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

We report on the first observation of time-reversal symmetry violation through a comparison of the probabilities of (K) over bar(0) transforming into K-0 and K-0 into (K) over bar(0) 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 --> K (+/-) pi (+/-) K-0 ((K) over bar(0)) 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 asymmetry [R((K) over bart=00 --> e+pi-nut=tau) - R(Kt=00 --> e-pi+t=tau)/R((K) over bart=00 --> e+pi-nut=tau) + R(Kt=00 --> e-pi+t=tau)] = (6.6 +/- 1.3stat +/- 1.0syst) X 10-3 was measured over the interval 1 tau(S) < tau < 20 tau(S), thus leading to evidence for time-reversal non-invariance. (C) 1998 Elsevier Science B.V. All righs reserved. Physics Letters B