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

Question information

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CalcHEP Edit question
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Solved by:
Andre Nepomuceno
Solved:
2021-03-03
Last query:
2021-03-03
Last reply:
2021-03-03
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.

Dear Andre,
sorry for slow reply,
could you quote your number (with or without cut)
so I will reproduce it number first.
I would like to do exact same thing as you did.
With PDF.
I assu me that you are running gui, not batch, right
Thanks
Alexander

P.S. Please let me know where did you take the information about "expected" results and what they are?

I guess I know thre reason fo your puzzle:
when you enter

G,G->e,E,l,L

thre are two diagrams: with G,G->Z,Z and G,G->W+,W-
so you need to remove the one with W+,W-
using "Exclude diagram" option when you enter the process

This should resolve your puzzle. You have been calculating
two processes G,G->Z,Z and G,G->W+,W- together instead of one

Best
Alexander

Andre Nepomuceno (asevedo) said : #6

  Hi Alexander
  Thanks for your reply. I am running GUI, with NNPDF23_lo, 14 TeV CME, and Qren = M12. For kinematics, I use in =12 and out1 = 34 and out2 = 56 (for the 4 leptons final state processes). I also included regularization "34" and "56" on MZ, wZ. The W and photon contributions are exclude from the calculation. Below I show the numbers, for each sub-processes, comparing qq(gg) ->e,E,ne,Ne xsec with xsec(qq -> ZZ)*Br(Z->ll)*Br(Z->nn). The first numbers have just soft cuts, and the second table has the stronger M(l,l) > 60 GeV. Without this cut, I see a weird bump in M(l,l) distribution from G,G->4l between 20 e 40 GeV.

1. Comparison with soft cuts [abs(eta) < 100, E(l) > 5, M(l,l) > 5 ]

   xsec(qq -> ZZ)*Br(Z->ll)*Br(Z->nn) pp->4l

  uubar

Andre Nepomuceno (asevedo) said : #7

   Sorry, I sent the message before finished it. Here are the numbers. The first column is xsec(qq -> ZZ)*2*Br(Z->ll)*Br(Z->nn), and the second is pp-> 4l

  1. Comparison with soft cuts [abs(eta) < 100, E(l) > 5, M(l,l) > 5 ] [fb]

    uubar 9,51 9,72
    ddbar 13,36 13,62
    ssbar 2,34 2,38
    gg 0,35 3,60

2. Comparison with stronger cut [abs(eta) < 100, E(l) > 5, M(l,l) > 60 GeV ] [fb]

    uubar ,51 9,47
    ddbar 13,36 13,20
    ssbar 2,34 2,34
    gg 0,35 0,35

  Thanks
  Andre

Andre Nepomuceno (asevedo) said : #8

         Sorry, I used comma instead of point. Here are the numbers again:

     uubar 9.51 / 9.72
    ddbar 13.36 / 13.62
    ssbar 2.34 / 2.38
    gg 0.35 / 3.60

2. Comparison with stronger cut [abs(eta) < 100, E(l) > 5, M(l,l) > 60 GeV ] [fb]

    uubar 9.51 / 9.47
    ddbar 13.36 / 13.20
    ssbar 2.34 / 2.34
    gg 0.35 / 0.35

    Cheers

Did yu remove W-boson (see my next message)?
Then you do not need any cuts to agree between yours and known numbers.
Please check.
regards,
Alexander

On 03/03/2021 18:45, Andre Nepomuceno wrote:
> Question #695702 on CalcHEP changed:
> https://answers.launchpad.net/calchep/+question/695702
>
> Andre Nepomuceno posted a new comment:
>
>
> Sorry, I used comma instead of point. Here are the numbers again:
>
> uubar 9.51 / 9.72
> ddbar 13.36 / 13.62
> ssbar 2.34 / 2.38
> gg 0.35 / 3.60
>
> 2. Comparison with stronger cut [abs(eta) < 100, E(l) > 5, M(l,l) > 60
> GeV ] [fb]
>
> uubar 9.51 / 9.47
> ddbar 13.36 / 13.20
> ssbar 2.34 / 2.34
> gg 0.35 / 0.35
>
> Cheers
>

--
______________________________________________________________________
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Andre Nepomuceno (asevedo) said : #10

  Yes, the W and photon contributions are excluded from the calculations.
  Cheers
  Andre

OK,
thanks
In general there is a difference between narrow width approximation and final widht approximation since Breight wigner allows to shift the shat for the process.
In addition
gg->llnn oes through the Higs -- it is very narrow and forces one z to be on-shell another off-mass shell , which also make Br approach not quite (within few percent) valid
Regards,
Alexander

Andre Nepomuceno (asevedo) said : #12

     Ok, thanks