Observation of Pontecorvo reactions with open strangeness: (p)over-bard ->Lambda K-0 and (p)over-bard -> Sigma K-0(0)

We report the first observation of antiproton annihilation on the deuteron into two-body channels with open strangeness. These reactions are of the Po ntecorvo type, designating annihilations kinematically not possible on a fr ee nucleon. With antiprotons stopped in a liquid deuterium target, a K-S(0) and Lambda enriched sample of events, corresponding to about 10(9) annihil ations, was recorded with the Crystal Barrel experiment, using a Silicon mi crostrip detector for triggering on secondary decays. The final states Lamb da K-S(0) and Sigma(0)K(S)(0), with Sigma(0) --> Lambda gamma, Lambda --> p pi(-), K-S(0) --> pi(+)pi(-), and the equivalent final states Lambda K-L(0 ) and Sigma(0)K(L)(0), with noninteracting K-L(0), were identified. The (p) over bar d branching fraction into Lambda K-0 and Sigma(0)K(0) are (2.35 /- 0.45) X 10(-6) and (2.15 +/- 0.45) X 10(-6), respectively. This similari ty disagrees strongly with two-step model predictions and supports the stat istical (fireball) model. (C) 1999 Published by Elsevier Science B.V. All r ights reserved.