Precise measurement of the K±→π±e+e−K±→π±e+e− decay

A sample of 7253 K±→π±e+e−(γ)K±→π±e+e−(γ) decay candidates with 1.0% background contamination has been collected by the NA48/2 experiment at the CERN SPS, which allowed a precise measurement of the decay properties. The branching ratio in the full kinematic range was measured to be BR=(3.11±0.12)×10−7BR=(3.11±0.12)×10−7, where the uncertainty includes also the model dependence. The shape of the form factor W(z)W(z), where z=(Mee/MK)2z=(Mee/MK)2, was parameterized according to several models, and, in particular, the slope δ   of the linear form factor W(z)=W0(1+δz)W(z)=W0(1+δz) was determined to be δ=2.32±0.18δ=2.32±0.18. A possible CP violating asymmetry of K+K+ and K−K− decay widths was investigated, and a conservative upper limit of 2.1×10−22.1×10−2 at 90% CL was established.