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g4me

Geant for me

Simulation framework

I did not manage to do all the order I wanted in my Geant4 photon-conversion framework. But I managed to put the current status on GitHub and what is there is something that works.
https://github.com/preghenella/g4me

The software is called g4me. There is support to build it with the aliBuild system. For that you need to get the g4me.sh receipt from my private alidist branch
https://github.com/preghenella/alidist/tree/g4me

If you manage to build it, here is how you run it. You can start from the example configurations, these ones

g4me/share/g4macro/init.mac          [configures the detector geometry and B field]
g4me/share/g4macro/pythia8.mac       [steers the Geant4 simulation with Pythia8]
g4me/share/py8config/pythia8_hi.cfg  [configures Pythia8 for Pb-Pb collisions]

I will have to explain you better all the details. Something is detailed in the following. Pease ask if it is unclear.
Put those files in the same directory and run

$ g4me pythia8.mac

It will generated 10 Pb-Pb collisions and perform Geant4 transport of photons and unstable particles. Eventually you should have a new file in the directory with the output of the simulation.

pythia8.000.root

There are switches in pythia8.mac to control which particles to transport

/stacking/transport gamma
/stacking/transport unstable

and you can control the number of events to be generated with

/run/beamOn 10

If you want to switch to pp inelastic collisions, put this file in your working directory

share/py8config/pythia8_inel.cfg

and modify the line

/pythia8/config pythia8_hi.cfg

Notice that in the default configuration only particles with |eta| < 0.8 are injected in Geant4. You can change that behaviour from pythia8.mac with

/pythia8/cuts/eta -0.8 0.8

Notice that in the default Pythia8 configuration all particles with proper lifetime ctau < 10 mm are decayed by the event generator. Therefore they are not injected in Geant4 and are not visible in the output. This is a limitation that will have to be fixed in a future version to attempt to keep in the output file also resonances and very short-lived particles.
The behaviour of Pythia8 can be modified in the pythia8_hi.cfg configuration file via the line

ParticleDecays:tau0Max = 10

This has to be done with care, because Geant4 might not know how to deal with the decay of some particles. This is another limitation that will have to be overcome in the future with the addition of the external decayer feature.

Analysis Framework

If you did not manage to run the simulation by yourself, you can find an example output on Dropbox
https://www.dropbox.com/s/rklbxjnvao4hbu2/pythia8.000.root

The output file pythia8.000.root is a plain ROOT Tree. So you can browse it and do whatever you like with it.

The interesting branch is the Tracks branch, the elements of which contain this information

n                [number of tracks]
id               [track id]
pgd              [pdg code]
parent           [parent id, -1 if primary injected]
px, py, pz, e    [four momentum vector at creation]
vx, vy, vz, vt   [position and time at creation]
status           [auxiliary status information]

There is an utility macro io.C to deal with the IO from the tree. Take that toghether with the example analysis electron.C

g4me/share/analysis/io.C
g4me/share/analysis/electron.C

and start a ROOT session for the analysis

root [0] .L electron.C
root [1] electron("pythia8.000.root")

This will create a histogram showing the log10(E) distribution of electrons. They are separated by

  • primary electrons
  • secondary from photon conversion
  • secondary from Compton scattering
  • secondary from other (i.e. from Dalitz decay, I still have to put a decent flag for it)

The macro should be self-exaplanatory enough to understand what is goin on. It shows you how to deal with the IO using the functionality provided by the io.C macro.

Notice that the structure of the IO is one of the things that is likely will be modified in the near future.