# setting QCD scale using the b-quark spectator jet in 4FS

Hello

I generate single top t-channel in 4FS, including the off-shell contribution. I'm interested in setting the dynamic scale to be Q=4*sqrt(pt^2+m^2) of a spectator b-jet at LO. Is there a way of doing it?

The process is

generate p p > w+ b b~ j $$ w+ w- a z h j QCD<=1 QED<=4

add process p p > w- b~ b j $$ w+ w- a z h j QCD<=1 QED<=4

I cannot use particle position since there are a few symmetric processes under b<->b~ exchange.

Maybe there is a way checking that the final state particle (b-quark) comes from a gluon splitting?

Regards,

Michael

## Question information

- Language:
- English Edit question

- Status:
- Solved

- Assignee:
- No assignee Edit question

- Solved by:
- Olivier Mattelaer

- Solved:
- 2021-02-24

- Last query:
- 2021-02-24

- Last reply:
- 2021-02-23

Hi,

This can be very complicated to do at the fortran level.

One solution would be to do this after event generation when you re-weight event generation by your new scale choice.

In that case most of the ambuiguity should be fixed and you can easily perform such computation.

One way to do that is to hack the file systematics.py and add a new method to compute your scale choice

Cheers,

Olivier

> On 23 Feb 2021, at 02:45, Michael Pitt <email address hidden> wrote:

>

> New question #695706 on MadGraph5_aMC@NLO:

> https:/

>

> Hello

>

> I generate single top t-channel in 4FS, including the off-shell contribution. I'm interested in setting the dynamic scale to be Q=4*sqrt(pt^2+m^2) of a spectator b-jet at LO. Is there a way of doing it?

> The process is

> generate p p > w+ b b~ j $$ w+ w- a z h j QCD<=1 QED<=4

> add process p p > w- b~ b j $$ w+ w- a z h j QCD<=1 QED<=4

>

> I cannot use particle position since there are a few symmetric processes under b<->b~ exchange.

>

> Maybe there is a way checking that the final state particle (b-quark) comes from a gluon splitting?

>

> Regards,

> Michael

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

Michael Pitt (mpitt82) said : | #2 |

Thanks Olivier Mattelaer, that solved my question.