First search for $K^{+}\to {\pi }^0\pi \mu e$ decays

The first search for the lepton number violating decay $K^{+}\to {\pi }^0{\pi }^{-}{\mu }^{+}{e}^{+}$ and lepton flavour violating decays $K^{+}\to {\pi }^0{\pi }^{+}{\mu }^{-}{e}^{+}$, $K^{+}\to {\pi }^0{\pi }^{+}{\mu }^{+}{e}^{-}$ has been performed using a dataset collected by the NA62 experiment at CERN in 2016-2018. Upper limits of $2.9\times {10}^{-10}$, $3.1\times {10}^{-10}$ and $5.0\times {10}^{-10}$, respectively, are obtained at 90% CL for the branching ratios of the three decays on the assumption of uniform phase-space distributions.