Speaker: Brigitte Block-Devaux
The NA48/2 collaboration has accumulated ~66000 semi-leptonic charged kaon decays Ke4(00) to pi0 pi0 e nu, increasing the world available statistics by several orders of magnitude. Background contamination at the one percent level and very good pi0 reconstruction allow the first accurate measurement of the Branching Fraction and decay Form Factor. The achieved precision makes possible the observation of small effects such as a deficit of events at low pi0 pi0 invariant mass which can be explained by charge exchange re-scattering in the pi pi system below the 2 m(pi+) threshold.
Future prospects include the observation of several ~1000 decays in similar muonic modes Kmu4(00) (never observed) and Kmu4(+-) (7 events observed). Such poorly known modes could be studied also in the forthcoming NA62 experiment currently under construction.