PDF Choice Ignored For Shower in MG5amcNLO + PY8.2

Asked by Richard Ruiz on 2017-07-31

Hi Folks,

I am playing with my PY8.2 parton shower settings for pp > tt~ [QCD] in MG5amcNLO + PY8.2 and am noticing that the default PDF used for the parton shower does not change when I select a new PDF for event generation with amcNLO. Specifically, I am talking about the "PDF:pSet = 13" switch in the file "MCatNLO/RUN_PYTHIA8_XX/Pythia8.cmd".

My understanding of MCatNLO_MadFKS_PYTHIA8.Script is that it will read run_card.dat and find the corresponding PDF set for the shower. However, after choosing in run_card.dat:
lhapdf = pdlabel
260000 = lhaid

and generating events, Pythia8.cmd still contains
PDF:pSet = 13 ! PDF set used in showers and multiparton interactions (13 = NNPDF2.3 QCD+QED)

I am unsure if this is a bug or feature, but I feel it can easily result in inconsistent treatment of the parton shower if a warning is not made somewhere in the shower_card.dat.

cheers,
richard

Question information

Language:
English Edit question
Status:
Solved
For:
MadGraph5_aMC@NLO Edit question
Assignee:
No assignee Edit question
Solved by:
Olivier Mattelaer
Solved:
2017-08-07
Last query:
2017-08-07
Last reply:
2017-07-31

Hi Richard,

The choice of the PDF is made from the shower_card (as you know)
i.e.
#***********************************************************************
# PDFs and non-perturbative modelling *
#***********************************************************************
pdfcode = 0 # 0 = internal, 1 = same as NLO, other = lhaglue

So if pdfcode=0, then we use the PDF internal to the Parton-shower (this typically correspond to the tuning PDF)
If you set it it "1" then you will use the PDF from the run_card.

Did you set that parameter to "1"?

Cheers,

Olivier

PS: We are discussing to set this parameter to "1" by default (if lhapdf is present on the machine)

> On 31 Jul 2017, at 11:44, Richard Ruiz <email address hidden> wrote:
>
> New question #654259 on MadGraph5_aMC@NLO:
> https://answers.launchpad.net/mg5amcnlo/+question/654259
>
> Hi Folks,
>
> I am playing with my PY8.2 parton shower settings for pp > tt~ [QCD] in MG5amcNLO + PY8.2 and am noticing that the default PDF used for the parton shower does not change when I select a new PDF for event generation with amcNLO. Specifically, I am talking about the "PDF:pSet = 13" switch in the file "MCatNLO/RUN_PYTHIA8_XX/Pythia8.cmd".
>
> My understanding of MCatNLO_MadFKS_PYTHIA8.Script is that it will read run_card.dat and find the corresponding PDF set for the shower. However, after choosing in run_card.dat:
> lhapdf = pdlabel
> 260000 = lhaid
>
> and generating events, Pythia8.cmd still contains
> PDF:pSet = 13 ! PDF set used in showers and multiparton interactions (13 = NNPDF2.3 QCD+QED)
>
> I am unsure if this is a bug or feature, but I feel it can easily result in inconsistent treatment of the parton shower if a warning is not made somewhere in the shower_card.dat.
>
> cheers,
> richard
>
>
>
>
> --
> You received this question notification because you are an answer
> contact for MadGraph5_aMC@NLO.

Richard Ruiz (rruiz) said : #2

Thanks Olivier Mattelaer, that solved my question.

Richard Ruiz (rruiz) said : #3

... oh, I feel very stupid. Sorry for the trouble.

best,
Richard