Observation of B0-B0 mixing

Abstract Using the ARGUS detector at the DORISII storage ringwe have searched in three different ways for B 0 - B 0 mixing in ϒ (4S) decays. One explicitly mixed event, a decay ϒ (4S)→B 0 B 0 , has been completely reconstructed. Furthermore, we observe a 4.0 standard deviationsignal of 24.8 events with like-sign lepton pairs and a 3.0 standard deviationsignal of 4.1 events containing one reconstructed B 0 ( B 0 ) and an additional fast l + (l) − . This leads to the conclusion that B 0 - B 0 mixing is substantial. For the mixing parameter we obtain r =0.21±0.08.