CalcHEP

Negative cross section for processes involving G,G->G

Asked by Angus Browne

Hi
I have had problems with the process G,G->~g,~g in the MSSM model specifically the diagram for G,G->G*->~g,~g , when computing this cross section i get the message "negative points 100%" and get a negative cross section. This same problem also arises when trying to calculate the cross section for any process that goes like G,G->G*->x,X in both the MSSM and SM models.

Is this perhaps a problem of applying the correct cuts???

Question information

Language:
English Edit question
Status:
Solved
For:
CalcHEP Edit question
Assignee:
No assignee Edit question
Solved by:
Alexander Pukhov
Solved:
Last query:
Last reply:
Revision history for this message
Best Alexander Pukhov (pukhov) said : 		#1

CalcHEP  uses Feynman gauge where gluon has   6 polarization states

1)  two physical transverse polarization states.

2)  temporal and longitudinal ones  with negative and positive norm
respectively

3) two Faddeev-Popov ghosts, one with positive norm  and second with
negative.

In case of gauge invariant interaction  sum of contributions  of all
unphysical states is zero. Selecting  special set of  diagram you break
gauge invariance and get negative result.   One has to keep all diagrams
or a gauge invariant subset.

Best

    Alexander Pukhov

On 03/24/2018 07:52 AM, Angus Browne wrote:
> New question #667172 on CalcHEP:
> https://answers.launchpad.net/calchep/+question/667172
>
> Hi
> I have had problems with the process G,G->~g,~g in the MSSM model specifically the diagram for G,G->G*->~g,~g , when computing this cross section i get the message "negative points 100%" and get a negative cross section. This same problem also arises when trying to calculate the cross section for any process that goes like G,G->G*->x,X in both the MSSM and SM models.
>
> Is this perhaps a problem of applying the correct cuts???
>

Revision history for this message
Angus Browne (angusccb) said : 		#2

Thanks Alexander Pukhov, that solved my question.