Model independent measurement of the leptonic kaon decay $K^{\pm}\to \mu^{\pm}\nu_{\mu}e^{+} e^-$ with the NA48/2 experiment

A measurement of the branching ratio of the rare leptonic kaon decay K ± → μ ± ν μ e + e − is pre- sented using data collected by the NA48/2 experiment in 2003 and 2004. The measurement is performed in the region M ee > 140 MeV/c 2 . In this particular region low energy QCD contri- butions become important and can be calculated in the framework of Chiral Perturbation Theory (ChPT). From a total number of 1 . 56 × 10 11 recorded kaon decays, the branching ratio is mea- sured to be B ( K ± → μ ± ν μ e + e − | M ee > 140 MeV / c 2 ) = ( 7 . 8 ± 0 . 2 ) × 10 − 8