Kaon experiments at CERN: recent results and prospects

The NA48/2 and NA62-\(R_K\) experiments at the CERN SPS collected large samples of charged kaon decays in flight in 2003--07. The data analysis is still on-going (with over 20 publications produced so far); the recent results from these experiments are presented. A new upper limit on the rate of a lepton number violating decay \(K^\pm\to\pi^\mp\mu^\pm\mu^\pm\) is reported: \(\mathcal{B} < 8.6\times 10^{-11}\) at 90\% CL. Searches for heavy sterile neutrino \(N_4\) and neutral scalar resonances (\(\chi\)) in \(K^\pm\to\pi\mu\mu\) decays are reported. 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 \(10^{-10}\) to \(10^{-9}\) for resonance lifetimes up to 100~ps. A preliminary measurement of the electromagnetic transition form factor slope of the \(\pi^0\) from \(1.05\times 10^6\) fully reconstructed \(\pi^0\to\gamma e^+e^-\) decays is presented: the obtained result \(a = (3.70 \pm 0.53_\text{stat} \pm 0.36_\text{syst})\times 10^{-2}\) represents the first observation of a non-zero slope in the time-like region of momentum transfer.