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 K0 transforming into K0 and K0 into K0 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 pp→K±π∓K0(K0) at rest, whereas the strangeness of the kaon at the decay time t=τ was tagged by the lepton charge in the final state. An average decay-rate asymmetry〈R(K0t=0→e+π−νt=τ)−R(K0t=0→e−π+νt=τ)R(K0t=0→e+π−νt=τ)+R(K0t=0→e−π+νt=τ)〉=(6.6±1.3stat±1.0syst)×10−3was measured over the interval 1 τS<τ<20 τS, thus leading to evidence for time-reversal non-invariance.