First observation of the hyper superheavy hydrogen Λ6H

Three candidate events of the neutron-rich hypernucleus Λ6H were uniquely identified in the FINUDA experiment at DANE, Frascati, by observing π+ mesons from the (K− ,π+) production reaction on stop 6Li targets, in coincidence with π− mesons from Λ6H → 6He + 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/K− for the 6H stop Λ production rate times the two-body π− weak decay branching ratio. The Λ6H binding energy with respect to 5H+Λ was determined jointly from production and decay to be BΛ = (4.0±1.1) MeV, assuming that 5H is unbound with respect to 3H + 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 0+g.s. → 1+ excitation. The consequences of this assignment to Λ hypernuclear dynamics are briefly discussed.