The difference between ISR in Pythia8 and MGISR in madgraph

Asked by Reza Jafari on 2021-04-16

Dear Experts,

As I checked below questions,
https://answers.launchpad.net/mg5amcnlo/+question/668160
https://answers.launchpad.net/mg5amcnlo/+question/295954,
I calculated the cross-section of for example e- e+ > z h process to see a comparison between the effect of ISR in MGISR plugin and the ISR which is switched on in pythia8 by default.

1: corss-section with pythia8 ISR switched off (manually by adding PartonLevel:ISR = off in pythia card) inside madgraph = 0.01282 pb
2: corss-section with pythia8 ISR switched on (by default) inside madgraph = 0.01284 pb
3: corss-section in madgraph with MGISR plugin (and pythia8 is off completely) = 0.0163 pb
(uncertainty for all above results is in order of 10^-6 pb)

What is the reason for difference between results of number 2 and 3 ?

Why the difference between number 1 and 2 is very small ?

For a complete and reliable simulation and analysis should I switch on both ISR with MGISR and pythia8 ?
If we switch on the ISR effect in only pythia8, I think the effect of ISR in the cross-section does not be considered.

Thanks,

Best Regards,
Reza

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The Parton-Shower is a unitary procedure (i.e. it does not change the cross-section)
therefore 1 and 2 should be the same.
On the other-hand using the MGISR, you do change the spectrum of energy and have an impact on the cross-section.

I do not think that this is problematic to have both Py8 and MGISR on the same sample.
But I'm not really an expert in such type of simulation.

Cheers,

Olivier

Reza Jafari (reza13) said : #2

Dear Olivier,

Thanks a lot for your answer and explanation.

I would be appreciated if you let me know your idea about one more precise question.

I generated samples with default pythia8 card and performed my analysis. Now, I want to add the effect of ISR in my work. Do you think it is enough to consider the effect of MGISR on the cross-sections only? Or does MGISR affect on sample events and I have to repeat data generation while the ISR effect was on by default in pythia card for the previous samples?

Regards,
Reza

I’m not expert enough in that type of physics to give you a correct answer on that.
Even if I would have been, the answer is likely something like “it is dependent of the analysis that you plan to perform”.
I would bet that it does not matter for some observables but does a lot for some other.
The most secure way to know is to generate both and compare.

Cheers,

Olivier

> On 17 Apr 2021, at 13:01, Reza Jafari <email address hidden> wrote:
>
> Question #696605 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/696605
>
> Status: Answered => Open
>
> Reza Jafari is still having a problem:
> Dear Olivier,
>
> Thanks a lot for your answer and explanation.
>
> I would be appreciated if you let me know your idea about one more
> precise question.
>
> I generated samples with default pythia8 card and performed my analysis.
> Now, I want to add the effect of ISR in my work. Do you think it is
> enough to consider the effect of MGISR on the cross-sections only? Or
> does MGISR affect on sample events and I have to repeat data generation
> while the ISR effect was on by default in pythia card for the previous
> samples?
>
> Regards,
> Reza
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Reza Jafari (reza13) said : #4

Dear Olivier,

Thanks a lot for your prompt answer.

Then I generate both and compare the results.

Regards,
Reza

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