First measurement of the CP violation parameter eta+-(gamma) in neutral kaon decays

Interference between [ital K][sub [ital S]] and [ital K][sub [ital L]] decays into the final state [pi][sup +][pi][sup [minus]][gamma] has been observed in experiment 731 at Fermilab. By fitting the distribution of decays downstream of a regenerator, a new CP-violation parameter [eta][sub +[minus][gamma]], analogous to [eta][sub +[minus]], has been measured, with its magnitude equal to (2.15[plus minus]0.26[plus minus]0.20)[times]10[sup [minus]3], and [phi][sub +[minus][gamma]], the associated phase angle, equal to (72[plus minus]23[plus minus]17)[degree]. Assuming that any difference between this fit value for [eta][sub +[minus][gamma]] and the previously measured value for [eta][sub +[minus]] can be attributed to direct CP violation in this decay, a limit of [vert bar][ital e][sub +[minus][gamma]][sup [prime]][vert bar]/[epsilon][lt]0.3 at the 90% confidence level has been made.