# What is the exact meaning of "PDF variation"?

Hi,

when I'm generating some process in MadGraph5_aMC@NLO, I usually see this in the logs:

22:29:17 INFO: Running Systematics computation

22:29:23 INFO: # events generated with PDF: NNPDF23_

22:29:23 INFO: #Will Compute 145 weights per event.

22:29:23 INFO: # currently at event 0 [elapsed time: 0.061 s]

22:29:40 INFO: # currently at event 2500 [elapsed time: 17 s]

22:29:57 INFO: # currently at event 5000 [elapsed time: 34 s]

22:30:15 INFO: # currently at event 7500 [elapsed time: 51 s]

22:30:32 INFO: #******

22:30:32 #

22:30:32 # original cross-section: 2.217067e-05

22:30:32 # scale variation: +11.8% -10.1%

22:30:32 # central scheme variation: +12.9% -9.5e-10%

22:30:32 # PDF variation: +9.86% -9.86%

22:30:32 #

22:30:32 # dynamical scheme # 1 : 2.40165e-05 +12.3% -10.4% # \sum ET

22:30:32 # dynamical scheme # 2 : 2.23752e-05 +11.8% -10.1% # \sum\sqrt{m^2+pt^2}

22:30:32 # dynamical scheme # 3 : 2.50262e-05 +12.5% -10.6% # 0.5 \sum\sqrt{m^2+pt^2}

22:30:32 # dynamical scheme # 4 : 2.21707e-05 +11.8% -10.1% # \sqrt{\hat s}

22:30:32 #******

What is the meaning of the "PDF variation" line? And how can I figure out which PDF variations were actually performed?

Thanks!

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Yury (letra1947) said : | #1 |

Hi,

I didn't see any reply, maybe my question wasn't clear enough: how exactly are the numbers in the line below (taken from my example) are calculated? What is a PDF variation? How is it done exactly?

22:30:32 # PDF variation: +9.86% -9.86%

Those are computed by the lhadpf library.

In principle the lhapdf is computing such error in a different way for the various type PDF.

For NNPDF, the various PDF correspond to various replica so I guess this is a standard-deviation computation.

The evaluation of the cross-section for the 100 different PDF needed for computation of the systematics is done by MG5aMC via a re-weighting procedure. ( see 1801.08401 <https:/

Cheers,

Olivier

> On 22 Apr 2021, at 11:30, Yury <email address hidden> wrote:

>

> Question #696647 on MadGraph5_aMC@NLO changed:

> https:/

>

> Yury posted a new comment:

> Hi,

>

> I didn't see any reply, maybe my question wasn't clear enough: how

> exactly are the numbers in the line below (taken from my example) are

> calculated? What is a PDF variation? How is it done exactly?

>

> 22:30:32 # PDF variation: +9.86% -9.86%

>

> --

> You received this question notification because you are an answer

> contact for MadGraph5_aMC@NLO.

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