New measurement of the K+- --> pi+-mu+mu- decay

A sample of 3120 $K^\pm\toπ^\pmμ^+μ^-$ decay candidates with $(3.3\pm0.7)%$ background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a detailed study of the decay properties. The branching ratio was measured to be ${\rm BR}=(9.62\pm0.25)\times 10^{-8}$. The form factor $W(z)$, where $z=(M_{μμ}/M_K)^2$, was parameterized according to several models. In particular, the slope of the linear form factor $W(z)=W_0(1+δz)$ was measured to be $δ=3.11\pm0.57$. Upper limits of $2.9\times 10^{-2}$ and $2.3\times 10^{-2}$ on possible charge asymmetry and forward-backward asymmetry were established at 90% CL. An upper limit ${\rm BR}(K^\pm\toπ^\mpμ^\pmμ^\pm)<1.1\times 10^{-9}$ was established at 90% CL for the rate of the lepton number violating decay.