Force certain decay without requiring on-shell

Hi,

Concerning the Z, the easiest is to keep your syntax, and put the parameter bw_cut (of the run_card) to a very large value.
Actually, I'm surprised that the code returns a cross-section in the case of the photon.
Note that in addition to be not physical, in the case of the photon, your result is also not gauge invariant.
(and then the result computed by MG is unlikely to be Lorentz invariant).
In consequence, I can't -- and don't want to-- guarantee that the cross-section for the photon case is the result that you expect.

Cheers,

Olivier

On May 30, 2013, at 10:01 PM, Will Flanagan <email address hidden> wrote:

> New question #229945 on MadGraph5:
> https://answers.launchpad.net/madgraph5/+question/229945
>
> Hi Madgraph experts,
>
> I am attempting to give some numerical confirmation to some analytical calculations by our group. In our studies, we are seeing huge interference between these two diagrams:
> http://hepr8.physics.tamu.edu/will/MadGraph/Madgraph130530.pdf
>
> As a result, I was hoping to get numerical results for cross sections with these two processes together and separate:
> mg5>import model mssm ; generate p p > p p z $$ w+ w- / p a QCD=0, (z > mur+ mur-) ; display diagrams
> mg5>import model mssm ; generate p p > p p a $$ w+ w- / p a QCD=0, (a > mur+ mur-) ; display diagrams
>
> This syntax works fine for the photon, but requires the Z to be on-shell (which is kinematically forbidden). Are there changes to syntax which allows only this diagram, but doesn't force the Z to be on-shell?
>
> Of course this syntax is unphysical!! I am just hoping to numerically confirm an analytical calculation.
>
> Any suggestions are much appreciated!
> Will
>
> (This question is somewhat related to my previous discussions with Olivier:
> https://answers.launchpad.net/madgraph5/+question/226332
> https://answers.launchpad.net/madgraph5/+question/226583 )
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.

Hi Olivier,

I still get the error: "Zero result detected: No Phase Space. Please check particle masses." when trying various bw cuts (10,1e6,1e20,-1).

I can't think of any syntax to narrow down to these two diagrams in order to check an analytic result.

Anyway, thanks for the help,
Will

Hi Will,

> I still get the error: "Zero result detected: No Phase Space. Please
> check particle masses." when trying various bw cuts (10,1e6,1e20,-1).

Sorry, I forget that we were checking this point at the Fortran level (this is kind of new).

Then the easiest is to remove by hand to diagram that you don't want.
I have create such function, but this is not (and will never be) part of the official version.
Please send me your email (either here or send one to me at omatt … illinois.edu), and I will send you instructions.

Cheers,

Olivier

On Jun 3, 2013, at 4:31 AM, Will Flanagan <email address hidden> wrote:

> Question #229945 on MadGraph5 changed:
> https://answers.launchpad.net/madgraph5/+question/229945
>
> Will Flanagan posted a new comment:
> Hi Olivier,
>
> I still get the error: "Zero result detected: No Phase Space. Please
> check particle masses." when trying various bw cuts (10,1e6,1e20,-1).
>
> I can't think of any syntax to narrow down to these two diagrams in
> order to check an analytic result.
>
> Anyway, thanks for the help,
> Will
>
> --
> You received this question notification because you are a member of
> MadTeam, which is an answer contact for MadGraph5.