Precise Measurement of the K - to Pi -E E- Decay

A sample of 7253 K{sup {+-}} {yields} {pi}{sup {+-}}e{sup +}e{sup -}({gamma}) decay candidates with 1.0% background contamination has been collected by the NA 48/2 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) x 10{sup -7}, where the uncertainty includes also the model dependence. The shape of the form factor W(z), where z = (M{sub ee}/M{sub K}){sup 2}, was parameterized according to several models, and, in particular, the slope {delta} of the linear form factor W(z) = W{sub 0}(1 + {delta}z) was determined to be {delta} = 2.32 {+-} 0.18. A possible CP violating asymmetry of K{sup +} and K{sup -} decay widths was investigated, and a conservative upper limit of 2.1 x 10{sup -2} at 90% CL was established.