Search for Direct CP Violation in Neutral Kaon Decays by a Measurement of Epsilon'.

The CP violation parameter (epsilon)', defined as (epsilon)'/(epsilon) (TURNEQ) (1/6)(1 - (VBAR)(eta)(,00)(VBAR)('2)/(VBAR)(eta)(,+ -)(VBAR)('2) where (VBAR)(eta)(,00)(VBAR)/(VBAR)(eta)(,+ -)(VBAR) = (Amp(K(,L) (--->) (pi)('0)(pi)('0))/(Amp(K(,S) (--->) (pi)('0)(pi)('0)))/(Amp(K(,L) (--->) (pi)('+)(pi)(' -))/Amp(K(,S) (--->) (pi)('+)(pi)('-))') has been measured at the Brookhaven AGS in an attempt to observe a deviation from zero in the value of this parameter. Such a deviation would indicate the presence of (VBAR)(DELTA)S(VBAR) = 1 CP violating amplitudes absent in superweak models of CP violation but generally present in gauge theory realizations of CP non-conservation. Two pion decays of neutral kaons were observed in a detector consisting of scintillation counters and proportional wire chambers arranged about a spectrometer magnet and a large, finely segmented lead glass array. The detector was situated around a 0(DEGREES) neutral beam line equipped with a carbon K(,S) regenerator which could be remotely moved in and out of the beam. Charged and neutral decays were observed simultaneously under two beam conditions: from an almost pure K(,L) beam (regenerator removed) and from a beam composed largely of K(,S) mesons (regenerator in the beam). Over 500 of the difficult K(,L) (--->) (pi)('0)(pi)('0) events were collected giving a measured value of (epsilon)'/(epsilon) = 0.015 (+OR-) .018, consistent with the best previous measurement of (epsilon)'/(epsilon) = 0.00 (+OR-) .02. This result places serious constraints of Higgs exchange models of CP violation which generally predict (VBAR)(epsilon)'/(epsilon)(VBAR) (GREATERTHEQ) .02. A pending data collection run should allow the sensitivity of this measurement to be increased by approximately a factor of three providing a severe test of Higgs exchange models of CP violation as well as probing deeply the region of predictions of the most popular gauge theory model of CP violation due to Kobayashi and Maskawa.