EW V + jets NLO

Asked by negin on 2018-06-26

Dear authors,

Is amC@NLO well suited for calculating EW V + jj with NLO corrections ?

with aMC v254 I ran
generate p p > z j j QCD=0 [QCD]

using
  1.0 = jetalgo ! FastJet jet algorithm (1=kT, 0=C/A, -1=anti-kT)
  0.4 = jetradius ! The radius parameter for the jet algorithm
20.0 = ptj ! Min jet transverse momentum
  5.0 = etaj ! Max jet abs(pseudo-rap) (a value .lt.0 means no cut)

and got
aMCatNLOError: Poles do not cancel, run cannot continue

Could you kindly let me know how I should proceed, if possible ?

Best Regards

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
marco zaro Edit question
Last query:
2018-10-16
Last reply:
2018-10-16

This question was reopened

marco zaro (marco-zaro) said : #1

Hi,
you can generate EW Z-production plus two jets at NLO QCD in two ways:
- with the standard (2.6.x) branch, you will not generate loop diagrams featuring EW bosons, and you have to turn off the check poles.
You can find the details (and how to turn them off) e.g. in this paper, where VBS has been studied
https://arxiv.org/pdf/1803.07943.pdf
- with the new, EW-capable branch (3.0.x), you can compute the full thing but only at fixed order (no event genration, no shower), because some corrections stem from EW corrections on top of the alpha^2 alpha_s interference. I fear you need to decay the Z in that case, I would advise you to start with the first option, which should normally be good enough.

Let us know
Cheers,

Marco

negin (neginsh) said : #2

Thanks marco zaro, that solved my question.

negin (neginsh) said : #3

Hi ,
thanks for your respond , I have tried that way and it worked but still one problem, I have large differences in cross-section of NLO and LO. I think 51.80 pb is large for NLO corrections as for LO is 37.42 pb! what do you think?

Cheers
Negin

marco zaro (marco-zaro) said : #4

Hi,
is 51.80 the NLO cross section or just the NLO corrections?
Here
https://arxiv.org/pdf/hep-ph/0310156.pdf
with the renormalization and factorization scales set to mz (and VBF-type cuts), one gets 10% corrections...
can you try to set the scale fixed to mz (you can do this in the run_card)?
Can you please also attach the process definition you used, just to cross-check?
thanks!

cheers,

Marco

negin (neginsh) said : #5

Hi,
 is 51.80 the NLO cross section or just the NLO corrections?
yes, it is NLO cross section( included NLO corrections)

I have attached The cards in following address for cross-check.
"https://github.com/nshafiei/VBF/"
Best regards
Negin

negin (neginsh) said : #6

Hi,

Sorry for sending you again the question because i have not received any respond. would you please check the cards and give me suggestion for this problem?!

Best
Negin

negin (neginsh) said : #7

Hi,

Sorry for sending you again the question because i have not received any respond. would you please check the cards and give me suggestion for this problem?!

Best
Negin

marco zaro (marco-zaro) said : #8

Hi Negin,
I was on hoidays in the past days.
Can you try setting the scale fixed to mz (you can do this directly from the run_card) and see how large the K-factor is?
In any case,the numbers you quoted does not seem pathological...
Let me know
cheers

Marco

negin (neginsh) said : #9

Hi Marco,

Thanks for your kind support.
I have run as you said with fixed scales and i have seen small differences between Fixed one and dynamical one

NLO/LO Fixed is 5.362e01/37.45
NLO/LO Dynamical is 5.184e01/37.42

So the NLO/LO difference is still a bit large.

I also have tried to change the drjj cut running LO (with fixed scales),,
but strangely I see no effect on the total cross-section, for example
LO Fixed is 37.4 with drjj=0.4 and 37.76 with drjj=0.0.

Do you know why the drjj cut does not change the results ?

Cheers
Negin

marco zaro (marco-zaro) said : #10

Hello Negin,
the reason of the large (40%) K factor is that, unless you require large jet-pair invariant mass, then you are dominated by
p p > z z with a Z decaying to jj (also contributions with a virtual photon are there)
Both zz and az production have rather large k-factor, see e.g. table 2 in https://arxiv.org/pdf/1405.0301.pdf
For the drjj, one should have some divergence if drjj->0, because of the contribution due to the virtual photon.
Best,
Marco

negin (neginsh) said : #11

Hello Marco,
Thanks so much , yes the problem was the k factor .

But for the drjj, I don't have have any divergence if drjj->0, i have number 37.76pb. Maybe as result of the cut that i have put for ptj=20?

Best,
Negin

marco zaro (marco-zaro) said : #12

Hi Negin,
I do not know, it can also be that the divergence is not seen by the
integrator or that the divergent contribution is rather small...
Just to understand, when you say LO you generate with or without [QCD]?
best,

Marco

On Wed, Sep 5, 2018 at 2:53 PM negin <email address hidden>
wrote:

> Question #670477 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670477
>
> negin gave more information on the question:
> Hello Marco,
> Thanks so much , yes the problem was the k factor .
>
> But for the drjj, I don't have have any divergence if drjj->0, i have
> number 37.76pb. Maybe as result of the cut that i have put for ptj=20?
>
> Best,
> Negin
>
> --
> You received this question notification because you are subscribed to
> the question.
>

negin (neginsh) said : #13

Hello Marco,
when i say LO my mean is without [QCD].
 generate p p > z jj QCD=0

Best
Negin

negin (neginsh) said : #14

Dear Marco,

As you said me couple of months ago , for adding a Mjj for NLO in cut.f it seems doesn't work. what do you think?

double precision sumdot

  ! MJJ > 200 GeV
      if (dsqrt(sumdot(pjet(0,1),pjet(0,2),1d0)).lt.200d0) then
        passcuts_user=.false.
        return
      endif

Mjj=10 , 51.84pb AND Mjj= 200 , 52.58pb.

Cheers
Negin

marco zaro (marco-zaro) said : #15

Dear Negin, at which point of cuts.f do you add this cut?
Let me know

best,

Marco

negin (neginsh) said : #16

Dear Marco,
 I have added it in" DeltaR and invariant mass cuts " .

Cheers
Negin

marco zaro (marco-zaro) said : #17

Hi Negin,
DeltaR and invariant mass are for the leptons.
You should add the cut after the fastjet clustering.
In particular right after (before 122 continue)

         call amcatnlo_fastjetppgenkt_etamax_timed(
     $ pQCD,nQCD,rfj,sycut,etaj,palg,pjet,njet,jet)
c
c******************************************************************************

c Apply the jet cuts
         if (njet .ne. nQCD .and. njet .ne. nQCD-1) then
            passcuts_user=.false.
            return
         endif
      endif

This way you will be sure to have at least two jets.
Let me know if this works.

Cheers,

Marco

> On 11 Sep 2018, at 09:42, negin <email address hidden> wrote:
>
> Question #670477 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670477
>
> Status: Needs information => Open
>
> negin gave more information on the question:
> Dear Marco,
> I have added it in" DeltaR and invariant mass cuts " .
>
> Cheers
> Negin
>
> --
> You received this question notification because you are subscribed to
> the question.

negin (neginsh) said : #18

Dear Marco,
Thanks so much, that approach of mjj cut seems to work well. Still one problem: for mjj >120 with reject the di-bosons
the NLO cross-section is 23.54pb and LO one is 14.23 . (still the k-faktor is very big) What do you think ?

Cheers
Negin

marco zaro (marco-zaro) said : #19

Hi Negin,
I guess what is happening there is that at LO you kill diboson, but at NLO you are dominated by real emission configurations where the two tagging jets are the extra parton and one of those coming from the decaying boson. Note that in this way at NLO the diboson can be on shell, while at LO it cannot…
Can you try with tighter cuts?
cheers,

Marco

> On 13 Sep 2018, at 12:27, negin <email address hidden> wrote:
>
> Question #670477 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670477
>
> Status: Answered => Open
>
> negin is still having a problem:
> Dear Marco,
> Thanks so much, that approach of mjj cut seems to work well. Still one problem: for mjj >120 with reject the di-bosons
> the NLO cross-section is 23.54pb and LO one is 14.23 . (still the k-faktor is very big) What do you think ?
>
> Cheers
> Negin
>
> --
> You received this question notification because you are subscribed to
> the question.

negin (neginsh) said : #20

Hi Marco,
Thanks so much, I have tried tighter cut as 500 and 700 mjj but for 1000 it takes long time do you have any suggestion for improving that ? The results for 500 and 700 are
MJJ>500 LO=6.68 NLO=7.45pb
MJJ>700 LO=4.77 NLO=5.05pb
so indeed the NLO/LO factor decreases as you predicted.

Stil I have a question about your suggested reason for large NLO/LO corrections with low MJJ.
Since I produce the Z jj final state, with no Z decay, is it that the Z boson is decayed anyhow with MG5_amcnlo
so that hadronic Z modes can contribute to the MJJ condition with kt jets ?
Otherwise the Z decay products would not contribute to the MJJ calculation, correct ?

Thank you for clarifying

Cheers
Negin

marco zaro (marco-zaro) said : #21

Hi Negin,

> On 9 Oct 2018, at 18:12, negin <email address hidden> wrote:
>
> Question #670477 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670477
>
> Status: Answered => Open
>
> negin is still having a problem:
> Hi Marco,
> Thanks so much, I have tried tighter cut as 500 and 700 mjj but for 1000 it takes long time do you have any suggestion for improving that ? The results for 500 and 700 are
> MJJ>500 LO=6.68 NLO=7.45pb
> MJJ>700 LO=4.77 NLO=5.05pb
> so indeed the NLO/LO factor decreases as you predicted.
the long time is due to the fact that the harder the mjj cut the more difficult is for the phase space to find the region that passes cuts.
>
> Stil I have a question about your suggested reason for large NLO/LO corrections with low MJJ.
> Since I produce the Z jj final state, with no Z decay, is it that the Z boson is decayed anyhow with MG5_amcnlo
> so that hadronic Z modes can contribute to the MJJ condition with kt jets ?
> Otherwise the Z decay products would not contribute to the MJJ calculation, correct ?
no, that z remain stable. However, the two jets that you have can come from another Z or W boson which can become resonant.

Cheers,

Marco

>
>
> Thank you for clarifying
>
> Cheers
> Negin
>
> --
> You received this question notification because you are subscribed to
> the question.

negin (neginsh) said : #22

Dear Marco,

Ok thank you. I understand with the NLO configuration more diboson events can pass because
one of the quarks from a Z or W decay can make a big mass with a ISR parton, right ?

I wanted still to ask you, when I implement the mjj cut as you suggested with

 if (dsqrt(sumdot(pjet(0,1),pjet(0,2),1d0)).lt.200d0) then
        passcuts_user=.false.

how are the pjets ordered ? Am I picking the two with the largest pt values ?
I mean are the pjet(0,1) and pjet(0,2) the two pt leading jets of the event ?

Thank you

marco zaro (marco-zaro) said : #23

Hi Negin,

> On 16 Oct 2018, at 18:07, negin <email address hidden> wrote:
>
> Question #670477 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/670477
>
> Status: Answered => Open
>
> negin is still having a problem:
> Dear Marco,
>
> Ok thank you. I understand with the NLO configuration more diboson events can pass because
> one of the quarks from a Z or W decay can make a big mass with a ISR parton, right ?

yes, this is correct
>
> I wanted still to ask you, when I implement the mjj cut as you suggested
> with
>
> if (dsqrt(sumdot(pjet(0,1),pjet(0,2),1d0)).lt.200d0) then
> passcuts_user=.false.
>
> how are the pjets ordered ? Am I picking the two with the largest pt values ?
> I mean are the pjet(0,1) and pjet(0,2) the two pt leading jets of the event ?

the jets are ordered in pt, so, those are the two with largest pt in the event, as you write.

cheers,

marco
>
> Thank you
>
> --
> You received this question notification because you are subscribed to
> the question.

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