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We report on the first measurement of a hypernuclear $\ensuremath{\gamma}$-transition probability. $\ensuremath{\gamma}$ rays emitted in the $E2(5/{2}^{+}\ensuremath{\rightarrow}1/{2}^{+})$ transition of ${}_{\ensuremath{\Lambda}}^{7}\mathrm{Li}$ were detected by a large-acceptance germanium detector array (Hyperball), and the lifetime of the parent state ( $5/{2}^{+}$) was determined by the Doppler shift attenuation method. The obtained result, ${5.8}_{\ensuremath{-}0.7}^{+0.9}\ifmmode\pm\else\textpm\fi{}0.7\mathrm{ps}$, was then converted into the reduced transition probability [ $B(E2)$] to be $B({E2;5/2}^{+}\ensuremath{\rightarrow}{1/2}^{+})\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}3.6\ifmmode\pm\else\textpm\fi{}{0.5}_{\ensuremath{-}0.4}^{+0.5}{e}^{2}{\mathrm{fm}}^{4}$. Compared with the $B(E2)$ of the corresponding $E2({3}^{+}\ensuremath{\rightarrow}{1}^{+})$ transition in the ${}^{6}\mathrm{Li}$ nucleus, our result gives evidence that the size of the ${}^{6}\mathrm{Li}$ core in ${}_{\ensuremath{\Lambda}}^{7}\mathrm{Li}$ is smaller than the ${}^{6}\mathrm{Li}$ nucleus in the free space.

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.

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.

With a germanium detector array (Hyperball), we observed two gamma-ray peaks corresponding to the two transitions (5/2(+)--1/2(+) and 3/2(+)--1/2(+)) in the (9)(Lambda)Be hypernucleus which was produced by the 9Be(K-,pi(-)) reaction. The energies of the gamma rays are 3029 +/- 2 +/- 1 keV and 3060 +/- 2 +/- 1 keV. The energy difference was measured to be 31.4(+2.5)(-3.6) keV, which indicates a very small Lambda-spin-dependent spin-orbit force between a 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.

A double-hyperfragment event has been found in a hybrid-emulsion experiment. It is identified uniquely as the sequential decay of ( 6)(LambdaLambda)He emitted from a Xi(-) hyperon nuclear capture at rest. The mass of ( 6)(LambdaLambda)He and the Lambda-Lambda interaction energy DeltaB(LambdaLambda) have been measured for the first time devoid of the ambiguities due to the possibilities of excited states. The value of DeltaB(LambdaLambda) is 1.01+/-0.20(+0.18)(-0.11) MeV. This demonstrates that the Lambda-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.

We report on the first measurement of a hypernuclear gamma-transition probability. gamma rays emitted in the E2(5/2(+)--1/2(+)) transition of (7)(Lambda)Li were detected by a large-acceptance germanium detector array (Hyperball), and the lifetime of the parent state ( 5/2(+)) was determined by the Doppler shift attenuation method. The obtained result, 5.8(+0.9)(-0.7)+/-0.7 ps, was then converted into the reduced transition probability [ B(E2)] to be B(E2;5/2(+)--1/2(+)) = 3.6+/-0.5(+0.5)(-0.4) e(2) fm(4). Compared with the B(E2) of the corresponding E2(3(+)--1(+)) transition in the 6Li nucleus, our result gives evidence that the size of the 6Li core in (7)(Lambda)Li is smaller than the 6Li nucleus in the free space.

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.