Pythia's input parameters in mg?

Asked by Andrew McAdams

Hi!

I have tried to generate events with Bs2*, Bs1 mesons produced in pp collisions at the LHC

What I have done is

generate p p > b b~ [QCD],

and ran the simulation without any other changes.

From the run with 10^4 events, I expected to see around 10-10^2 of events containing Bs2*, however I have not seen any of those (I saw however events with Bs*, B* mesons).

From inspecting the web, I have found that the appearance of these mesons depends on values of pythia input parameters parj(16), parj(17), which by default are set to zero.

I am not an expert in pythia8, but I naively expect that these parameters may be set up in some run cards generated by mg. Could you please tell me how can I do this?

Question information

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Status:
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For:
MadGraph5_aMC@NLO Edit question
Assignee:
Paolo Torrielli Edit question
Solved by:
Paolo Torrielli
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Paolo Torrielli (paolotorriell) said :
#1

Dear Andrew,

are you using Pythia6 or Pythia8? When you write 'parj(16)' this reminds me of Pythia6,
but at the end of the message you write Pythia8.

Assuming you're using Pythia8 (Pythia6 is not maintained any longer), and you shower
your events through the internal MG5_aMC interface, i.e. with the ./bin/shower run_xx
command, you can set the B hadrons to be stable in shower_card.dat, with the option

b_stable = T

This correspond to the following commands in the Pythia8 input card:

511:maydecay = false ! stable B hadrons
521:maydecay = false ! stable B hadrons
531:maydecay = false ! stable B hadrons
541:maydecay = false ! stable B hadrons
551:maydecay = false ! stable B hadrons
553:maydecay = false ! stable B hadrons
5112:maydecay = false ! stable B hadrons
5122:maydecay = false ! stable B hadrons
5132:maydecay = false ! stable B hadrons
5222:maydecay = false ! stable B hadrons
5232:maydecay = false ! stable B hadrons
5332:maydecay = false ! stable B hadrons

I hope this helps.
Best regards.
Paolo

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Andrew McAdams (mcadams1994) said :
#2

Dear Paolo,

thanks, but my question is about how to set up the possibility of appearing of particles such as Bs2*, Bs1, which is handled (at least in pythia6, yes) by parameters parj(16), parj(17), and I wanted to set their values to appropriate ones.

Revision history for this message
Andrew McAdams (mcadams1994) said :
#3

Dear Paolo,

in pythia8, instead of PARJ we have something like StringFlav:mesonUDvector, StringFlav:mesonUDvector (this is for PARJ(11),PARJ(12)). So I need to modify these values. Could you please tell me whether it is possible via mg cards?

Revision history for this message
Andrew McAdams (mcadams1994) said :
#4

I have finally found the desired parameters: these are StringFlav:mesonBL1S1J2 and StringFlav:mesonBL1S1J1.

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Andrew McAdams (mcadams1994) said :
#5

I cannot just paste
StringFlav:mesonBL1S1J2 = 0.2986
in the shower_card.dat, this option is unknown.

Revision history for this message
Best Paolo Torrielli (paolotorriell) said :
#6

Dear Andrew,

modifying inputs in the shower_card.dat acts on the script
MCatNLO/Script/MCatNLO_MadFKS_PYTHIA8.Script, which is the
one that writes the actual input card for the shower, i.e.
MCatNLO/RUN_PYTHIA8_xx/Pythia8.cmd.

I would suggest the following: set
b_stable = T # set B hadrons stable
in shower_card.dat.

Go in MCatNLO/Script/MCatNLO_MadFKS_PYTHIA8.Script,
look for ‘B_STABLE_PY8’, and, inside the ‘if' statement
'if [ $B_STABLE_PY8 = ".TRUE." ]’, just add your
'StringFlav:mesonBL1S1J2 = 0.2986’ command.
(You may also comment out some '5xxx:maydecay = false’ commands
in that ‘if’ statement, if relevant).

Then, you can check if the MCatNLO/RUN_PYTHIA8_xx/Pythia8.cmd
file has your desired command in it.

Best.
Paolo

> On 8 Nov 2021, at 10:45, Andrew McAdams <email address hidden> wrote:
>
> Question #699389 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/699389
>
> Andrew McAdams gave more information on the question:
> I cannot just paste
> StringFlav:mesonBL1S1J2 = 0.2986
> in the shower_card.dat, this option is unknown.
>
> --
> You received this question notification because you are subscribed to
> the question.

Revision history for this message
Andrew McAdams (mcadams1994) said :
#7

Thanks! Alternatively, I may insert the strings

  pythia.readString("StringFlav:mesonBL1S1J2 = 0.2986");

in .cc files located in the folder srcPythia8.

Revision history for this message
Andrew McAdams (mcadams1994) said :
#8

Thanks Paolo Torrielli, that solved my question.