lab frame in cuts.f

Hi,

The identity of the beam are not yet assigned so this is not possible at that stage of the computation (at least not without huge and deep change to the code, change that would need days of work for me to figure it out).

Cheers,

Olivier

On May 25, 2014, at 1:06 AM, John Doe <email address hidden> wrote:

> New question #249223 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/249223
>
> Hi MG5 developers, in the function passcuts within the file cuts.f the 4-momenta P(0:3,nexternal) are defined relative to the partonic center-of-mass frame, as is easy to check. I would like to know if there is a way to access the 4-momenta of the external particles in the hadronic lab frame within that function.
>
> (By hadronic lab frame I mean the frame defined by the initial beam types and energies: lpp1, lpp2, ebeam1, ebeam2. I understand that the kinematic variables defined in the run_card, and the momenta written to the .lhe.gz files, are defined in that frame.)
>
> Thanks a lot!
>
> --
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> contact for MadGraph5_aMC@NLO.

Hi Olivier, thanks for your answer. I don't have in mind any changes to the code at all. So maybe I didn't phrase my question clearly enough.

When at least one of the initial particles has PDF=0, one knows its energy in the lab frame, which one chooses when defining the process. One also knows the energy of that particle in the partonic cm frame, which is given by p(0:1) (or p(0:2)). So one can easily find the boost connecting the two frames, and at least in that case the problem is solvable.

My question refers to the case when both initial particles have PDF=+/-1. In that case one knows the energy in the cm frame of, say, the first particle: p(0,1). In order to obtain the boost to the original lab frame one would need to know the energy of that particle in that frame. That information should be known at all times, since it is the initial data to the scattering process.

Thanks again!

By the way, when I wrote "in that case the problem is solvable", I meant "solvable by the user".

Oh well, it was a trivial question. The variables ebeam(1:2) and xbk(1:2) included through run.inc are the obvious answer. Thanks again for your help.

Hi John,

If you use such number then you have to use the option
set group_subprocesses False.

Otherwise xbk(1) and xbk(2) might be flipped just before the writing of the events.

Cheers,

Olivier

Hi Olivier, thanks a lot! Pure wisdom!