Is the cross-section and helicity calculated in the lab frame or the centre-of-mass frame of the partons ?

Asked by Songshaptak De on 2019-12-19

I have two queries ->
Firstly, is the cross-section shown in the .lhe files calculated in the lab frame or the center of mass frame of the partons ?
Secondly, does the SPINUP variable store the value of helicity in the lab frame or the centre of mass frame of the partons ?

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
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Solved by:
Olivier Mattelaer
Solved:
2019-12-23
Last query:
2019-12-23
Last reply:
2019-12-19

Hi,

> Firstly, is the cross-section shown in the .lhe files calculated in the lab frame or the center of mass frame of the partons ?

The cross-section is typically a lorentz invariant quantity and in that case your question does not make sense. But it is true that in general you have multiple source of the breaking of the lorentz invariance (the most comon one being the cuts, then the explicit breaking of gauge invariance, ...) and in that case you have a frame dependence.
However since all the cuts (or more generaly all the source of lorentz invariance breaking) are not in the same frame, you can not claim that your cross-section is associated to a unique frame.
So I can not answer that question.

> Secondly, does the SPINUP variable store the value of helicity in the lab frame or the centre of mass frame of the partons ?

At LO, this is done in the cente of mass frame.
At NLO, when using the special madspin mode to fill those information, I do not know the answer for sure. If you need that information I can take a look at the code.

Cheers,

Olivier

PS: I guess that the following paper would be very useful for you:
Xiv:1912.01725 <http://arxiv.org/abs/arXiv:1912.01725>

> On 19 Dec 2019, at 12:58, Songshaptak De <email address hidden> wrote:
>
> New question #687306 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/687306
>
> I have two queries ->
> Firstly, is the cross-section shown in the .lhe files calculated in the lab frame or the center of mass frame of the partons ?
> Secondly, does the SPINUP variable store the value of helicity in the lab frame or the centre of mass frame of the partons ?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Songshaptak De (zaptac) said : #2

Of coyrse, the final amplitude in Lorentz invariant, but internally when madgraph evaluates the amplitude, which frame they are evaluated in ?

> Of coyrse, the final amplitude in Lorentz invariant, but internally when
> madgraph evaluates the amplitude, which frame they are evaluated in ?

At LO and before version 2.7.0, the amplitude is evaluated is evaluated in the partonic center of mass frame. Starting in 2.7.0, the user can choose the frame in which the evaluation of the
amplitude is done such that it is possible to use polarised amplitude for the first time in MG5aMC. (as indicated in the paper pointed before)

Cheers,

Olivier

> On 19 Dec 2019, at 14:47, Songshaptak De <email address hidden> wrote:
>
> Question #687306 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/687306
>
> Songshaptak De posted a new comment:
> Of coyrse, the final amplitude in Lorentz invariant, but internally when
> madgraph evaluates the amplitude, which frame they are evaluated in ?
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Songshaptak De (zaptac) said : #4

For higher version (> 2.7.0) of MadGraph, if the user doesn't choose any frame, what default frame does Madgraph take for internal evaluation of amplitude ?

Default are not fixed in MG5aMC, they depend of the process.

This allow to mitigate the typical error of beginner that does not understand all the parameter and just "trust" us to have sensitive default.

As you can imagine, we also have hidden parameter in the run_card (parameter not present in the run_card by default but that you can add). The idea here is obviously to avoid the presence of very complex parameter by default but still allowing advanced user to use such parameters.

We actually also have a set of auto-hidden parameter. Those are parameters which depending of the process are sometimes hidden and sometimes visible. The "me_frame" entry of the run_card is within that category. When not written inside the run_card it means that at LO the center of mass frame is used for the computation at NLO it means that various frame at used for the computation).
If another frame is selected by default then it will always be written in the run_card_default.dat and in the run_card (but the user can obviously remove it from the run_card or edit it).

So the safest answer to your answer is look at run_card.

Cheers,

Olivier

Songshaptak De (zaptac) said : #6

Thanks Olivier Mattelaer, that solved my question.