MadGraph5_aMC@NLO

ttbar matching with jet cuts

Asked by Matthew Low

Hi,

I would like to match the process:

generate p p > t t~, t > b l+ vl, t~ > b~ l- vl~ @1
add process p p > t t~, t > b j j, t~ > b~ l- vl~ @2
add process p p > t t~, t > b l+ vl, t~ > b~ j j @3
add process p p > t t~, t > b j j, t~ > b~ j j @4

add process p p > t t~ j, t > b l+ vl, t~ > b~ l- vl~ @5
add process p p > t t~ j, t > b j j, t~ > b~ l- vl~ @6
add process p p > t t~ j, t > b l+ vl, t~ > b~ j j @7
add process p p > t t~ j, t > b j j, t~ > b~ j j @8

Here I am decaying all the tops because I would like to apply the ptj1min=80 cut (and maxjetflavor=5) because I would like either of the b jets or jets from top decay to pass the ptj1min cut. When I do this, however, the cross-section for the matched process is much lower than the unmatched +0-jet (i.e. processes 1-4 above). Even if I remove the jet cut and use maxjetflavor=4 the matched and unmatched cross-sections are very discrepant. I've noticed that setting (cut_decays=F) seems to increase the matched cross-section to a reasonable value.

Are there implicit/explicit cuts somewhere that I need to remove in order to generate this process properly? For reference I am using version 5.1.5.14.

Thanks for your help,
- Matthew

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Olivier Mattelaer
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Best Olivier Mattelaer (olivier-mattelaer) said : 		#1

Hi Matthew,

In the documentation (https://cp3.irmp.ucl.ac.be/projects/madgraph/wiki/Matching), you have the following statement:

Note: For most processes, the generation speed can be improcved by setting ptj and mjj to xqcut, which is done automatically if the flag auto_ptj_mjj is set to T. If some jets should not be restricted this way (as in single top or vector boson fusion (VBF) production, where some jets are not radiated from QCD), auto_ptj_mjj should be set to F.

This is actually your case as well if you have cut_decays=T. Since you didn't mention this point, I guess that this is your problem.

Cheers,

Olivier

On Jan 25, 2014, at 9:41 PM, Matthew Low <email address hidden> wrote:

> New question #242763 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/madgraph5/+question/242763
>
> Hi,
>
> I would like to match the process:
>
> generate p p > t t~, t > b l+ vl, t~ > b~ l- vl~ @1
> add process p p > t t~, t > b j j, t~ > b~ l- vl~ @2
> add process p p > t t~, t > b l+ vl, t~ > b~ j j @3
> add process p p > t t~, t > b j j, t~ > b~ j j @4
>
> add process p p > t t~ j, t > b l+ vl, t~ > b~ l- vl~ @5
> add process p p > t t~ j, t > b j j, t~ > b~ l- vl~ @6
> add process p p > t t~ j, t > b l+ vl, t~ > b~ j j @7
> add process p p > t t~ j, t > b j j, t~ > b~ j j @8
>
> Here I am decaying all the tops because I would like to apply the ptj1min=80 cut (and maxjetflavor=5) because I would like either of the b jets or jets from top decay to pass the ptj1min cut. When I do this, however, the cross-section for the matched process is much lower than the unmatched +0-jet (i.e. processes 1-4 above). Even if I remove the jet cut and use maxjetflavor=4 the matched and unmatched cross-sections are very discrepant. I've noticed that setting (cut_decays=F) seems to increase the matched cross-section to a reasonable value.
>
> Are there implicit/explicit cuts somewhere that I need to remove in order to generate this process properly? For reference I am using version 5.1.5.14.
>
> Thanks for your help,
> - Matthew
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Revision history for this message
Matthew Low (mattlow) said : 		#2

Thanks Olivier Mattelaer, that solved my question.