First direct observation of time-reversal violation

Using its unique capability of strangeness tagging at K0 production in pp→K∓π±K0(K0) and at decay with the lepton charge in semileptonic decays CPLEAR measured the semileptonic decay-rate asymmetry R(K0→e+π−ν)−R(K0→e−π+ν)R(K0→e+π−ν)+R(K0→e−π+ν). The asymmetry, fitted over the eigentime interval 1–20 τS, yielded a non-zero result of (6.6±1.3stat±1.1syst)×10−3. A thorough phenomenological anaylsis identifies T violation in K0 mixing and/or CPT violation in semileptonic decays as possible interpretations. A confrontation with world data on neutral kaon decays, however, excludes the latter with sufficient precision to establish the result as the first direct observation of time reversal non-invariance.