Measurements of CP violation and neutral kaon charge radius using long-lived kaon going to pion-antipion-positron-electron decays

CP violation and K0 charge radius were measured using KL → pi+pi -e+e - decays. Specifically, a unique CP-violating decay-plane asymmetry was measured along with the parameters of individual contributions to the decay invariant amplitude: (i) CP-conserving magnetic dipole direct emission form factor, (ii) CP-conserving K0 charge radius transition amplitude and (iii) an upper limit for the CP-violating electric dipole direct emission amplitude. The measurements were obtained from the data sample accumulated by KTeV experiment at Fermilab.; KTeV had two major goals: the measurement of direct CP violation parameter Re (epsilon'/epsilon) and the study of rare kaon decays. The state of the art detector was constructed, commissioned, operated and maintained by an international collaboration of scientists from fourteen institutions. The KL → pi +pi-e+e- data was accumulated over the 1997 and 1999 running periods. During that time hundreds of billions KL decays took place in the KTeV fiducial decay region.; In the analysis of the rare decay mode KL → pi +pi-e+e- 5241 candidate events were selected from the entire KTeV dataset. This data sample included an estimated background of n = 204 +/- 14 events. The data was analyzed using the method of maximum likelihood to obtain parameters of the decay invariant amplitude. Namely, coefficients of the magnetic dipole (M1) direct emission form factor g&d5;M1=1.11+/-0 .12stat+/-0.08 syst a1a2= -0.744+/-0.027stat+/- 0.032syst GeV2 and the amplitude of K0 charge radius (CR) transition gCR=0.163+/- 0.014stat+/-0.023 syst were measured; and an upper limit for the CP-violating electric dipole (E1) direct emission process gE1 gM1 <0.0490%CL and gE1E ★g<0.03 90%CL was determined. The result for gCR allowed to determine the approximate value of the mean square charge radius of K0 r2K0 =-0.077+/-0.007stat +/-0.011syst fm2 in a novel way. Using the measured values for g&d5;M1 and a1a2 the average of gM1 (over the observed range of energies of the e +e- pair was calculated gM1 E★g =0.74+/-0.04. Finally, using the above values for the individual contributions in the invariant amplitude of the decay, the CP-violating decay-plane asymmetry was measured A= 13.6+/-1.4stat +/-1.5syst %. This is the most precise measurement of this unique asymmetry. It is the largest CP violation effect ever observed and the only one ever observed in an angular variable.