Big difference in Inclusive-EWK-QCD and Interference only

Asked by Jing Chen on 2017-08-09

Dear experts,
I generated p p > l+ l- vl vl~ j j Inclusive, EWK, QCD and Interference only processes. Here's my settings:
import model sm-no_b_mass
define p = g u c d s b u~ c~ d~ s~ b~
define j = g u c d s b u~ c~ d~ s~ b~
define l+ = e+ mu+
define l- = e- mu-
define vl = vt
define vl~ = vt~
generate p p > l+ l- vl vl~ j j QED=6 QCD=0
...
For different process, I used the following orders:
EWK: QED=6 QCD=0
QCD: QED=4 QCD=2
Inclusive: QCD=2
Interference: QCD^2==2

In EWK process I set drjj 0, mmjj 0. In QCD, inclusive and interference processes, I set drjj 0.2, mmjj 50. Each sample I generated 100k. After parton shower, I also did some pre-selection(mjj>50,drjj>0.2, 76<mll<106...) to keep the same phase space in these samples, then I compared interference and Inclusive-EWK-QCD. The interference is 7.10e-06 +- 3.02e-08. Inclusive-EWK-QCD is 2.33e-05 +- 1.63e-06.

Do you have any idea about this?
Thanks in advance,
Jing

Question information

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Last query:
2017-08-09
Last reply:
2017-08-09

Hi,

This difference is related to the scale choice. Since you did not mention it, I guess that you use the default scale choice of MG.
However that choice is process specific and therefore you do not have the same scale choice for your four generation.
Making that the difference is kind of meaningless and even more meaningless to compare with the interference term.
(At least it is meaningless to compare such number without quoting the scale/PDF uncertainties)

Cheers,

Olivier

> On 9 Aug 2017, at 11:46, Jing Chen <email address hidden> wrote:
>
> New question #655501 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/655501
>
> Dear experts,
> I generated p p > l+ l- vl vl~ j j Inclusive, EWK, QCD and Interference only processes. Here's my settings:
> import model sm-no_b_mass
> define p = g u c d s b u~ c~ d~ s~ b~
> define j = g u c d s b u~ c~ d~ s~ b~
> define l+ = e+ mu+
> define l- = e- mu-
> define vl = vt
> define vl~ = vt~
> generate p p > l+ l- vl vl~ j j QED=6 QCD=0
> ...
> For different process, I used the following orders:
> EWK: QED=6 QCD=0
> QCD: QED=4 QCD=2
> Inclusive: QCD=2
> Interference: QCD^2==2
>
> In EWK process I set drjj 0, mmjj 0. In QCD, inclusive and interference processes, I set drjj 0.2, mmjj 50. Each sample I generated 100k. After parton shower, I also did some pre-selection(mjj>50,drjj>0.2, 76<mll<106...) to keep the same phase space in these samples, then I compared interference and Inclusive-EWK-QCD. The interference is 7.10e-06 +- 3.02e-08. Inclusive-EWK-QCD is 2.33e-05 +- 1.63e-06.
>
> Do you have any idea about this?
> Thanks in advance,
> Jing
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Jing Chen (jingchen) said : #2

Dear experts,
I also use u d > l+ l- u d to check this, I only change the "pp" and "jj" to "ud" and set drjj 0.2, mjj 50 in each sample. I compared the parton level results. Interference is 5.67e-05 +- 4.31e-08. Inclusive-EWK-QCD is 6.29e-05 +- 4.90e-07. The difference seems smaller than the previous pp>llvvjj process.

I also try to study the interference cased by WW process, so I generate p p > e+ e- ve ve~ j j. The QCD, Inclusive, interference process were generated successfully. But when I generate EWK process I got a strange results in gridpack:
  === Results Summary for run: run_01 tag: tag_1 ===

     Cross-section : 34.3 +- 8.54e+04 pb
     Nb of events : 0

Best,
Jing

Jing Chen (jingchen) said : #3

Hi Olivier,
I think you are right, I used the default scale choice of MG. How could I keep the scale same? Is the any parameters I should set in the run card?
Best,
Jing

You have those parameters:

#*********************************************************************
# Renormalization and factorization scales *
#*********************************************************************
  False = fixed_ren_scale ! if .true. use fixed ren scale
  False = fixed_fac_scale ! if .true. use fixed fac scale
  91.188 = scale ! fixed ren scale
  91.188 = dsqrt_q2fact1 ! fixed fact scale for pdf1
  91.188 = dsqrt_q2fact2 ! fixed fact scale for pdf2
  -1 = dynamical_scale_choice ! Choose one of the preselected dynamical choices
  1.0 = scalefact ! scale factor for event-by-event scales

You can either decide to use a fix_scale (should be quite clear)
Or use one of the other dynamical scale: See here for more details:
https://answers.launchpad.net/mg5amcnlo/+faq/2014

Cheers,

Olivier

> On 9 Aug 2017, at 12:10, Jing Chen <email address hidden> wrote:
>
> Question #655501 on MadGraph5_aMC@NLO changed:
> https://answers.launchpad.net/mg5amcnlo/+question/655501
>
> Jing Chen gave more information on the question:
> Hi Olivier,
> I think you are right, I used the default scale choice of MG. How could I keep the scale same? Is the any parameters I should set in the run card?
> Best,
> Jing
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

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