Searches for Lepton Number Violation and Resonances in K±→πμμ Decays

The NA48/2 experiment at CERN collected a large sample of charged kaon decays to final states with multiple charged particles in 2003--2004. A new upper limit on the rate of the lepton number violating decay $K^pm opi^mpmu^pmmu^pm$ is reported: $mathcalB(K^pm opi^mpmu^pmmu^pm)<8.6 imes 10^-11$ at 90% CL. Searches for two-body resonances $X$ in $K^pm opimumu$ decays (such as heavy neutral leptons $N_4$ and inflatons $chi$) are also presented. In the absence of signals, upper limits are set on the products of branching fractions $mathcalB(K^pm omu^pmN_4)mathcalB(N_4 opimu)$ and $mathcalB(K^pm opi^pmX)mathcalB(X omu^+mu^-)$ for ranges of assumed resonance masses and lifetimes. The limits are in the $(10^-11,10^-9)$ range for resonance lifetimes below 100 ps.