Flavour scheme and heavy quark mass

Hi,

For five flavor computation it is better to use a model where the bottom is set to zero from the start.
(This allows that a lot of parameter are set correctly to five flavor from the start and allow a lot of optimization)

So I would suggest to use
import model sm-no_b_mass
The annoying part is that such model also removes the yukawa coupling.
So I would suggest to define your own model restriction to keep the yukawa but set the pole mass to zero:
See: https://answers.launchpad.net/mg5amcnlo/+faq/2312 <https://answers.launchpad.net/mg5amcnlo/+faq/2312>

Concerning
> Also, when I am trying to generate the following process in 5 flavor scheme:
> define p = p b b~
> generate p p > b b~ mu+ mu- QED=99
> add process p p > b b~ mu+ mu- j QED=99

This is not a valid computation in five flavor because you have b in the final state (and they can be comming from a gluon splitting). This will not be consistent with the running of alpha_s and will create miss-match between the various sample and therefore the sum of the sample will not be consistent.

If you want to do this in five flavor this should be
> generate p p > mu+ mu- QED=99
> add process p p > mu+ mu- j QED=99

 add process p p > mu+ mu- j jQED=99
> add process p p > mu+ mu- j j j QED=99

and then you should ask yourself if you really want QED=99 here.
matching and merging is only done for QCD. So double counting in Electroweak will not be removed here.
so you should rather do

> generate p p > mu+ mu-
> add process p p > mu+ mu- j

 add process p p > mu+ mu- j j
> add process p p > mu+ mu- j j j

Cheers,

Olivier

> On 23 Jul 2021, at 16:41, Antara Sarkar <email address hidden> wrote:
>
> New question #698123 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/698123
>
> Dear MG5 experts,
>
> I am trying to produce higgs and Z using in the sm model:
> define p = p b b~
> generate p p > z h, h > b b~, z > l+ l-
> I am using running b quark yukawa by entering by hand (in the model parameters file) the bottom mass at the factorisation scale. Is it the correct way to do this?
>
> Also, when I am trying to generate the following process in 5 flavor scheme:
> define p = p b b~
> generate p p > b b~ mu+ mu- QED=99
> add process p p > b b~ mu+ mu- j QED=99
>
> To generate the above process I use sm model.
>
> When I try to do matching upto 1 jet with "JetMatching:nJetMax = 1 " in pythia card, it generates the following error message:
>
> "PY8_parallelization/run_PY8.sh 0 launch ends with non zero status: 139. Stop all computation
> Please report this bug on https://bugs.launchpad.net/mg5amcnlo"
>
> Could you please help me understand the reason for the error ? With 4 flavour scheme, the error does not arise.
>
> Best regards,
> Antara
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

We study the sensitivity of our recent MSTW NLO and NNLO PDF analyses to the values of the charm- and bottom-quark masses, and we provide additional public PDF sets for a wide range of these heavy-quark masses.

We quantify the impact of varying m_c and m_b on the cross sections for W, Z and Higgs production at the Tevatron and the LHC.

 We generate 3- and 4-flavour versions of the (5-flavour) MSTW 2008 PDFs by evolving the input PDFs and alpha_S determined from fits in the 5-flavour scheme, including the eigenvector PDF sets necessary for calculation of PDF uncertainties.
https://zewawise.com/

As an example of their use, we study the difference in the Z total cross sections at the Tevatron and LHC in the 4- and 5-flavour schemes.
Significant differences are found, illustrating the need to resum large logarithms in Q^2/m_b^2 by using the 5-flavour scheme. The 4-flavour scheme is still necessary, however, if cuts are imposed on associated (massive) b-quarks, as is the case for the experimental measurement of Z b bbar production and similar processes.