First observation of the hyper superheavy hydrogen H _Λ6

Three candidate events of the neutron-rich hypernucleus H _Λ ⁶ 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 H _Λ ⁶→He ⁶+π- weak decay. Details of the experiment and the analysis of its data are reported, leading to an estimate of (2.9±2.0){dot operator}10 ^-6/Kstop- for the H _Λ ⁶ production rate times the two-body π ^- weak decay branching ratio. The H _Λ ⁶ binding energy with respect to H ⁵+ _Λ was determined jointly from production and decay to be B _Λ=(4.0±1.1) MeV, assuming that ⁵H is unbound with respect to H ³+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.