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

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 × 10$^{6}$ $K^{\pm }\to \pi ^{\pm }{\pi ^{0D}}$ 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.