pythia8 card does not keep ordering for some lines in the pythia8_card

Bug #1897700 reported by Olivier Mattelaer
20
This bug affects 2 people
Affects Status Importance Assigned to Milestone
MadGraph5_aMC@NLO
Won't Fix
Undecided
Unassigned

Bug Description

Dear MG5 Team members,

I'm facing a problem with the MG5-PY8 framework while generating heavy flavor pair production process, especially p p > b b~. I want to study di-leptonic events from this process only and want to switch ON the leptonic modes of the b-hadrons to generate events efficiently. To do so, I write the following lines at the end of the PY8 card in MG5 Cards directory,

!================================
511:onMode = off
511:onIfAny = 11 13 15 -11 -13 -15

521:onMode = off
521:onIfAny = 11 13 15 -11 -13 -15

531:onMode = off
531:onIfAny = 11 13 15 -11 -13 -15

541:onMode = off
541:onIfAny = 11 13 15 -11 -13 -15
!===================================

Now, after PY8 run is over, the log and cmnd files print the following

!==============================
541:onMode=off
541:onIfAny=11 13 15 -11 -13 -15
521:onIfAny=11 13 15 -11 -13 -15
521:onMode=off
531:onIfAny=11 13 15 -11 -13 -15
531:onMode=off
511:onMode=off
511:onIfAny=11 13 15 -11 -13 -15
!=================================
i.e, the order of decay statements is changed while the order is very important for such forced decay. This happens almost for any given list of b-hadrons; some are correctly printed while the order is changed for others. Of course, it changes the expected final state lepton multiplicity significantly. For example, the decay of particles with ID 521 and 531 are OFF completely (I have checked the Pythia log file too, it is indeed a stable particle) in stead of leptonic decays.

It would be nice if you kindly provide some help to solve this.

Thank you in advance.

Best regards,
Amit Chakraborty

Revision history for this message
Olivier Mattelaer (olivier-mattelaer) wrote :

Hi,

I believe that if you run with python3, you will not face that issue since the dictionary are by default ordered. Since python2.7 is out of support (by python developer), it does not make sense to develop a specific code fixing for python2.7 while this is automatically handle correctly in any future version of python.

By the way, we plan to remove our support for python2.7 in late 2021.

Cheers,

Olivier

PS: For python3 support within MG5aMC, you need to use version 2.8.0 at minimum (or some beta version of the 2.7.x branch)

Changed in mg5amcnlo:
status: New → Won't Fix
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