Double Lambda and Xi hypernuclei

Nuclei with double strangeness (S $=$ -2) provide the key information to understand Baryon-Baryon interaction under the SU(3)$_f$ symmetry. Therefore we have carried out the experiments at KEK for quarter a century. Recently, the interaction in S $=$ -2 sector is noted to derive the information of the EOS of neutron star. The Lambda-Lambda interaction has been presented to be weak attractive by NAGARA event which showed the production and decay of 6He double-hypernucleus. The event also presented the lower mass limit of H dibaryon. In other five events, we obtained the knowledge about an excitation level of 10Be double-hypernucleus under the consistency with NAGARA event. Moreover, very recently, we have discovered a Xi-14N system which was deeply bound far from the atomic 3D level (0.17 MeV) for a captured Xi hyperon. Since a 8Li nucleus was associated with the decay of one of twin-hypernuclei, the event was uniquely identified as Xi- $+$ 14N $\Rightarrow$ 10Be$_L$ $+$ 5He$_L$. The system was selected from 8 million pictures on the test running for development of ``Overall Scanning'' to be used in the coming experiment. This is the first evidence of Xi hypernucleus to be bound and it is impacting for the study of Xi-N interaction. At J-PARC facility, for the further study of hyperon-hyperon interaction, we plan to perform the E07 experiment at J-PARC. In the workshop, we will review the above knowledge obtained by the experiments at KEK-PS, and discuss developed technologies to detected 10$^2$ or more double-hypernuclei in the E07 experiment at J-PARC.