MadGraph5_aMC@NLO

shower_scale_factor affects results when shower is OFF at nlo

Asked by Jasmina

I am running stable ttW at NLO with the shower setting turned off. All else being equal I tested changing shower_scale_factor in run_card.dat and I got a different events.lhe file. I would have expected the results to be completely independent of this parameter.

Can you please explain why it does depend on it?

Best,
Jasmina

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

Hi,

As you likely know, If you run at NLO+PS with parton-shower = OFF, you still have to define which parton shower you are going to use. The reason is that MC@NLO method is using a counterterm which substract the probability of emission as done by the parton-shower. This is we have to know what the parton-shower you are going to run (such that we perform the correct substraction).

I did not investigate your issue, but it seems natural that such counter-term/probability depends of the starting scale of the parton-shower and therefore of that parameter.

Cheers,

Olivier

> On 12 Aug 2021, at 10:01, Jasmina <email address hidden> wrote:
>
> New question #698337 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/698337
>
> I am running stable ttW at NLO with the shower setting turned off. All else being equal I tested changing shower_scale_factor in run_card.dat and I got a different events.lhe file. I would have expected the results to be completely independent of this parameter.
>
> Can you please explain why it does depend on it?
>
> Best,
> Jasmina
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Jasmina (jasmin9) said : 		#2

Thanks Olivier Mattelaer, that solved my question.