Can loop_qcd_qed_sm_Gmu be used for QCD only?

Hi,

I have done some calculations using LUXPDF and the model loop_qcd_qed_Gmu.

After doing so, I was asked to calculate QCD corrections only. We want to check if loop_qcd_qed_sm_Gmu can be run in a meaningful way, even if we are only interested in QCD corrections.

It works fine for QCD+EW. I got 642.5 pb for the p p > ttbar cross section using this model and LUXpdf. This is exactly the result from the paper we used as reference.

In order to consider QCD calculations only, I have used the syntax p p > t t~ aEW=0 aS=2 [QCD]. So, I tried to minimize the aEW coupling as much as possible.

To activate loop_qcd_qed_sm I have followed the following steps:
MG5 aMC> set complex mass scheme true
MG5 aMC> import model loop qcd qed sm Gmu
MG5 aMC> define p = g d d~ u u~ s s~ c c~ b b~ a
MG5 aMC> define j = g d d~ u u~ s s~ c c~ b b~ a

The cross section was at NLO 640.2+- 0.072. Meaning that it was reduced by 2 pb when considering QCD only. This was the first approach we came up with.

My thesis advisor tried to recreate this, since we both expected an increase in cross section of atleast 10 %. I have only achieved a small decrease in cross section.

She used a different approach. She used the standard MG5 (with loop_sm[which is the default]) in order to only consider the QCD corrections.
Meaning, no new model, no complex mass scheme, the photon was not added. The same parameters were used, for the run and param card. She got 645.2 +- 0.072 pb.

So we were wondering, what approach would be the correct one, if we want to consider QCD corrections only. We expected both approaches to be correct.

I have made some plots. If necessary, I can try to send them to you. Maybe it is also possible to upload my plots to the institutes website.