Measurement of the Mass Difference between Top and Antitop

We discuss a measurement of the mass difference ($\Delta_m$) between $t$ and $\overline{t}$ quarks in lepton+jets final states of $t\overline{t}$ events in data collected with the D0 detector at Fermilab's Tevatron. $\Delta_m$ is measured using the Matrix Element approach developed at D0. We report the most likely $\Delta_m$ and an upper limit on $\Delta_m$ at 95\% confidence. This is the first measurement of a mass difference between a quark and its antiquark.