First direct observation of time-reversal non-invariance in the neutral-kaon system

Abstract

We report on the first observation of time-reversal symmetry violation through a comparison of the probabilities of transforming into K0 and K0 into as a function of the neutral-kaon eigentime t. The comparison is based on the analysis of the neutral-kaon semileptonic decays recorded in the CPLEAR experiment. There, the strangeness of the neutral kaon at time t=0 was tagged by the kaon charge in the reaction at rest, whereas the strangeness of the kaon at the decay time t=[tau] was tagged by the lepton charge in the final state. An average decay-rate asymmetrywas measured over the interval 1 [tau]S<[tau]<20 [tau]S, thus leading to evidence for time-reversal non-invariance.