9}\mathrm{Be}$ hypernucleus which was produced by the ${}^{9}\mathrm{Be}({K}^{\ensuremath{-}},{\ensuremath{\pi}}^{\ensuremath{-}})$ reaction. The energies of the $\ensuremath{\gamma}$ rays are $3029\ifmmode\pm\else\textpm\fi{}2\ifmmode\pm\else\textpm\fi{}1\mathrm{keV}$ and $3060\ifmmode\pm\else\textpm\fi{}2\ifmmode\pm\else\textpm\fi{}1\mathrm{keV}$. The energy difference was measured to be ${31.4}_{\ensuremath{-}3.6}^{+2.5}\mathrm{keV}$, which indicates a very small $\ensuremath{\Lambda}$-spin-dependent spin-orbit force between a $\ensuremath{\Lambda}$ and a nucleon. This is the smallest level splitting by far ever measured in a hypernucleus.

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.

An experiment demonstrating the production of double-Lambda hypernuclei in (K(-),K(+)) reactions on (9)Be was carried out at the D6 line in the BNL alternating-gradient synchrotron. The technique was the observation of pions produced in sequential mesonic weak decay, each pion associated with one unit of strangeness change. The results indicate the production of a significant number of the double hypernucleus (4)(double Lambda)H and the twin hypernuclei (4)(Lambda)H and (3)(Lambda)H. The relevant decay chains are discussed and a simple model of the production mechanism is presented. An implication of this experiment is that the existence of an S = -2 dibaryon more than a few MeV below the double Lambda mass is unlikely.