  In this brief report, we propose a new definition of charge asymmetry in top
pair production at the LHC, namely the edge charge asymmetry (ECA). ECA
utilizes the information of drifting direction only for single top (or
anti-top) with hadronically decay. Therefore ECA can be free from the
uncertainty arising from the missing neutrino in the $t\bar{t}$ event
reconstruction. Moreover rapidity $Y$ of top (or anti-top) is required to be
greater than a critical value $Y_{\rm C}$ in order to suppress the symmetric
$t\bar{t}$ events mainly due to the gluon-gluon fusion process. In this paper
ECA is calculated up to next-to-leading order QCD in the standard model and the
choice of the optimal $Y_{\rm C}$ is investigated.
