MadGraph5_aMC@NLO

choice of asrwgtflavor

Asked by xxwan

Hi,

I do the matching of pp > b b~ h +0, 1jet, and take 4-flavor scheme, p = j = g u c d s u~ c~ d~ s~, xqcut =10, qcut=40

generate p p > b b~ h
add process p p > b b~ h j

1 = ickkw ! 0 no matching, 1 MLM, 2 CKKW matching
 1 = highestmult ! for ickkw=2, highest mult group
 1 = ktscheme ! for ickkw=1, 1 Durham kT, 2 Pythia pTE
 1 = alpsfact ! scale factor for QCD emission vx
 False = chcluster ! cluster only according to channel diag
 True = pdfwgt ! for ickkw=1, perform pdf reweighting
 5 = asrwgtflavor ! highest quark flavor for a_s reweight
 True = clusinfo ! include clustering tag in output
 3.0 = lhe_version ! Change the way clustering information pass to shower.

4 = maxjetflavor

I find that asrwgtflavor is taken as 4 or 5 by some people in the other matching process.
However, the results of matching of pp > b b~ h +0, 1jet have sizable difference for asrwgtflavor = 4 and 5.

For asrwgtflavor = 5
    Cross-section : 2.934 +- 0.004383 pb
     Nb of events : 50000
     Matched Cross-section : 1.447 +- 0.006905 pb
     Nb of events after Matching : 24669

For asrwgtflavor = 4
     Cross-section : 1.705 +- 0.002669 pb
     Nb of events : 50000
     Matched Cross-section : 0.8311 +- 0.004027 pb
     Nb of events after Matching : 24372

How to understand the difference? which value of asrwgtflavor is correct?

Many thanks.

Question information

Language:
English Edit question
Status:
Answered
For:
MadGraph5_aMC@NLO Edit question
Assignee:
Rikkert Frederix Edit question
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Last reply:
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Rikkert Frederix (frederix) said : 		#1

Dear xxwan,

Effectively, the difference between the two choices of asrwgtflavor is a different value for the renormalisation scale.

When running without MLM merging, the default renormalisation is calculated by clustering *all* particles (not only the QCD ones) to get a 2->2 system. The transverse energy in that system defines the central renormalisation scale.

When turning on the MLM merging, that central scale is effectively changed by the 'alpha_s reweighting': for each QCD clustering, one power of alpha_s, computed at the central scale, is divided out, and the result is multiplied by one power of alpha_s, computed at the scale of that clustering. Effectively, this results in a smaller renormalisation scale.

When setting asrwgtflavor to 4 the b quarks are not included in this clustering to determine the alpha_s reweighting. Which is formally the correct thing to do, because the b quarks/jets do not enter in the merging at all. However, the central renormalisation scale, defined by the transverse energy in the 2->2 system, is also a very poor central scale for this process, because it is way too large. By setting asrwgtflavor to 5, this poor central scale is somewhat compensated.

So, both scales are correct: at LO there is simply a very large uncertainty due to the scale choice, and this is particularly enhanced for hbb~ production. To get a better approximation of the total cross section, I would suggest to include the NLO corrections, (see also arXiv:1409.5301) which is rather straight-forward in mg5_aMC@NLO, even though it significantly increases the run time.

Best regards,
Rikkert

Revision history for this message
xxwan (xxwanphy) said : 		#2

Hi Rikkert,

Thank you for your reply. asrwgtflavor is definitely taken as 5 in 5-flavor scheme. So, according to your explanation. 5-flavor scheme seems to be more correct than 4-flavor scheme?

Best

Revision history for this message
Rikkert Frederix (frederix) said : 		#3

Dear xxwan,

It depends. If you use the default renormalisation and factorisation scales for the central process, then maybe "yes", the 5-flavor scheme is the better choice in this case. If you improve that to better reflect the hbb~ process, then the 4-flavor scheme is the better choice.

Best regards,
Rikkert

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