Pythia 6/8 unable to shower events produced by MG5 with eps_ijk color vertex

Asked by Chris Brust on 2012-03-27

Hi there,

   I'm attempting to generate and decay events in MG5 and pass them to Pythia for showering and hadronization. The model contains a color triplet scalar with an eps_ijk color coupling to quarks. Event generation seems to go smoothly, and the .lhe files look reasonable. Using the built-in pythia-pgs package, I obtain the following error in the pythia.log: "(PYCTTR:) colour tag tracing failed".

   Reading a similar question asked yesterday seemed to indicate that Pythia 6 can't handle this sort of vertex, and I should instead use Pythia 8. Upon using "output pythia8" in my proc_card, MG5 tells me that Pythia 8 can only handle up to 2 -> 3 processes, and I'm interested in a 2 -> 4 process.

   My next step was to write code using the Pythia 8 library to read the unweighted lhe file with 'pythia.init("myevents.lhe")' that MG produced; Pythia 8 now complains with "PYTHIA Error in ProcessLevel::checkColours: incorrect colour assignment".

   Any thoughts? Thank you in advance, Chris

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Solved by:
Johan Alwall
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Best Johan Alwall (johan-alwall) said : #1

Hello Chris,

"output pythia8" means to output *matrix elements* for use internally in Pythia 8. To shower events in Pythia 8, that you have generated in MadGraph, just output and run as usual, and feed the resulting .lhe file to Pythia 8. I understand from the last part of your question that you tried this, and that Pythia 8 complains about the color assignment. Please make sure you are using a recent version of both MadGraph 5 and Pythia 8, since this has been fixed fairly recently in both. If you still fail, please attach the gzipped .lhe file here, so we can take a look at it.

All the best,

Chris Brust (cpbrust) said : #2


   Thank you for your reply. Switching to just pythia 8.1.62 didn't help, but switching to MG5 1.4.3 as well did. Thank you! Chris

Chris Brust (cpbrust) said : #3

Thanks Johan Alwall, that solved my question.