WHIZARD

Understanding the cross-section of Drell-Yan of whizard vs MG

Asked by Shafakat Arifeen

Hello!

I am using a mili-charged dark pion model (UFO) in whizard. The model is used in conjunction to SM, so the Drell-Yan process is:

proton ,proton => dpip, dpim

Where dpip and dpim are milicharged dark pions. This process usually has the virtual photon and Z as propagators, but in my model, we also have a dark Z included. I compared the result with another event generator (Madgraph).

Here is the SINARDIN script:
#############################
model = pidm_UFO_v1 (ufo)
alias proton = u:U:d:D:s:S
process testdy = proton ,proton => dpip, dpim
beams = p,p => lhapdf, lhapdf
$lhapdf_file = "NNPDF23_nlo_as_0119"
?rebuild_library = true
sqrts = 14 TeV
n_events = 1000
$phs_method = "wood"
?write_raw = false
?rebuild_events = true
scan Mdpim=(1 GeV => 100 GeV /+ 1 GeV){
    MD=MU
    MS=MU
    kappadark = 0.01 GeV
    fdecay = 100 GeV
    compile
    $run_id = sprintf "testdy100_%1.3f" (Mdpim)
    $sample = sprintf "testdy100_%1.3f" (Mdpim)
    sample_format = lhef
    simulate(testdy)
    show(results)
}
#############################

fdecay is the dark pion decay constant. kappadark is the mixing between SM and DS. I set the width of dark Z to be 9.914352e-06 GeV to ensure both event generators have the same width. The PDF for the process is the standard nnlo23 PDF (pdlabel = 230000). The mass of dark Z is set to be half the mass of Z (MZ' = 45 GeV). I had to set the masses of the quarks to be equal because otherwise alias fails.

I'm not sure if it is possible to attach an image so I uploaded it to imgur: https://i.imgur.com/ACsd7L0.png

As you can see, MG shows a steep drop in cross-section once it crosses half of MZ' (~22 GeV), we do not see that in WHIZARD. This can be explained by the on-shell contribution by the dark Z before resonance. Why do we not see that in WHIZARD?

Am I missing something or not including something in the SINDARIN script?

Thank you.

Question information

Language:
English Edit question
Status:
Solved
For:
WHIZARD Edit question
Assignee:
Juergen Reuter Edit question
Solved by:
Juergen Reuter
Solved:
Last query:
Last reply:
Revision history for this message
Juergen Reuter (j.r.reuter) said : 		#1

Dear Shafakat,
without the UFO file it is almost impossible to reproduce what is going on. From the curves you showed it seems that the dark Z resonance has not been found. Again, without the UFO file I cannot check the input parameters. Could you send us the UFO file via email to <whizard (at) desy.de >, please ? I don't know how the dark photon couples to quarks but if this goes via the quark masses, the setting of the quark masses might be a problem.
Cheers,
    JRR (Juergen)

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Shafakat Arifeen (arifeen) said : 		#2

Dear Juergen,

I have sent the email. Thank you for responding.

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Juergen Reuter (j.r.reuter) said : 		#3

Dear Shafakat.
you mentioned the width of the dark Z but it doesn't seem to be set in your input file above, I would expect something like
WZ1 = 9.9 keV
or so, and if I set this then for a a mass of 10 GeV of the phip I get a cross section of 3.4 fb. This is roughly the ballpark number from your plot (~ 0.003 pb). The integration and the setting of the parameters looks ok to me.
My suspicion is that there is some parameter of the model that is set differently between the two MCs.
A
show (model)
in the Whizard input file might help you getting a clou.
Cheers,
    JRR

Revision history for this message
Juergen Reuter (j.r.reuter) said : 		#4

We quickly checked the cross section for the corresponding process within MG5 and it seems there also a cross section of 3.7-3.8 fb appears for a mass of the pip/pim of 10 GeV, pretty consistent with Whizard. It really seams something went wrong in your MG5 run.
Could you take a look, please, and then close the issue as 'solved'. Thanks a lot in advance.

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Shafakat Arifeen (arifeen) said : 		#5

Hi Juergen,

Thank you very much for your verification. I do not think there are parameters set differently in MG5, since I'm loading the same model file. As a test, I tried doing e+ e- => dpip dpim (ee EE => dpip dpim in the model). Here's the plot I get: https://i.imgur.com/zExGD6f.png. Note that it's only 1000 events so there are fluctuations but essentially the process has the same propagators as the DY process, but the cross-sections do agree.

This leads me to believe the issue could be how MG5 and whizard reads the PDF, since there is no PDF in the e+e- process. Is there a paper or a link for me to go through how whizard reads the PDF?

Revision history for this message
Shafakat Arifeen (arifeen) said : 		#6

Hi Juergen,

Can you please tell me what MG5 version did you use? And what was the script for the process? This may be a version issue in that case.

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Shafakat Arifeen (arifeen) said : 		#7

Hi Juergen,

On further evaluation, I noticed that the WHIZARD cross-section on my end is completely off. With the parameters that you used, I get a cross-section of 3.6323595065e-08 pb. The MG5 cross-section is correct and agrees with your result. I am scaling the cross-section by dividing with the effective charge. Could you please send the SINDARIN script for that?

Thank you very much, checking MG5 and WHIZARD really helped me figure out what the problem was (messing up WHIZARD).

Revision history for this message
Best Juergen Reuter (j.r.reuter) said : 		#8

Hi Shafakat,
sorry, but this is not correct what you are writing, The script below gives a cross section of 3,x fb for the DY process for dark pions.

model = pidm_UFO_v1 (ufo)

show (model)

alias proton = u:U:d:D:s:S

process testdy = proton ,proton => dpip, dpim

beams = p,p => lhapdf, lhapdf

$lhapdf_file = "NNPDF23_nlo_as_0119"

sqrts = 14 TeV

scale = massZ1

    MD = 0
    MU = 0
    MS= 0
    MC = 0
    MB = 0
    kappadark = 0.01 GeV
    fdecay = 100 GeV
    Mdpim = 10 GeV
    WZ1 = 9.9 keV

integrate (testdy)

So I consider this issue to be solved.
Thanks,
   Juergen

Revision history for this message
Shafakat Arifeen (arifeen) said : 		#9

Dear Juergen,

Thank you. This helped me solve my problem. Just one question: what does "scale = massZ1" do in this case? That's what I was missing in my script.

Sincerely
Shafakat

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Shafakat Arifeen (arifeen) said : 		#10

Thanks Juergen Reuter, that solved my question.

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Juergen Reuter (j.r.reuter) said : 		#11

Dear Shafakat,
the setting of
scale = <something>
sets the factorization and renormalization scale for the PDFs and the running alpha_s. The second is irrelevant here as there is no power of alpha_s in the process.
As a general remark, if you rerun a process with a change please make sure to do --rebuild-grids as an option to the Whizard command as sometimes Whizard doesn't recognize that some setting has changed and is just reusing a previous result. This could sometimes lead to confusions.
Cheers,
    JRR