# Proper madgraph settings for 5-flavor vs 4-flavor PDFs

Asked by Shin-Shan Eiko Yu on 2017-08-09

I would like to know more details about the proper madgraph settings for 4-flavor vs 5-flavor PDFs. From my previous understanding, in order to use a 5-flavor PDF (say, LHAPDF 263000), I need to do the following changes according to this thread [1]:

1) modify the definition of proton and jet in process card
2) modify the PDF, maxjetflavor, asrwgtflavor in run card
3) either modify b quark mass to zero in the parameter card or define a restrict card and put it in the UFO directory, import the model MODEL-no_bmass

However, while I try this change for a few mono-Higgs models, I notice a feature and I am not sure if it's OK: since the b quark mass is set to zero, for the (pseudo-) scalar mediators that have yukawa-like coupling, its coupling to b quark becomes zero. This also reduces the width of (pseudo-) scalar mediators slightly, not a lot. But now I wonder if it makes sense.

I mean I assume we only want to modify the mass of incoming b quarks, not the mass of the b quarks in the decays (outgoing b quarks)? Is there a way to do so in madgraph?

In addition, I also find for these models, no matter what I put in process card/run card/b quark mass, as long as LHAPDF is 5F PDF 263000, they give the same cross sections xxxx, when LHAPDF is 4 F PDF 263400, madgraph returns another cross section value yyyy, independent of the setting of b quark mass or proton/jet definition. I assume this is because for these models, the b-quark initiated process has very small contribution in the cross section to start with. So perhaps for these models, if there is no way to change only the mass of incoming b quarks, we could leave b quark mass at 4.7 GeV while using the 5F PDF?

Thanks!!
Shin-Shan Eiko Yu

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2017-08-09
 Olivier Mattelaer (olivier-mattelaer) said on 2017-08-09: #1

Hi,

If you are using a recent enough version of the code. Importing a model with massless/massive b will automatically set up the following value:
> 1) modify the definition of proton and jet in process card

> maxjetflavor, asrwgtflavor in run card

Note that we modify those for the default value. If you overwrite the run_card, we will keep your value.
(Same for the proton/jet definition if you explicitly redefine it)
Note that we do not change the default value for the PDF

> However, while I try this change for a few mono-Higgs models, I notice a feature and I am not sure if it's OK: since the b quark mass is set to zero, for the (pseudo-) scalar mediators that have yukawa-like coupling, its coupling to b quark becomes zero. This also reduces the width of (pseudo-) scalar mediators slightly, not a lot. But now I wonder if it makes sense.

At LO, You can also decide to keep the Yukawa non zero even if the b mass is zero. At NLO it is also possible but need to be handle at the time of the generation of the model).

> I mean I assume we only want to modify the mass of incoming b quarks, not the mass of the b quarks in the decays (outgoing b quarks)? Is there a way to do so in madgraph?

No this is not possible and I do not think that it makes sense, since the log that you want to resum (or not) should be of the same level wether the particle is in initial or final state. So the mass should be the same for the full event.

> In addition, I also find for these models, no matter what I put in process card/run card/b quark mass, as long as LHAPDF is 5F PDF 263000, they give the same cross sections xxxx, when LHAPDF is 4 F PDF 263400, madgraph returns another cross section value yyyy, independent of the setting of b quark mass or proton/jet definition. I assume this is because for these models, the b-quark initiated process has very small contribution in the cross section to start with

Difficult to comment on that without seeing your runs. But your comment seems reasonable.

Cheers,

Olivier

> On 9 Aug 2017, at 12:34, Shin-Shan Eiko Yu <email address hidden> wrote:
>
> New question #655505 on MadGraph5_aMC@NLO:
>
>
>
> I would like to know more details about the proper madgraph settings for 4-flavor vs 5-flavor PDFs. From my previous understanding, in order to use a 5-flavor PDF (say, LHAPDF 263000), I need to do the following changes according to this thread [1]:
>
> 1) modify the definition of proton and jet in process card
> 2) modify the PDF, maxjetflavor, asrwgtflavor in run card
> 3) either modify b quark mass to zero in the parameter card or define a restrict card and put it in the UFO directory, import the model MODEL-no_bmass
>
> However, while I try this change for a few mono-Higgs models, I notice a feature and I am not sure if it's OK: since the b quark mass is set to zero, for the (pseudo-) scalar mediators that have yukawa-like coupling, its coupling to b quark becomes zero. This also reduces the width of (pseudo-) scalar mediators slightly, not a lot. But now I wonder if it makes sense.
>
> I mean I assume we only want to modify the mass of incoming b quarks, not the mass of the b quarks in the decays (outgoing b quarks)? Is there a way to do so in madgraph?
>
> In addition, I also find for these models, no matter what I put in process card/run card/b quark mass, as long as LHAPDF is 5F PDF 263000, they give the same cross sections xxxx, when LHAPDF is 4 F PDF 263400, madgraph returns another cross section value yyyy, independent of the setting of b quark mass or proton/jet definition. I assume this is because for these models, the b-quark initiated process has very small contribution in the cross section to start with.
>
>
> [1]