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}$(\chi\to\mu^+\mu^-)$aresetintherange$10^{-10}$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.