Measurements of CP Violation and K Charge Radius using K- Decays.

CP violation and K 0 charge radius were measured using K L → π + π − 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) CPconserving K 0 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 (ε ′ /ε ) 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 K L → π + π − e + e − data was accumulated over the 1997 and 1999 running periods. During that time hundreds of billions K L decays took place in the KTeV fiducial decay region. In the analysis of the rare decay mode K L → π + π − 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 iv factor ˜g M1 = 1.11 ± 0.12(stat) ± 0.08(syst) a 1 a 2 = [−0.744 ± 0.027(stat) ± 0.032(syst)] (GeV ) 2 and the amplitude of K 0 charge radius (CR) transition |g CR | = 0.163 ± 0.014(stat) ± 0.023(syst) were measured; and an upper limit for the CP-violating electric dipole (E1) direct emission process |g E1 | |g M1 | < 0.04 (90% CL) and 〈|g E1|〉 E ⋆ γ < 0.03 (90% CL) was determined. The result for g CR allowed to determine the approximate value of the mean square charge radius of K 0 〈 r 2 K 0 〉 = [−0.077 ± 0.007(stat) ± 0.011(syst)] (fm) 2 in a novel way. Using the measured values for ˜g M1 and a 1 a 2 the average of |g M1 | over the observed range of energies of the e + e − pair was calculated 〈|g M1 |〉 E ⋆ γ = 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 ± 1.4(stat) ± 1.5(syst)] % . 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.

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