The NA48/2 experiment at CERN collected a large sample of charged kaon decays into final states with multiple charged particles in 2003--2004. A new upper limit on the rate of the lepton number violating decay $K^{ \pm}\to\pi^{ \mp}\mu^{ \pm}\mu^{ \pm}$ obtained from this sample is reported: $\mathcal{ B}(K^{ \pm}\to\pi^{ \mp}\mu^{ \pm}\mu^{ \pm})<8.6 \times 10^{ -11}$ at 90\% CL. Searches for two-body resonances in the $K^{ \pm}\to\pi\mu\mu$ decays (including heavy neutral leptons~$N_4$ and inflatons~$\chi$) in the accessible range of masses and lifetimes are also presented. In the absence of a signal, upper limits are set on the products of branching ratios~$\mathcal{ B}(K^{ \pm}\to\mu^{ \pm}N_4)\mathcal{ B}(N_4\to\pi\mu)$ and $\mathcal{ B}(K^{ \pm}\to\pi^{ \pm}\chi)\mathcal{ B}(\chi\to\mu^+\mu^-)$ as functions of the resonance mass and lifetime. These limits are in the $10^{ -10}-10^{ -9}$ range for resonance lifetimes below 100~ps.
Categories: hep-ex