The Search for the Emission of a CP-Violating E1 Photon in the K<sub>L</sub> → π<sup>+</sup>π<sup>-</sup>γ Decay

Abstract

A search for the CP-violating electric dipole (E1) direct emission contribution to the K{sub L} {yields} {pi}{sup +}{pi}{sup -}{gamma} decay is performed using data from the 1997 KTeV/E832 experiment. Because the K{sub L} {yields} {pi}{sup +}{pi}{sup -}{gamma} decay mode is massively dominated by the CP-violating inner bremsstrahlung (IB) and the CP-conserving magnetic dipole (M1) direct emission processes, previous analyses have neglected the E1 contribution. Therefore, this measurement is the first attempt to directly quantify the size of the E1 decay process. This E1 transition is one of the very few CP-violating processes that is accessible to experiment and, in principle, will produce new insights into the structure of the neutral kaon. The result of this analysis is that the E1 contribution is below the threshold of sensitivity, and therefore an upper bound of |g{sub E1}| < 0.14 (90% CL) is reported. In the process of obtaining this upper limit, high resolution measurements of fit parameters ({tilde g}{sub M1} and a{sub 1}/a{sub 2}) associated with the size and shape of the M1 direct emission peak are also extracted. The fit results for these parameters: {tilde g}{sub M1} = 1.229 {+-} 0.035 (stat) {+-} 0.087 (syst); a{sub 1}/a{sub 2} = -0.733 {+-} more » 0.007 (stat) {+-} 0.014 (syst) are in strong agreement with previous measurements. « less