how to generate polarized top quark

Asked by Seulgi Kim on 2019-05-29

Hi,

I wanted to generate p p > t(right-handed) t~(right-handed), so I tried this.
(I referred to https://answers.launchpad.net/mg5amcnlo/+faq/2243, and I used MG5 v2.6.5)

generate p p > t t~
output lhtop

after changing SubProcesses/P*/matrix*.f files,
before:
       DATA (NHEL(I, 2),I=1,4) /-1,-1,-1,-1/
      DATA (NHEL(I, 3),I=1,4) /-1,-1, 1, 1/
      DATA (NHEL(I, 4),I=1,4) /-1,-1, 1,-1/
      DATA (NHEL(I, 5),I=1,4) /-1, 1,-1, 1/
      DATA (NHEL(I, 6),I=1,4) /-1, 1,-1,-1/
      DATA (NHEL(I, 7),I=1,4) /-1, 1, 1, 1/
      DATA (NHEL(I, 8),I=1,4) /-1, 1, 1,-1/
      DATA (NHEL(I, 9),I=1,4) / 1,-1,-1, 1/
      DATA (NHEL(I, 10),I=1,4) / 1,-1,-1,-1/
      DATA (NHEL(I, 11),I=1,4) / 1,-1, 1, 1/
      DATA (NHEL(I, 12),I=1,4) / 1,-1, 1,-1/
      DATA (NHEL(I, 13),I=1,4) / 1, 1,-1, 1/
      DATA (NHEL(I, 14),I=1,4) / 1, 1,-1,-1/
      DATA (NHEL(I, 15),I=1,4) / 1, 1, 1, 1/
      DATA (NHEL(I, 16),I=1,4) / 1, 1, 1,-1/
      DATA IDEN/256/

      INTEGER NCOMB
      PARAMETER ( NCOMB=16)

after:
      DATA (NHEL(I, 1),I=1,4) /-1,-1, 1, 1/
      DATA (NHEL(I, 2),I=1,4) /-1, 1, 1, 1/
      DATA (NHEL(I, 3),I=1,4) / 1,-1, 1, 1/
      DATA (NHEL(I, 4),I=1,4) / 1, 1, 1, 1/
      DATA IDEN/256/

      INTEGER NCOMB
      PARAMETER ( NCOMB=4)

launch
shower = Pythia8
detector = Delphes
madspin = ON
reweight = OFF
done
./lhtop/Cards/delphes_card_CMS.dat
done

I checked the result, but I think the helicity wasn't reflected. Is there something wrong in my procedure?

Thanks,
Seulgi

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
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Last query:
2019-05-29
Last reply:
2019-05-29

Hi,

If you check the lhe file, you should see if MG5aMC correctly polarised the top.
Now I do not think that pythia8 is using that information and your top decay will be unpolarised.

For the moment, the only solution that I know is to use the OLD (and not supported anymore) DECAY package that was part of Magraph_MadEvent_version4. (another solution might be BRIDGE but this does not support the UFO format and is also not supported anymore).

As you have seen in the FAQ, we are updating MG5aMC to automate what you have done but this project is also looking for a way to have polarised top propagator. While the first feature is currently working --but not fully validated--, the second is not working at all and I have no clue why.

Cheers,

Olivier

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