Forced semi-leptonic decay of b-hadrons

Asked by Amit Chakraborty

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:onIfAny=11 13 15 -11 -13 -15
521:onIfAny=11 13 15 -11 -13 -15
531:onIfAny=11 13 15 -11 -13 -15
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

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Olivier Mattelaer (olivier-mattelaer) said :

Do I understand that if you run outside of MG5 framework, you do not have that issue?



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Amit Chakraborty (amitchakraborty) said :

Hi Olivier,

Thanks fr the quick reply.

Yes, if I take the LHE file and run a stand-alone pythia (using v8.2.43), it is working fine. The b-hadrons are decaying leptonically always.

Only when running PY8 interfaced with MG5 (v 2.6.6 or v2.7.2), I am getting the problem.

I guess, it must be something to do with the Interface part.
Let me know your comments & suggestions on this.

Also, let me know if you need any further information.
Thank you.

Best regards,

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