CalcHEP

Negative cross-section for gg->g->e8E8 (Color octet lepton)

Asked by Umit Kaya

I have a lagrangian L=(gs/2Lambda)*G_munu[e8sigma_munu(eta_L*e_L+eta_R*e_R)]+h.c. (http://arxiv.org/pdf/1211.6394v2.pdf)

In this type of lagrangian I have 5 feynman diagrams for pp->e8E8 pair production. You can see those from above link. In Fig1., I am getting negative cross-section for first diagram . But last four diagrams give positive. (I am using 3.6.23 version)

What's wrong?

Thanks...

Umit

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Alexander Pukhov (pukhov) said : 		#1

One diagram can give negative contribution because Calchep sums over physical and unphysical incoming gluon states.
Unphysical states are longitudial and temporal gluon components and Faddeev-Popov ghost/anti_ghost. Two of unphysical states have negative norm and can lead to negative cross section. For gauge invariant Lagrangian sum of unphysical states has to be zero. But you can see it only after summation of all diagrams. So [(a)+ (c)+(d)]^2 has to be positive, but [(a)]^2 can be negative.
    Best
   Alexander Pukhov

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Alexander Belyaev (alexander.belyaev) said : 		#2

Dear Umit, did you solve the problem and checked gauge-invariance of your model?

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Umit Kaya (umitkaya0) said : 		#3

It did not solve the problem. Quadratic G_\mu *G_\nu term gets 2 more vertices (e8 E G G and E8 e G G). We do not want to add these vertices. Because these gives one more gluon for our final states.

Best Regards...

Umit KAYA

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