First observation of the hyper superheavy hydrogen HΛ6

Three candidate events of the neutron-rich hypernucleus HΛ6 were uniquely identified in the FINUDA experiment at DAΦNE, Frascati, by observing π+ mesons from the (Kstop−,π+) production reaction on 6Li targets, in coincidence with π− mesons from HΛ6→He6+π− weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of (2.9±2.0)⋅10−6/Kstop− for the HΛ6 production rate times the two-body π− weak decay branching ratio. The HΛ6 binding energy with respect to H5+Λ was determined jointly from production and decay to be BΛ=(4.0±1.1) MeV, assuming that 5H is unbound with respect to H3+2n by 1.7 MeV. The binding energy determined from production is higher, in each one of the three events, than that determined from decay, with a difference of (0.98±0.74) MeV here assigned to the 0g.s.+→1+ excitation. The consequences of this assignment to Λ hypernuclear dynamics are briefly discussed.