RECIPE FOR GENERATING W/Z POLARIZED SAMPLES WITH MADGRAPH5
Madgraph outputs events in the LHE format which already contains the particle polarization information. So , if you generate a process like pp > qqWW , where the W is the final state particle,
the polarization information is already there. When the W/Z are not the final state and you use the full matrix element calculation or Madspin to decay the W/Z bosons it removes the polarization information of these intermediate bosons from the LHE . In order to keep the W/Z polarization information in the LHE you need to decay them with an older decay package called DECAY, which is not part of the Madgraph5 release anymore. Bellow you will find a recipe on how to generate your polarized sample in two steps, using an old DECAY package.
Polarized vector boson sample generation
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Event generation:
First you need to specify your process in
proc_card.dat up to the generation of the W/Z boson , without specifying their decay. After running Madevent you will get a LHE file containing the W/Z
as final states and its polarization information.
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W/Z decay:
After the W/Z generation step you need to download the DECAY package from an older
MadGraph5 (v1.5.14) at the following link:
DECAY
You also need to replace the original decay_couplings.f and decay.f files by the ones attached here:
decay_couplings.f
decay.f
Then you must compile the DECAY code and use the executable to perfom the W/Z decay. You can find here a script
decay_lhe.py to help to process the decay in case you have many LHE files.
Useful links
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MG5aMCnlo Launchpad
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MG4 Launchpad
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AndreSznajder - 2015-03-30