Model independent measurement of the leptonic kaon decay K+- ->mu+- nu e+ e- with the NA48/2 experiment

A measurement of the branching ratio of the rare leptonic kaon decay $K^\pm \rightarrow \mu^\pm \nu_{\mu} e^+ e^- $ is presented 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 contributions become important and can be calculated in the framework of Chiral Perturbation Theory (ChPT). From a total number of $1.56 \times 10^{11}$ recorded kaon decays, the branching ratio is measured to be $\mathcal{B} \left( K^\pm \rightarrow \mu^\pm \nu_{\mu} e^+ e^- | M_{ee} > 140~ MeV/c^2 \right) = (7.8 \pm 0.2) \times 10^{-8} $.