Rare K sup + decays in flight: Latest results and future plans

E777 at the Brookhaven AGS has published a final branching ratio limit for the lepton family violating decay K{sup +} {yields} {pi}{sup +}{mu}{sup +}e{sup {minus}} of 2.1 {times} 10{sup {minus}10} (90% C.L.). New results based on approximately 500 events of the G.I. M. suppressed decay K{sup +} {yields} {pi}{sup +}e{sup +}e{sup {minus}} have yielded a branching ratio of (2.75 {plus minus} 0.23 {plus minus} 0.13) {times} 10{sup {minus}7} assuming a vector interaction with, a form factor = 0.105 {plus minus} 0.035 {plus minus} 0.015. The branching ratio times decay probability for K{sup 0} {yields} {pi}{sup +} X{sup 0}, ,{sup 0} {yields} e{sup +}e{sup {minus}} was found to less than 1.5 {times} 10{sup {minus}8} (99% C.L.) in the e{sup +}e{sup {minus}} invariant mass range of 150 to 340 MeV/c. An approved second generation experiment sensitive to K{sup +} {yields} {mu}{sup +}e{sup {minus}} at the 10{sup {minus}12} level is discussed. 5 refs., 3 figs.