Problem with outputting to pythia8

Asked by Philippe Giguere

Hi,

I've been trying to use MadGraph5 (v 1.4.8.4) with Pythia8 (8.1) to do some LHC SUSY searches. I seem to run into problems when I try to output to Pythia8:

mg5>output pythia8
Command "output pythia8 " interrupted in sub-command:
"output pythia8" with error:
InvalidCmd : Pythia 8 cannot handle incoming flavors set([(-6, 6)])

Here the processes that were generated are q qbar > go go ; with q q being a multiparticle label for quarks (qbar for antiquarks), and go - gluinos. Any help here would be greatly appreciated.

On another note, I'm new to the physics of parton events, showers, and so on, and don't quite understand the difference between how MadEvent and Pythia handle these. If anyone could suggest a good reference(other than the Pythia8 manual), that would be great. Essentially I'm trying to understand the difference between generating events using madgraph's pythia8 output directly, and what happens if I run it through MadEvent and submit a LHE file to Pythia instead.

Thanks!

Phil

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MadGraph5_aMC@NLO Edit question
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Solved by:
Johan Alwall
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Best Johan Alwall (johan-alwall) said :
#1

Hello Phil,

It looks like you have included the top quark in your q and qbar definitions. This is not a parton, so can not be handled as initial-state particles by Pythia.

All the best,
Johan

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Philippe Giguere (philippe-giguere2) said :
#2

Thanks Johan Alwall, that solved my question.

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

Looks like Johan forget this part to your question:

> Essentially I'm trying to understand the difference between generating events using madgraph's pythia8 output directly, and what happens if I run it through MadEvent and submit a LHE file to Pythia instead.

Let's take a short example:
p p > e+ ve mu- vm~

Pythia will generate
 p p > w+ w-
and then will decay each w independently. You then loose the full spin-correlation between the electron and the muon.
MadGraph uses the full square matrix element and therefore takes care of those effects (as well as full width effect).

So using Madgraph and then pythia allows you to made less approximation and therefore to gain in sensitivity.

The difference between the two code are of course much bigger than that but this is the main point that you need to know to my point of view.

Cheers,

Olivier