GG->4leptons process in CalcHep

Asked by Andre Nepomuceno on 2021-02-22

 Dear colleagues
 I'd like to ask for your help to understand the following:
 I calculated the xsec for two processes in CalchHep 3.8.7, using the model SM(+hgg+h4G): pp -> ZZ and pp->e,E,ne,Ne, with 14 TeV CME and NNPDF pdf. For each subprocess, I compared xsec(qq -> ZZ)*Br(Z->ll)*Br(Z->nn) with qq -> e,E,ne,Ne. For quarks in the initial state, the results are compatible as expected. However, for gluon-gluon in the initial state, the xsec for the process gg->e,E,ne,Ne is 20 times greater than xsec(gg->ZZ)**Br(Z->ll)*Br(Z->nn)!! Could someone explain to me whats going on ?
  Many thanks

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Andre Nepomuceno (asevedo) said : #1

I forgot to mention that the W and photon are excluded from the calculations. Only qq (gg) -> ZZ and qq (gg) -> e,E,ne,Ne via ZZ, are considered.

Andre Nepomuceno (asevedo) said : #2

I verified that, if I apply a cut on M(l,l) and M(n,n) > 60 GeV, the two process agree. It seems there is a divergence in gg->4l process for M(l,l) < 40 GeV, which is not observed in qq->4l. Also, the problem does note appear if no PDF is chosen (PDF OFF), so it must be related to gluon PDF.

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