Recent results on kaon decays by the NA48/2 experiment at CERN

Abstract

The NA48/2 experiment reports the first observation of the rare decay K ^± → π ^± π ⁰ e ⁺ e ⁻, based on about 2000 candidates from 2003 data. The preliminary branching ratio in the full kinematic region is $\mathcal {B}(K^{\pm } \to \pi ^{\pm }\pi ^{0}e^{+}e^{-})=(4.06\pm 0.17)\cdot 10^{-6}$. A sample of 4.687 × 10⁶ $K^{\pm }\to \pi ^{\pm }{\pi ^{0}_{D}}$ events collected in 2003/4 is analyzed to search for the dark photon ($A^{\prime }$) via the decay chain K ^± → π ^± π ⁰, $\pi ^{0}\to \gamma A^{\prime }$, $A^{\prime }\to e^{+}e^{-}$. No signal is observed, limits in the plane mixing parameter ε ² versus its mass $m_{A^{\prime }}$ are reported.