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

Abstract

A sample of 3120 $K^{\pm }\to {\pi }^{\pm }{\mu }^{+}{\mu }^{-}$ decay candidates with $(3.3\pm 0.7)$ 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 $\mathrm{BR}=(9.62\pm 0.25)\times 10^{-8}$. The form factor $W(z)$, where $z=(M_{\mu \mu }/M_K{)}^2$, was parameterized according to several models. In particular, the slope of the linear form factor $W(z)=W_0(1+\delta z)$ was measured to be $\delta =3.11\pm 0.57$. 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 $\mathrm{BR}(K^{\pm }\to {\pi }^{\mp }{\mu }^{\pm }{\mu }^{\pm })<1.1\times 10^{-9}$ was established at 90% CL for the rate of the lepton number violating decay.

arXiv comment16 pages, 7 figures

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