Recent results from NA48/2 and NA62 experiments at CERN

The NA48/2 and $\text{NA62}\text{-}{\text{R}_\text{K}}$ experiments at the CERN SPS collected a large sample of charged kaon decays in flight. $\text{NA62}\text{-}{\text{R}_\text{K}}$ was running in 2007-08 with a highly efficient minimum bias trigger for decays into electrons. A preliminary measurement of the electromagnetic transition form factor slope of the $\pi^0$ from $1.05\times 10^{6}$ fully reconstructed $\pi^0$ Dalitz decays is presented. The obtained value $a = (3.70 \pm 0.53_\text{stat} \pm 0.36_\text{syst})\times 10^{-2}$ represents a $5.8\sigma$ observation of a non-zero slope in the time-like region of momentum transfer. An upper limit on the rate of a lepton number violating decay $K^\pm\to\pi^\mp\mu^\pm\mu^\pm$ is reported from $\sim 1.6\times 10^{11} K^\pm$ decays at NA48/2 in 2003-04: $\mathcal{B} < 8.6\times 10^{-11}$ at $90 \%$ CL. Searches for heavy sterile neutrino $N_4$ and inflaton $\chi$ resonances in $K^\pm\to\pi\mu\mu$ decays are reported. No signal is observed and upper limits on the products $\mathcal{B}(K^\pm\to\mu^\pm N_4)\mathcal{B}(N_4\to\pi^\mp\mu^\pm)$ and $\mathcal{B}(K^\pm\to\pi^\pm \chi)\mathcal{B}(\chi\to\mu^+\mu^-)$ are set in the range $10^{-10}-10^{-9}$ for resonance lifetimes up to $100~\text{ps}$. The result of a search for dark photon with the same sample of decays is also reported. In the absence of observed signal, the limits on the mixing parameter $\varepsilon^2$ in the range $9-70~\text{MeV/c}^{2}$ are improved.