MadGraph5_aMC@NLO

Generation p p > e+ e- e+ vl cross section

Asked by Melissa

Dear developers,

I've a question about generation of the following process:
 p p > e+ e- e+ vl
When I generate this at 13TeV energy, I get a cross section of ~0.03pb.

Then I demand a photon and a w+ boson to be generated first, which then decay into leptons and a neutrino:
p p > w+ a, w+ > e+ vl, a > e+ e-
Then I get a cross section of 2411505 pb. Why is this so much higher? I checked the diagrams which are generated, and it seems that only a subsection of the first process is generated (as expected). What am I not understanding here?

To see whether I understand things correctly, I have tried to generate p p > e+ e- as well, followed by p p > a > e+ e-. Here the cross sections do make sense (first gives 844 pb, second one 208 pb).

Thanks for your help.
Melissa

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

The syntax,

> , a > e+ e-

Does not make any sense since the photon is never on-shell and has a zero width.
Therefore since you force the photon to be on shell (up to N times the width but N times zero is zero)
I have no idea how much physical that number is.

Cheers,

Olivier

> On Oct 19, 2015, at 12:31, Melissa <email address hidden> wrote:
>
> New question #272595 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/272595
>
> Dear developers,
>
> I've a question about generation of the following process:
> p p > e+ e- e+ vl
> When I generate this at 13TeV energy, I get a cross section of ~0.03pb.
>
> Then I demand a photon and a w+ boson to be generated first, which then decay into leptons and a neutrino:
> p p > w+ a, w+ > e+ vl, a > e+ e-
> Then I get a cross section of 2411505 pb. Why is this so much higher? I checked the diagrams which are generated, and it seems that only a subsection of the first process is generated (as expected). What am I not understanding here?
>
> To see whether I understand things correctly, I have tried to generate p p > e+ e- as well, followed by p p > a > e+ e-. Here the cross sections do make sense (first gives 844 pb, second one 208 pb).
>
> Thanks for your help.
> Melissa
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Melissa (mcbeekveld) said : 		#2

Dear Olivier,

I was afraid it had something to do with the photon being on shell, but can you explain why p p > a > e+ e- does make sense? Is it a coincidence? Or is the syntax p p > a > e+ e- different from p p > a, a > e+ e-?

Cheers,
Melissa

Revision history for this message
Melissa (mcbeekveld) said : 		#3

Dear Olivier,

I have answered my own question, it is indeed different.

Thanks for your help,
Melissa

Revision history for this message
Melissa (mcbeekveld) said : 		#4

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#5

The two syntax do not have the same meaning:
> p p > a > e+ e-
 requires to have a photon in s-channel.
This syntax is typically bad since you apply some diagram selection where they are no good reason for doing it. (i.e. the interference are not negligible)

> p p > a, a > e+ e-?

requires to to have an on shell photon decaying into e+ e-

Cheers,

Olivier

> On Oct 19, 2015, at 12:51, Melissa <email address hidden> wrote:
>
> Question #272595 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/272595
>
> Melissa posted a new comment:
> Dear Olivier,
>
> I was afraid it had something to do with the photon being on shell, but
> can you explain why p p > a > e+ e- does make sense? Is it a
> coincidence? Or is the syntax p p > a > e+ e- different from p p > a, a
>> e+ e-?
>
> Cheers,
> Melissa
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.