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

* 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.

* 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 to help to process the decay in case you have many LHE files.

Useful links

* MG5aMCnlo Launchpad

* MG4 Launchpad

-- AndreSznajder - 2015-03-30

Topic attachments
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Unknown file formatf decay.f r1 manage 51.6 K 2018-01-22 - 01:11 AndreSznajder decay.f
Unknown file formatf decay_couplings.f r1 manage 21.3 K 2015-03-30 - 17:22 AndreSznajder decay_couplings.f
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Topic revision: r5 - 2018-01-22 - AndreSznajder
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