Search for the Lightest Double- Λ Hypernucleus, ΛΛ5 H, at J-PARC

Investigations of double-Λ hypernuclei, which contain two Λ hyperons in a nucleus, provide valuable information on baryon–baryon interaction in the strangeness - 2 sector, such as the Λ –Λ interaction. Whereas several double-Λ hypernuclei have been discovered in nuclear-emulsion experiments, an experimental observation of ΛΛ5H is awaited. It is not only because ΛΛ5H is theoretically expected to be the lightest bound state, but also because Λ Λ –Ξ N coupling may play an important role in its structure. On the other hand, the coupling effect may be suppressed in ΛΛ6He.We propose an experiment to investigate ΛΛ5H at J-PARC. ΛΛ5H will be produced by a decay of a Ξ hypernucleus, Ξ7H, which can be produced by the ⁷Li (K^-, K⁺) reaction. The mass of ΛΛ5H 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 ΛΛ5H.