Evidence for Heavy Hyperhydrogen 6H

Evidence for the neutron-rich hypernucleus 6 H is presented from the FINUDA experiment at DANE, Frascati, studying ðþ; Þ pairs in coincidence from the K stop þ 6Li ! 6 H þ þ production reaction followed by 6 H ! 6He þ weak decay. The production rate of 6 H undergoing this two-body decay is determined to be ð2:9 2:0Þ 106=K stop. Its binding energy, evaluated jointly from production and decay, is B ð6 HÞ ¼ ð4:0 1:1Þ MeV with respect to 5H þ . A systematic difference of ð0:98 0:74Þ MeV between B values derived separately from decay and from production is tentatively assigned to the 6 H 0þ g:s: ! 1þ excitation.