A Measurement of the K0 Charge Radius and a CP Violating Asymmetry Together with a Search for CP Violating E1 Direct Photon Emission in the Rare Decay KL->pi+pi-e+e-

TL;DR

This paper reports precise measurements of the K0 charge radius, CP violating asymmetry, and limits on CP violating E1 photon emission in the rare decay KL->pi+pi-e+e-, advancing understanding of CP violation and kaon structure.

Contribution

It provides the first experimental upper limit on E1 photon emission and detailed measurements of the K0 charge radius and CP asymmetry in KL decays.

Findings

Measured the K0 charge radius as (-0.077+-0.007(stat)+-0.011(syst)) fm^2.

Established an upper limit of 0.04 (90% CL) for the E1 to M1 photon emission ratio.

Observed a CP violating asymmetry of (13.6+-1.4(stat)+-1.5(syst))%.

Abstract

Using the complete KTeV data set of 5241 candidate KL->pi+pi-e+e- decays (including an estimated background of 204+-14 events), we have measured the coupling gCR=0.163+- 0.014(stat)+-0.023(syst) of the CP conserving charge radius process and from it determined a K0 charge radius of <r**2>(K0)=(-0.077+-0.007(stat)+-0.011(syst)) fm**2. We have also determined a first experimental upper limit of 0.04 (90% CL) for the ratio |g_{E1}|/|g_{M1}| of the coupling for the E1 direct photon emission process relative to the coupling for M1 direct photon emission process. We also report the measurement of its associated vector form factor |gM1`|(1+ (a_1/a_2)/(M(rho)**2-M(K)**2)+2M(K)E(gamma*)) where |gM1`|=(1.11+- 0.12(stat)+-0.08(syst) and a_1/a_2 = (-0.744+-0.027(stat)0.032(syst)) GeV**2/c**2. In addition, a measurement of the manifestly CP violating asymmetry of magnitude (13.6+- 1.4+-(stat)+-1.5(syst))% in the CP and T odd angle phi between the decay planes of the e+e- and pi+pi- pairs in the KL center of mass system is reported.