MadGraph5_aMC@NLO

Flavour Schemes and setting masses of quarks

Asked by Jihwan Bhyun

Dear Madgraph/MC@NLO experts

Hi, I have some questions related to flavour scheme and the mass of quarks in parameter card.
I'm searching for correct ways of using flavour scheme. And it seems like one should set b mass to zero in parameter card if one is to use 5FS, is that correct?
If this is correct, then what about other quarks? Do I need to set the mass of lighter quarks(u,d,c,s) to zero as well?

Jihwan Bhyun

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

Hi,

For five flavor, it is indeed advise to set all quark mass (but the top) to zero.
For four flavor, you should put all the mass to zero but for the bottom/top.

Additionally, you have to use a PDF consistent with the 4/5 flavor scheme that you choose,
Finally, you have to ensure that the cuts/alphas computation are set accordingly and define the proton/jet multi particle with the corresponding quark content.
For this last point, the default run_card would do that automatically for you if you use a model where the mass of the associated quark are forced by the model to be zero. This is actually strongly advised to do so since the code will then be more optimized.
In order to do that you can check the following link:
https://answers.launchpad.net/mg5amcnlo/+faq/2312

Cheers,

Olivier

> On Jun 2, 2016, at 05:42, Jihwan Bhyun <email address hidden> wrote:
>
> Question #294747 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/294747
>
> Description changed to:
> Dear Madgraph/MC@NLO experts
>
>
> Hi, I have some questions related to flavour scheme and the mass of quarks in parameter card.
> I'm searching for correct ways of using flavour scheme. And it seems like one should set b mass to zero in parameter card if one is to use 5FS, is that correct?
> If this is correct, then what about other quarks? Do I need to set the mass of lighter quarks(u,d,c,s) to zero as well?
>
>
> Jihwan Bhyun
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Jihwan Bhyun (jbhyun) said : 		#3

Thanks a lot! It solved most part of my question. But let me just ask a few more things. I see I need to set alphas in accordance with the choice of the PDF. But does the cuts you mentioned I need to set accordingly mean the cuts applied to jets or b jets? (Because in 5FS, same cuts as other jets are also applied to b jets) Or did you mean that there are other cuts I need to care regarding the choice of PDFs? (If this is the case, could you please mention which cuts they are?)

Cheers,
Jihwan Bhyun

Revision history for this message
Jihwan Bhyun (jbhyun) said : 		#4

Thanks Olivier Mattelaer, that solved my question.

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

Hi,

1) the alpha_s at MZ is automatically taken to the one of the PDF and not read from the param_card (independently of the flavor scheme that you use)
2) for the cuts, what is changed is the maxjetflavour parameter which is set to 4 for 4 flavor and to 5 for 5 flavor.
This defines if ptj (and other similar cut in the param_card) parameter are applied to b quark or not.
With that parameter on 4, the ptj is applied for u,d,s,c and the ptb is used for b quark
With that parameter on 5, the ptr is applied for u,d,s,c,b and the pub is not used for any particle.

3) Another parameter automatically set to either 4 or 5 with the flavor scheme is the asrwgtflavor which control the running of alpha_s for the matching.

Cheers,

Olivier

> On Jun 2, 2016, at 14:03, Jihwan Bhyun <email address hidden> wrote:
>
> Question #294747 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/294747
>
> Jihwan Bhyun posted a new comment:
> Thanks a lot! It solved most part of my question. But let me just ask a
> few more things. I see I need to set alphas in accordance with the
> choice of the PDF. But does the cuts you mentioned I need to set
> accordingly mean the cuts applied to jets or b jets? (Because in 5FS,
> same cuts as other jets are also applied to b jets) Or did you mean that
> there are other cuts I need to care regarding the choice of PDFs? (If
> this is the case, could you please mention which cuts they are?)
>
> Cheers,
> Jihwan Bhyun
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Jihwan Bhyun (jbhyun) said : 		#6

Dear Olivier

Thank you really much for the detailed explanation!

Cheers,

Jihwan Bhyun