Recent results from NA48/2 and NA62 Experiments at CERN

Abstract

The NA48/2 and \nark experiments at the CERN SPS collected a large sample of charged kaon decays in flight. \nark 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 \(\left(10^{-10}-10^{-9}\right)\) 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 \(\left(9-70\right) \text{MeV}/c^2\) are improved.