MadGraph5_aMC@NLO

Particle decays giving low number of events with bad widths

Asked by Russell

I'm playing with an effective model where I have a massive scalar particle that can decay to gluons with a vertex Sgg. If I tell MG to S > g g, I get the decay width that I expect. If I look at the decay S > g g g in MG through a diagram where I have two vertices, Sgg and the SM ggg vertex, the decay width from MG is significanly larger than the S>gg decay width when it should be suppressed.

When having the 3 gluon decay, I also have significanly less events generated than what I asked for, ~5 of the 10k that were supposed to be generated which is why I get the weird width, but I cannot figure out why I am getting so few events and was hoping someone could shed some light on why this is happening for me.

Thanks,
-Russell

Question information

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

Hi Russell,

When you compute a width with Madgraph all cut are removed.
So if you just ask
S > g g g to MG it will generate diagram like
S > g ( g > g g )
which are divergent at LO accuracy.

Therefore your cross-section blows up and the maximal weight for the unweighting for the event is infinite (numerically extremelly large) preventing to efficiently generate events.

To compute width, I would suggest to use MadWidth: http://arxiv.org/abs/arXiv:1402.1178
Note that in order to include the
S > g g g four point vertex in the computation, it should have it's own coupling order (as explained in the paper).

Cheers,

Olivier

Can you help with this problem?

Provide an answer of your own, or ask Russell for more information if necessary.

To post a message you must log in.