# help choosing model file

Asked by Rameswar Sahu on 2021-03-09

Hi,
I want to study a process in which i have b quarks in the initial state. The process that i want to generate is P P > final state... i want to know which of the following method is correct,
1) work with four flavour scheme
define p = p b b~
generate p p > final state
OR
2) Work in five flavour scheme
generate p p > final state

whichever is the answer can you please give me a good reason...

My only doubt is , if i am generating a process for p p collision where i have b quark in initial state, then should i take the b quark to be massless???
best

## Question information

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Solved by:
Olivier Mattelaer
Solved:
2021-03-09
Last query:
2021-03-09
2021-03-09
 Olivier Mattelaer (olivier-mattelaer) said on 2021-03-09: #1

Hi,

PDF are computed in either the four or five flavour scheme so if you want to have the b quark in the initial state it is
more consistent to use a massless b quark and therefore a five flavour model.

So I typically do not encourage to do
> 1) work with four flavour scheme
> define p = p b b~
> generate p p > final state

This being said people are doing that and claim that the theoretical error due to the missmatch is small enough/...
I have never study that in details to be able to check up to which point this is a True statement (for sure it is for some process and/or in some limit).

Now for many processes you have theoretical paper comparing the computation within the four/five flavour scheme to determine which is the most appropriate for a given process, I can only suggest you to look at the litterature about that.

> My only doubt is , if i am generating a process for p p collision where i have b quark in initial state, then should i take the b quark to be massless???

In that case it is better to start from a model with 5 massless flavor.

Cheers,

Olivier

> On 9 Mar 2021, at 12:45, Rameswar Sahu <email address hidden> wrote:
>
> New question #695939 on MadGraph5_aMC@NLO:
>
> Hi,
> I want to study a process in which i have b quarks in the initial state. The process that i want to generate is P P > final state... i want to know which of the following method is correct,
> 1) work with four flavour scheme
> define p = p b b~
> generate p p > final state
> OR
> 2) Work in five flavour scheme
> generate p p > final state
>
> whichever is the answer can you please give me a good reason...
>
> My only doubt is , if i am generating a process for p p collision where i have b quark in initial state, then should i take the b quark to be massless???
> best
>
> --