MadGraph5_aMC@NLO

Cross sections for two-photon processes pp → pXp

Asked by Mamutjan

Dear MadGraph Team,

I wanted to calculate the cross sections for (a a > l+ l-) and (a a > w+ w-) in two photon process at 14TeV
my syntax

>generate a a > l+ l-
>output
>lauch
>set lpp 2
>set ebeam 7000

total cross section is 2.4 pb which is 39.4 pb in Table I of arXiv:0912.5161

and for a a > w+ w- my syntax is:

>generate a a > w+ w-
>output
>lauch
>set lpp 2
>set ebeam 7000

result is 0.1023 pb which is 96.5 fb in Table I of arXiv:0912.5161
My question: Why am I getting the different results? especially for leptonic decays?

Best regards,
Mamutjan.

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

Hi,

We are currently investigating numerical instability for photon-photon process.
But when I see such large discripancy in your number this can only come from the fact that you are not using the latest version of the code, since we fix a bug recently which was dropping the a a cross-section.

Cheers,

Olivier

Revision history for this message
Mamutjan (mamutjan) said : 		#2

Hi ,

Thank you for such a quick reply.
I downloaded the MG (the latest version) just now and got the same results that I had mensioned in my question above.

Revision history for this message
Mamutjan (mamutjan) said : 		#3

I mean my MG was the latest one, and there are still differences.

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Olivier Mattelaer (olivier-mattelaer) said : 		#4

Could you be more precise?

Revision history for this message
Mamutjan (mamutjan) said : 		#5

sorry,

I got the latest MG,(downloaded it just now)

my syntax:
>generate a a > l+ l-
>output
>lauch
>set lpp 2
>set ebeam 7000

total cross section given by MG is 2.4 pb which is 39.4 pb in Table I of arXiv:0912.5161

>generate a a > w+ w-
>output
>lauch
>set lpp 2
>set ebeam 7000

result is 0.1023 pb which given by MG is 96.5 fb in Table I of arXiv:0912.5161
My question: Why am I getting the different results? especially for leptonic decays?

Revision history for this message
Best Olivier Mattelaer (olivier-mattelaer) said : 		#6

Hi,

First they include the tau in their cross-section while he is not in the default l+ / l- definition.

Second, you should think to apply the same cut as them,
In particular they apply a pt cut at 5 (the default MG one is at 10)
and they do not seems to cut on eta (at least I didn’t see any cut in their paper) (the MG default is 2.5)
If you weaken those two default cut you get 42.38 for the l+l- case.

You can look at other cut and/or scale to discover the reason of the remaining difference.

Cheers,

Olivier

On 23 Dec 2014, at 12:01, Mamutjan <email address hidden> wrote:

> Question #259508 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/259508
>
> Status: Needs information => Open
>
> Mamutjan gave more information on the question:
> sorry,
>
> I got the latest MG,(downloaded it just now)
>
> my syntax:
>> generate a a > l+ l-
>> output
>> lauch
>> set lpp 2
>> set ebeam 7000
>
> total cross section given by MG is 2.4 pb which is 39.4 pb in Table I of
> arXiv:0912.5161
>
>> generate a a > w+ w-
>> output
>> lauch
>> set lpp 2
>> set ebeam 7000
>
> result is 0.1023 pb which given by MG is 96.5 fb in Table I of arXiv:0912.5161
> My question: Why am I getting the different results? especially for leptonic decays?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Mamutjan (mamutjan) said : 		#7

Thanks Olivier Mattelaer, that solved my question.