MadGraph5_aMC@NLO

MadOnia->Pythia8 Color Connections Problem

Asked by Reggie Bain

Hi,

I am trying to shower on ME's generated in MadOnia (or using sm_onium in MG4) using Pythia 8 by initializing pythia on a .lhe file created by Madgraph (the "onia_run_unweighted_events.lhe.gz" file for instance) but am getting the following error message:

 PYTHIA Error in ProcessLevel::checkColours: unphysical colour flow
 PYTHIA Abort from Pythia::next: processLevel failed; giving up

My understanding is that Pythia 8 was built to work with MG5 more so than MG4 but that this should still be possible. Unfortunately, I am unsure how to approach fixing this issue.

1. What is the source of the color connections issue?
2. Is this issue fixable using output from MadOnia/MG4?

Thank you.

Question information

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MadGraph5_aMC@NLO Edit question
Assignee:
Pierre Artoisenet Edit question
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Revision history for this message
Pierre Artoisenet (partois) said : 		#1

Hi,

Could you give me the process that you have requested in the run_card.dat ?

Cheers,
Pierre

Revision history for this message
Reggie Bain (rab59) said : 		#2

Absolutely! Although I selected the process in MadOnia/working_dir/Cards/proc_card.dat not in run_card.dat. In run_card.dat I've largely kept everything at default values except for a few that I set by hand such as:
ickkw = 1 and setting the seed so as to compare the output with other versions of MG.

The process in proc_card.dat was gg>g>gcc~[3S18to9900443]
QCD=3 and QED=2

Thank you!

Reggie

Revision history for this message
Reggie Bain (rab59) said : 		#3

Is this sufficient information?

What is the source of this colour issue?

Also, what was changed about these colour connections in order to make MG5 compatible with Pythia 8?

Thank you,

reggie

Revision history for this message
Pierre Artoisenet (partois) said : 		#4

From Pythia 8 manual:

"The unphysical colour-octet states follow the id scheme of 99 n_q n_s n_r n_L n_J where n_q is the quark flavour of the state andn_s is the colour-octet state type. Here 0 is 3S1, 1 is 1S0, and 2 is 3PJ. All remaining numbers follow the standard PDG numbering scheme. If a physical state is requested without a corresponding colour-octet state, a colour-octet state is automatically added to theParticleData when a colour-octet process is selected. The colour-octet state is created with a mass given by the mass of the physical state plus the singlet-octet mass splitting parameter Onia:massSplit, which is by default set at 200 MeV, and decays exclusively to a gluon and the physical state. If the user wishes to manually set the mass splitting for each colour-octet state individually then Onia:forceMassSplit can be set to off. By default the widths of the octet states are set to vanish. This is not realistic, given their presumably rather rapid decay, but a nonvanishing width is not likely to have any measurable consequences that go beyond what comes from viewing the singlet-octet mass splitting as an effective parameter."

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