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.

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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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Alexander Belyaev (alexander.belyaev) said :
#3

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

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Alexander Belyaev (alexander.belyaev) said :
#4

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

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Alexander Belyaev (alexander.belyaev) said :
#5

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

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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

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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

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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

Revision history for this message
Alexander Belyaev (alexander.belyaev) said :
#9

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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Revision history for this message
Andre Nepomuceno (asevedo) said :
#10

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

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Alexander Belyaev (alexander.belyaev) said :
#11

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

Revision history for this message
Andre Nepomuceno (asevedo) said :
#12

     Ok, thanks