Model independent measurement of the leptonic kaon decay $K^\pm \to \mu^\pm \nu_\mu e^+e^-$ and study of the $K^\pm \to \pi^\pm \pi^0 e^+e^-$ decay by the NA48/2 experiment at CERN

Two recent results on rare kaon decays are presented, based on $\sim 2 \times 10^{11} K^{\pm}$ decays recorded by the NA48/2 experiment at CERN SPS in 2003 and 2004. The branching ratio of the rare leptonic decay $K^{\pm} \to \mu^{\pm} \nu_{\mu} e^{+}e^{-}$ has been measured in the region of large $e^+ e^-$ invariant mass $M_{ee} \geq 140$ MeV/c$^2$, where low energy QCD contributions become important and can be calculated in the framework of Chiral Perturbation Theory (ChPT). This branching ratio is measured to be BR($K^{\pm} \to \mu^{\pm} \nu_{\mu} e^{+}e^{-} \mid M_{ee} \geq 140$ MeV/c$^2$) = $(7.8 \pm 0.2) \times 10^{-8}$. The $K^{\pm} \to \pi^{\pm} \pi^{0}e^{+}e^{-}$ rare decay has been observed for the first time, with about 5000 candidates and a 5% background. The branching ratio in the full kinematic region is measured to be BR($K^{\pm} \to \pi^{\pm}\pi^{0}e^{+}e^{-} = (4.22 \pm 0.15) \times 10^{-6}$, in good agreement with ChPT-based theoretical predictions.