Discrepancy in ttbar inclusive cross-section using MadGraph with it's theoretical value
I am trying to understand the inclusive cross-section(xsec) for ttbar production. When I generate 1M events of SM ttbar exclusive, I get xsec=447.5 pb. For exclusive SM ttbar+1jet, xsec=307.7 pb and for exclusive SM ttbar+2jets, I get xsec=141.7 pb with cuts on jets (jet pT>20GeV and jet \eta<2.5). The sum of these is xsec(inclusive till 2 jets)=896.9 pb. However, we know that theoretical calculations give xsec(inclusive)=832 pb which agrees with experimental measurements quite well. I am assuming that inclusive xsec includes ttbar + all possible jets. Of course, the latest theoretical calculation also has the loop contributions and thus, I was expecting that the number 896.9 pb would be less than 832 pb. Is this apparent discrepancy due to the large scale dependence (I have used dynamical scale choice = 1) of the number 896.9 pb and maybe negative contribution from the loops? Or am I missing some more basic physics issues?
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