Bundle PDFs with \(\alpha_s\) replicas

Using validphys it is possible to produce a bundled PDF set which accounts for a combined PDF + \(\alpha_s\) uncertainty. The procedure to generate such an LHAPDF set is to simply take a baseline PDF set and append the central replica from a list of \(\alpha_s\) variation fits.

The action to leverage this is validphys.replica_selector.alpha_s_bundle_pdf(). We specify the baseline PDF as the pdf key within the runcard and the pdfs list specifies the \(\alpha_s\) fits that are to be used. In the following example, the NNPDF31_nnlo_as_0118 PDF set is used as baseline and we append the central replica from NNPDF31_nnlo_as_0117 and NNPDF31_nnlo_as_0119.

pdf: NNPDF31_nnlo_as_0118

pdfs:
 - NNPDF31_nnlo_as_0117
 - NNPDF31_nnlo_as_0119

actions_:
 - alpha_s_bundle_pdf

Executing this runcard with validphys produces the bundled PDF set in the output folder, which by default will be name the same as the baseline, except for the _pdfas suffix being appended:

output/
├── NNPDF31_nnlo_as_0118_pdfas
│   ├── NNPDF31_nnlo_as_0118_pdfas.info
│   ├── NNPDF31_nnlo_as_0118_pdfas_0000.dat
│   ├── NNPDF31_nnlo_as_0118_pdfas_0001.dat
│   ├── NNPDF31_nnlo_as_0118_pdfas_0002.dat

The .info file now has replicas+as as the ErrorType and the NumMembers key has now been updated to reflect the additional replicas (in this example it has been incremented by two). The additional replicas also have the AlphaS_MZ key prepended at the top of the file so as to keep track of what PDF set they originated from.

The optional target_name key can be provided in the runcard so as to specify the name of the resulting PDF. The following runcard will generate the same bundled PDF as before, but with the name bundled_pdf:

pdf: NNPDF31_nnlo_as_0118

pdfs:
  - NNPDF31_nnlo_as_0117
  - NNPDF31_nnlo_as_0119

target_name: bundled_pdf

actions_:
  - alpha_s_bundle_pdf

output/
├── bundled_pdf
│   ├── bundled_pdf.info
│   ├── bundled_pdf_0000.dat
│   ├── bundled_pdf_0001.dat
│   ├── bundled_pdf_0002.dat

Note

Despite adding additional replicas, the central replica in the bundled PDF set is not recomputed: it is identical to that of the baseline.