Search for the Lightest Double- $$\Lambda $$ Λ Hypernucleus, $${ _{\varvec{\Lambda }\varvec{\Lambda }}^{\varvec{5}}}$$ Λ Λ 5 H, at J-PARC

Investigations of double- $$\Lambda $$ hypernuclei, which contain two $$\Lambda $$ hyperons in a nucleus, provide valuable information on baryon–baryon interaction in the strangeness $$-2$$ sector, such as the $$\Lambda $$ – $$\Lambda $$ interaction. Whereas several double- $$\Lambda $$ hypernuclei have been discovered in nuclear-emulsion experiments, an experimental observation of $${}_{\Lambda \Lambda }^{5}$$ H is awaited. It is not only because $$_{\Lambda \Lambda }^{5}$$ H is theoretically expected to be the lightest bound state, but also because $$\Lambda \Lambda $$ – $$\Xi N$$ coupling may play an important role in its structure. On the other hand, the coupling effect may be suppressed in $$_{\Lambda \Lambda }^{6}{\mathrm {He}}$$ .We propose an experiment to investigate $$_{\Lambda \Lambda }^{5}$$ H at J-PARC. $$_{\Lambda \Lambda }^{5}$$ H will be produced by a decay of a $$\Xi $$ hypernucleus, $$_\Xi ^{7}$$ H, which can be produced by the $$^7\mathrm {Li}(K^-,K^+)$$ reaction. The mass of $$_{\Lambda \Lambda }^{5}$$ H will be determined by means of decay pion spectroscopy; it can be calculated from the measured momentum of decay pions, owing to a two-body decay of $$_{\Lambda \Lambda }^{5}$$ H.