Precise measurement of the K+- --> pi+-e+e- decay

Abstract

A sample of 7253 $K^{\pm }\to {\pi }^{\pm }{e}^{+}{e}^{-}(\gamma )$ decay candidates with 1.0% background contamination has been collected by the NA48/2 experiment at the CERN SPS, allowing a precise measurement of the decay properties. The branching ratio in the full kinematic range was measured to be $\mathrm{BR}=(3.11\pm 0.12)\times 10^{-7}$, where the uncertainty includes also the model dependence. The shape of the form factor $W(z)$, where $z=(M_{ee}/M_K{)}^2$, was parameterized according to several models, and, in particular, the slope $\delta$ of the linear form factor $W(z)=W_0(1+\delta z)$ was determined to be $\delta =2.32\pm 0.18$. A possible CP violating asymmetry of $K^{+}$ and $K^{-}$ decay widths was investigated, and a conservative upper limit of $2.1\times 10^{-2}$ at 90% CL was established.

Version accepted for publication in Physics Letters B. 17 pages, 6 figures