Comment on the CMS search for charge-asymmetric production of W' boson in ttbar + jet events

A reanalysis is presented on the CMS result on a search for a W' boson that couples to the top and down quarks. The model is motivated by the Tevatron results on the forward-backward asymmetry of ttbar pair production. In the evaluation of the theoretical cross section of pp -> ttbar + j, the interference effect between the SM and W' amplitudes is shown to be important, though it is ignored in the CMS analysis. The lower mass bound on the W' boson is relaxed from 840 GeV to 740 GeV at the 95% C.L. due to the interference effect. The bound is also compared to the top forward-backward asymmetry.