Evidence forK+→π+π−+

On the history of pion-pion scattering

“…[The first K + → π + π − e + ν decay was reported in Ref. [32], with a camera lucida drawing of the event. ]…”

High statistics measurement ofKe4decay properties

“…Among the long list of possible charged kaon decays the rare K e4 decay branch (K ± → π + π − e ± ν e (ν e )) has received particular attention, because it was recognized [1], almost coincident with the observation of the first event for this decay 40 years ago [2], that it could provide important information on the structure of the weak hadronic currents and also on ππ scattering at low energies. The final state interaction of the two pions was expected to manifest itself in an angular correlation between the decay products, namely an asymmetry of the lepton distribution with respect to the plane formed by the two pion momenta.…”

Analysis of 3587τ+Decays

“…One of these events was shown to be an example of the decay mode K e i (K + -> w + +w~-\-e + +v), and has been previously reported. 38 The other two events are also consistent with this decay mode, but are not sufficiently complete to allow clear identification. The remaining sample of 3356 "r-like" events, together with 97 events from the Columbia stack C and 94 events from the Columbia stack D 39 were included in the detailed analysis described below.…”