# Implement cut on Bjorken-x

Dear madgraph developpers,

I'm running some e+p simulations, and p+p but then one proton emits only a photon (option 0 and 2 respectively, for the incoming particle. In this I assume btw that the photon-emitting proton does not break up in the process..).

For some reaction, I'd like to study the integrated cross section, but for Bjorken-x <=0.01.

Would you be able to advice me where and how to implement this cut? If I implement it, then the cross section that would be stated after "launch" would be the number I'm looking for, correct?

I browsed around in the code a while, but ultimately I can't figure how to implement it correctly.

Any help would be greatly appreciated.

Best wishes,

Merijn

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- Last query:
- 2019-01-03

- Last reply:
- 2019-01-07

Hi,

Did you check the following link:

https:/

Cheers,

Olivier

On 3 Jan 2019, at 14:32, Merijn van de Klundert <<email address hidden>

New question #677294 on MadGraph5_aMC@NLO:

https:/

Dear madgraph developpers,

I'm running some e+p simulations, and p+p but then one proton emits only a photon (option 0 and 2 respectively, for the incoming particle. In this I assume btw that the photon-emitting proton does not break up in the process..).

For some reaction, I'd like to study the integrated cross section, but for Bjorken-x <=0.01.

Would you be able to advice me where and how to implement this cut? If I implement it, then the cross section that would be stated after "launch" would be the number I'm looking for, correct?

I browsed around in the code a while, but ultimately I can't figure how to implement it correctly.

Any help would be greatly appreciated.

Best wishes,

Merijn

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Dear Olivier,

many thanks, I wasn't aware of these instructions. I managed to implement a cut vetoing all events by adapting dummy_cuts.

I didn't manage afer a few attempts to define the Bjorken-x in this function and apply a cut on it, but I'm not experienced in Fortran either.

If there is an easy rule to implement this cutoff I'd be interested; otherwise I'll have to reside to implementing an additional step, i.e. analysing the .lhe output files and implementing the x-cut there.

Best wishes and many thanks for the reply,

Merijn

Hi,

One trick is to use the following common block (define in run.inc)

double precision ebeam(2), xbk(2),q2fact(2)

common/

and then put your cut on xbk(1), xbk(2)

Note here that you might need to generate your process with "set group_subprocess False" (you need to check if it is needed or not)

setting that parameter will switch off a lot of optimisation (i.e. slower code). One of those worries me since this is related to the symmetry between the initial state particles (that you broke due to your cut). So if you found weird result without that flag retry with that flag on.

Cheers,

Olivier

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