deviation between madgraph outcome and own calculations

Asked by Steven Jeremies on 2021-06-08

Hello everyone,

I have a very strange problem when calculating the cross section of the u b > d t process. I calculated it with madgraph simply via

>> generate u b > d t
>> output ubdt
>> launch

I varied the beam energy to get multiple values to check and changed the pdf´s to 0 since I dont want to inclued them right now.
I also calculated the cross section by hand and with another MonteCarlo vegas algorithm. Here are some results:

sqrts [GeV] / madgraph [pb] / own calculations (MonteCarlo and analytic computation agree)[pb]

2000 / 50.85 / 53.85
5000 / 46.37 / 54.23
13000 / 24.94 / 54.33

I checked my own calculations by cross-checking the matrix element squared with madgraph which worked. So then I integrated the phase space by hand and via the MC integration so I would say there are no points where I could have messed up the same thing in both calculations since I basically just input the matrix element in my vegas algorithm. On the other hand it is quite odd that my cross section changes only slightly when varying sqrts pretty much,

In the end my question is wether there is any possibility that I maybe just calculated the wrong thing in madgraph for example I had to change another thing in one of the cards that I just forgot other than the beam energy and the value of the lpp´s?

Question information

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English Edit question
Status:
Solved
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MadGraph5_aMC@NLO Edit question
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Solved by:
Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said :
#1

Hi,

I do not know enough to comment especially since I do not reproduce your number for MG5.

In my case I have run the following command file:

> generate u b > d t
> output
> launch
> set no_parton_cut
> set fixed_scale 100
> set lpp 0
> set ebeam 1000
> launch
> set ebeam 2500
> launch
> set ebeam 6500
> launch -i
> print_results --path=./cross_section.txt --format=short

and the content of cross_section.txt is then:

> # run_name tag cross error Nb_event cross_after_matching nb_event_after matching
> run_01 tag_1 54.093 0.047335 10000
> run_02 tag_1 54.399 0.067666 10000
> run_03 tag_1 54.528 0.071512 10000

So clearly I have number that seems close to yours.

Olivier

> On 8 Jun 2021, at 13:01, Steven Jeremies <email address hidden> wrote:
>
> New question #697447 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/697447
>
> Hello everyone,
>
> I have a very strange problem when calculating the cross section of the u b > d t process. I calculated it with madgraph simply via
>
>>> generate u b > d t
>>> output ubdt
>>> launch
>
> I varied the beam energy to get multiple values to check and changed the pdf´s to 0 since I dont want to inclued them right now.
> I also calculated the cross section by hand and with another MonteCarlo vegas algorithm. Here are some results:
>
> sqrts [GeV] madgraph [pb] own calculations (MonteCarlo and analytic computation agree)[pb]
>
> 2000 50.85 53.85
> 5000 46.37 54.23
> 13000 24.94 54.33
>
> I checked my own calculations by cross-checking the matrix element squared with madgraph which worked. So then I integrated the phase space by hand and via the MC integration so I would say there are no points where I could have messed up the same thing in both calculations since I basically just input the matrix element in my vegas algorithm. On the other hand it is quite odd that my cross section changes only slightly when varying sqrts pretty much,
>
> In the end my question is wether there is any possibility that I maybe just calculated the wrong thing in madgraph for example I had to change another thing in one of the cards that I just forgot other than the beam energy and the value of the lpp´s?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Steven Jeremies (minits) said :
#2

Thanks Olivier Mattelaer, that solved my question.

Revision history for this message
Shaw Mett (shaw442) said (last edit ):
#3

I think this correction is what you want. You should replace the following incorrect code:

public double MeanAbsoluteDeviation
{
    get
    {
        //return _window.Average(e => Math.Abs(e - Average));

        // https://stackoverflow.com/questions/5336457/how-to-calculate-a-standard-deviation-array
        var n = N;
        var sumOfDifferences = _window.Sum(e => Math.Abs(e - Average));
        return n > 1 ? sumOfDifferences / (n - 1) : 0.0;
    }
}

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