Nonmesonic decay of the Lambda-hyperon in hypernuclei produced by p+Au collisions

The lifetime of the Lambda -hyperon for the nonmesonic decay LambdaN --> NN has been determined by a measurement at COSY Julich of the delayed fission of heavy hypernuclei produced in proton-Au collisions at T-p = 1.9 GeV. It is found that heavy hypernuclei with mass numbers A approximate to (180 +/ - 5) and atomic numbers Z approximate to (74 +/- 2) fission with a lifetime tau (Lambda) = 130 +/- 13 (stat.) +/-15 (syst.) ps. This value together wi th the results obtained for other heavy hypernuclei in previous investigati ons indicates -on the confidence level of 0.9- a violation of the phenomeno logical DeltaI = 1/2 rule for the LambdaN --> NN transitions as known from the weak mesonic decays of kaons and hyperons.