LIFETIMES OF LAMBDA-HYPERNUCLEI UP TO FE-LAMBDA

We have observed that the global behavior of the lifetime of a Lambda hyperon in nuclear matter is almost mass independent. Its lifetime dec reases slightly from that of a free Lambda and quickly saturates at si milar to 80% of tau(Lambda) in free space as the mass number increases . We have determined the lifetimes of three Lambda hypernuclei as tau( C-12(Lambda)g.s.) = 231 +/- 15 ps, tau(Si-28(Lambda)g.s.) = 206 +/- 11 2 ps and tau({Fe-56(Lambda)}) = 215(-13)(+14) ps. The hypernuclei were produced by the (pi(+), K+) reaction and identified by the SKS spectr ometer at the KEK 12 GeV PS. The lifetimes were obtained by a direct m easurement of the production time of a hypernucleus and its decay time with two sets of timing/range counters. These are the first lifetime results of Lambda hypernuclei over this broad mass range with the expl icit identification of the produced hypernuclei.