Direct measurement of the mass difference between top and antitop quarks

We present a direct measurement of the mass difference between top and antitop quarks (Δm) in lepton+jets t(bar t) final states using the 'matrix element' method. The purity of the lepton+jets sample is enhanced for t(bar t) events by identifying at least one of the jet as originating from a b quark. The analyzed data correspond to 3.6 fb-1 of p(bar p) collisions at √s = 1.96 TeV acquired by D0 in Run II of the Fermilab Tevatron Collider. The combination of the e+jets and m+jets channels yields Δm = 0.8 ± 1.8 (stat) ± 0.5 (syst) GeV, which is in agreement with the standard model expectation of no mass difference.