We present a direct measurement of the mass difference between top and antitop quarks ($\Delta m$) in $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ $t\overline{t}$ final states using the “matrix element” method. The purity of the $\mathrm{\text{lepton}}+\mathrm{\text{jets}}$ sample is enhanced for $t\overline{t}$ events by identifying at least one of the jets as originating from a $b$ quark. The analyzed data correspond to $3.6{\mathrm{fb}}^{-1}$ of $p\overline{p}$ collisions at $\sqrt{s}=1.96\mathrm{TeV}$ acquired by D0 in Run II of the Fermilab Tevatron Collider. The combination of the $e+\mathrm{\text{jets}}$ and $\mu +\mathrm{\text{jets}}$ channels yields $\Delta m=0.8\pm 1.8(\mathrm{stat})\pm 0.5(\mathrm{syst})\mathrm{GeV}$, which is in agreement with the standard model expectation of no mass difference.

• Received 13 June 2011

DOI:https://doi.org/10.1103/PhysRevD.84.052005