Search for the Lightest Double- Λ Hypernucleus, _ΛΛ^5 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 _ΛΛ^5 H is awaited. It is not only because _ΛΛ^5 H is theoretically expected to be the lightest bound state, but also because ΛΛ – Ξ N coupling may play an important role in its structure. On the other hand, the coupling effect may be suppressed in _ΛΛ^6He .We propose an experiment to investigate _ΛΛ^5 H at J-PARC. _ΛΛ^5 H will be produced by a decay of a Ξ hypernucleus, _Ξ ^7 H, which can be produced by the ^7Li(K^-,K^+) reaction. The mass of _ΛΛ^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 _ΛΛ^5 H.