Experimental knowledge of doubly-strange hypernuclei and prospects

We applied the Emulsion-Counter "Hybrid-method" linking real-time detectors and nuclear emulsion to search for doubly-strange hypernucleus in the KEK-E176 experiment. Then, the existence of double-Λ hypernucleus was confirmed with sequential weak decay at a Ξ^- hyperon captured point. In the E373 (KEK) experiment, the uniquely identified event of ⁶_ΛΛHe was successfully detected with upgraded hybrid method for the first time. In the more improved E07 (J-PARC) experiment, we detected 33 candidate events of doubly-strange hypernuclei. The 47 events from the above three experiments yielded the follwoing findings, such as 1) the interaction between two Λ hyperons was weakly attractive, 2) the binding energy of two Λ hyperons to a nucleus seems to have linear dependence on the nuclear mass number, 3) the Ξ hypernucleus existed, thus 4) the attractive interaction works between the Ξ^- and nucleus. Besides, the level structure can be seen for the ¹⁵_ΞC hypernucleus. Far from Hybrid-method, the development is on-going for "Overall-scanning method" as an effectively detecting method by searching for doubly-strange hypernuclei in the whole volume of the emulsion with applying a machine learning model. In the detection of hypertriton (³_ΛH), the Overall-scanning method is working very well. This development must give us a large number of double-Λ hypernuclei produced at the K- reaction point as well as the Ξ^- absorption. We also expect to detect many Ξ hypernuclei not only of ¹⁵_ΞC but also ¹³_ΞB and ¹⁷_ΞN at Ξ^- absorption point in the emulsion. Regarding ¹²_ΞBe, the E70 experiment will be able to present level structure of Ξ^- hyperon at J-PARC, soon. The E75 experiment at J-PARC is going to search for ⁷_ΞH. By doping ¹⁰B in the emulsion gel, it will be possible to search for ¹¹_ΞBe hypernuclei. Such experimental approaches must provide important and reliable information to understand baryon-baryon interaction.