MadGraph5_aMC@NLO

problem with W+ W- cross sections

Asked by Proklova Nadezhda

Hi,
I was trying to generate 2 processes with W+/- and I think I got some problems with their cross sections.
I'm using the commands:
generate p p > W- a a QED=4, W- > e- ve~
generate p p > W+ a a QED=4, W+ > e+ ve
I have equal run cards for them, but when I'm trying to get their cross sections, I receive 2 different numbers:
0.0004133 +- 1.432e-06 pb for W- and
0.0006521 +- 2.211e-06 pb for W+.
I believed that they must be equal in sm,
so may be I'm wrong somewhere.
Can you please help me?
Thanks in advance,

Regards,
Nadezhda

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Olivier Mattelaer
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Olivier Mattelaer (olivier-mattelaer) said : 		#1

Dear Nadezhda,

Those number are correct, those process are not equal in the SM due to presence of the PDF. (more exactly due to the valence quark)

Cheers,

Olivier

On Sep 24, 2014, at 7:03 PM, Proklova Nadezhda <email address hidden> wrote:

> New question #254956 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/254956
>
> Hi,
> I was trying to generate 2 processes with W+/- and I think I got some problems with their cross sections.
> I'm using the commands:
> generate p p > W- a a QED=4, W- > e- ve~
> generate p p > W+ a a QED=4, W+ > e+ ve
> I have equal run cards for them, but when I'm trying to get their cross sections, I receive 2 different numbers:
> 0.0004133 +- 1.432e-06 pb for W- and
> 0.0006521 +- 2.211e-06 pb for W+.
> I believed that they must be equal in sm,
> so may be I'm wrong somewhere.
> Can you please help me?
> Thanks in advance,
>
> Regards,
> Nadezhda
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Proklova Nadezhda (alenorr) said : 		#2

Thank you for the answer, but can you please just specify what did you mean? I can't understand clearly why and how can PDF have such kind of influence... I'm using 'cteq6l1' in both run cards btw.

Regards,
Nadezhda

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#3

If you try to do this:
generate p p > w+

You will see that you will have this processes:
INFO: Trying process: u d~ > w+ WEIGHTED=2
INFO: Process has 1 diagrams
INFO: Trying process: c s~ > w+ WEIGHTED=2
INFO: Process has 1 diagrams
INFO: Process d~ u > w+ added to mirror process u d~ > w+
INFO: Process s~ c > w+ added to mirror process c s~ > w+

While for p p > w-
INFO: Trying process: d u~ > w- WEIGHTED=2
INFO: Process has 1 diagrams
INFO: Trying process: s c~ > w- WEIGHTED=2
INFO: Process has 1 diagrams
INFO: Process u~ d > w- added to mirror process d u~ > w-
INFO: Process c~ s > w- added to mirror process s c~ > w-

Since the valence quark of the proton are the quark u (times 2) and the quark down, you expect that the contribution of the
d~ u > w+ process is bigger than the one of the d u~ > w-

When I run the above computation (with the option set group_subprocesses False)
I have the following output:
P0_dxu_wp 4.182e+04
P0_udx_wp 4.179e+04
P0_dux_wm 3.017e+04
P0_uxd_wm 3.006e+04

So this goes as you can expect.

Cheers,

Olivier

On Sep 24, 2014, at 10:33 PM, Proklova Nadezhda <email address hidden> wrote:

> Question #254956 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/254956
>
> Status: Answered => Open
>
> Proklova Nadezhda is still having a problem:
> Thank you for the answer, but can you please just specify what did you
> mean? I can't understand clearly why and how can PDF have such kind of
> influence... I'm using 'cteq6l1' in both run cards btw.
>
> Regards,
> Nadezhda
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Proklova Nadezhda (alenorr) said : 		#4

Thank you very much!

Regards,
Nadezhda

Revision history for this message
Proklova Nadezhda (alenorr) said : 		#5

Thanks Olivier Mattelaer, that solved my question.