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

Abstract

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($K^\pm \to \mu^\pm \nu_\mu e^+e^- | M_{ee} \geq 140$ 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.