MadGraph5_aMC@NLO

Width calculation in Feynman gauge

Asked by Kim Berghaus

I wrote a model in Feynrules introducing two neutral scalars, p1 and p2 that couple to g0 in Feynman gauge. I am generating the process: p p > p1 p1~, p1 > g0 p2. I set the width in the param_card to Auto, but upon launching, the calculation takes forever and I get cross-sections that are order of magnitude higher than for p p > p1 p1~. I would expect the cross sections to be the same since there is only one possible decay vertex.

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Kim Berghaus
Solved:
Last query:
Last reply:
Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#1

hi,

What is the mass of p1,p2, g0
and the width computed via Auto?

Cheers,

Olivier

Revision history for this message
Kim Berghaus (kimbe1) said : 		#2

Hello,
my masses are p1 = 300, p2 = 20, and g0 = 91 (g0 is the goldstone boson associated with the Z boson mass). The width computed via Auto is 0. When computing the width with Feynrules it is also 0. I am trying to couple the two scalars p1 and p2 to only the longitudinal mode of the Z boson. Thank you for your assistance.

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) said : 		#3

Hi,

The problem is the width, since a width to zero makes the cross-section to be infinite.
Now,
1) in principle you should never have goldstone particles in the final states.
So your process definition is weird.
2) why do you not decay p1 > p2 z ? (and/or why this decay is not possible?)

Cheers,

Olivier

Revision history for this message
Kim Berghaus (kimbe1) said : 		#4

Thank you. I realized that my process was ill defined. Decaying directly to the z and modifying allowed helicity states works for me now.

Cheers,
Kim