maxjetflavor giving unexpected results

Asked by Samuel Homiller on 2018-09-26

Dear Madgraph Experts,

I've become a bit confused about the functionality of the tag "maxjetflavor" in the run card for MG5. My understanding was that this simply determined whether or not the jet cuts (ptj, etc.) applied to b-jets or not. However, when I run a process with the same cuts applied to the jets and the b-jets, and change maxjetflavor from 4 to 5, I get a different result for the cross section.

As an example, I've tried:
import model heft
generate p p > h b b~
set ebeam1 13500.0
set ebeam2 13500.0
ptj 20.0
ptb 20.0
etaj 2.5
etab 2.5
drjj 0.4
drbb 0.4
drbj 0.4

with all other jet and b-jet cuts turned off. Instead of getting the same result, however, I find:
0.246 pb for maxjetflavor = 4
0.2054 pb for maxjetflavor = 5

which is quite a substantial difference. I've tried editing the parameters in the model file (to set the bottom quark to be massless, for instance), but this doesn't seem to have much affect. Is there something that maxjetflavor is doing that I'm not understanding?

Thanks very much,
Sam

Question information

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Last query:
2018-09-26
Last reply:
2018-09-28

Hi,

The difference is actually within theoretical error.
As you can see here:

> INFO: #Will Compute 145 weights per event.
> INFO: #***************************************************************************
> #
> # original cross-section: 0.245656690362
> # scale variation: +25.3% -19.1%
> # central scheme variation: + 0% -29.3%
> # PDF variation: +2.96% -2.96%
> #
> # dynamical scheme # 1 : 0.203962 +22.3% -17% # \sum ET
> # dynamical scheme # 2 : 0.190185 +21.2% -16.8% # \sum\sqrt{m^2+pt^2}
> # dynamical scheme # 3 : 0.23057 +24.2% -18.4% # 0.5 \sum\sqrt{m^2+pt^2}
> # dynamical scheme # 4 : 0.173765 +20.4% -16.3% # \sqrt{\hat s}
> #***************************************************************************

The error is obviously quite large (as you should expect for loop-induced processes).

So the difference is actually linked to our default choice for the scale where we use the that parameter in order
to choose our scale.

You can check that if you use a fix scale, you do not have any difference between the two predictions.
Same if you choose a simpler (purely kinematical) scale choice ( I have tested with HT/2).

Cheers,

Olivier

> On 26 Sep 2018, at 17:22, Samuel Homiller <email address hidden> wrote:
>
> New question #674448 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/674448
>
> Dear Madgraph Experts,
>
> I've become a bit confused about the functionality of the tag "maxjetflavor" in the run card for MG5. My understanding was that this simply determined whether or not the jet cuts (ptj, etc.) applied to b-jets or not. However, when I run a process with the same cuts applied to the jets and the b-jets, and change maxjetflavor from 4 to 5, I get a different result for the cross section.
>
> As an example, I've tried:
> import model heft
> generate p p > h b b~
> set ebeam1 13500.0
> set ebeam2 13500.0
> ptj 20.0
> ptb 20.0
> etaj 2.5
> etab 2.5
> drjj 0.4
> drbb 0.4
> drbj 0.4
>
> with all other jet and b-jet cuts turned off. Instead of getting the same result, however, I find:
> 0.246 pb for maxjetflavor = 4
> 0.2054 pb for maxjetflavor = 5
>
> which is quite a substantial difference. I've tried editing the parameters in the model file (to set the bottom quark to be massless, for instance), but this doesn't seem to have much affect. Is there something that maxjetflavor is doing that I'm not understanding?
>
> Thanks very much,
> Sam
>
> --
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> contact for MadGraph5_aMC@NLO.

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