# Crosssections for uu~>tt~ scattering

Asked by Sho Iwamoto on 2011-04-10

I have a question on the cross section calculated by MG5.

In the following procedure, we can obtain uu~ > tt~ scattering in "Tevatron-like" setup,
and cross section is calculated:
| ./bin/mg5
| > generate u u~ > t t~ QED=2 QCD=2
| > output UUTT
| > launch UUTT
| and here edit "run_cards.dat" as: lpp1=0, lpp2=0, beam1=980, beam2=980.

In MG4, the cross section was 1.89pb, while MG5 gives 1.66pb.

I doubt this result and I tried another choice: I did the same procedure as above, but with QED=0 QCD=2.
Here MG4 gives 1.87pb, and MG5 is 1.87pb.

These results seems that MG5 does some weird things with multiple diagrams.

I am sorry that I could not find the bug, but I have two clues:
(1)
In QED=2 QCD=2 case, we have 3 diagrams: s-channel with gluon, photon, and Z.
but the calculation gives only G1 and G2; there is no G3.
(2)
In QED=0 QCD=2 case, the generated "configs.inc" has
DATA SPROP(-1,1)/21/
(21 is gluon PDG!)
and QED=2 QCD=2 case, it is
DATA SPROP(-1,1)/22/
DATA SPROP(-1,2)/23/
that is, only photon and Z; here no gluon.
I doubt this file, and modified "22" to "21" by hand.
Then the cross section is calculated as 1.88pb, and the result seems to get better.

So now I think that
* one diagram (glu-exchange) lacks, or

I am sorry that I did not investigate the source code.
I am glad if you could give some ideas.

Best Regards,

## Question information

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Solved by:
Johan Alwall
Solved:
2011-04-11
Last query:
2011-04-11
2011-04-10
 Johan Alwall (johan-alwall) said on 2011-04-10: #1

Dear Sho,

Many thanks for this bug report, and you good error tracking. The problem is that the scale choice becomes different when there is a photon line instead of the gluon line. We will fix this in the next version.

All the best,
Johan

 Sho Iwamoto (misho) said on 2011-04-11: #2

Dear Johan,

Thank you for your prompt reply. I think it would solve this cross section problem.

Also I would like to ask another question:
why in this case (QED=QCD=2), only TWO diagrams are generated?
while in MG5 ver.0.7.0, this procedure generates G1, G2, and G3,
MG5 ver.1.0.0 release candidate branch generates G1, G1a, ..., G1l, G2.

Best regards,
Sho

 Sho Iwamoto (misho) said on 2011-04-11: #3

Thanks Johan Alwall, that solved my question.