# k-factor, scale uncertainties and pp->z>tt~ at NLO

Hi,

I am interesting in the NLO QCD correction to the production of top quark pairs through the Z boson exchange (pp---> z---> tt~). In MadGraph, one can generate this reaction by

(1) generate p p > t t~ QCD<=0 / a w- w+ [QCD]

or by

(2) generate p p > z> t t~ [QCD]

These commands allow to generate the same Born, the same one-loop and the same qq~->tt~g real emission Feynman diagrams. The only difference is in the qg->tt~q and q~g>tt~q~ real emission diagrams, where some diagrams are discarded if we use the method (2).

The cross sections, the uncertainties and the k-factor obtained by (1) and (2) are:

(1) sigma_LO=1.373e-01 +- 5.0e-04 pb,+0.2% -0.7%,

sigma_

k-factor=17.4

(2) sigma_LO= 1.369e-01 +- 5.0e-04 pb, +0.2% -0.7%,

sigma_

k-factor=1.37

In the first one, we get huge k-factor and the scale uncertainties at NLO are not reduced.

Which method is the correct one? and why the k-factor is very large in (1)?

Cheers,

Sadek

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- marco zaro Edit question

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