PDF set storage and interpolation

LHAPDF is a C++ library that evaluates PDFs by interpolating the discretised PDF ‘grids’ that PDF collaborations produce. It also gives its users access to proton and nuclear PDF sets from a variety of PDF collaborations, including NNPDF, MMHT and CTEQ. A list of all currently available PDF sets can be found on their website. Particle physics programmes that typically make use of PDFs, such as Monte Carlo event generators, will usually be interfaced with LHAPDF, to allow a user to easily specify the PDF set that they wish to use in their calculations. You can read more about LHAPDF by reading the paper that marked their latest release.

PDF evolution

The evolution of PDFs is fully handled in the EKO code, and the way QCD evolution is calculated is described in great detail here.

PDF compression

PDF compression seeks to maintain the statistical accuracy of a large sample of replicas produced by a fit when using a PDF set with a smaller number of replicas (and thus fewer convolutions required to compute cross sections with PDF uncertainties). For example the main published PDFs are typically based on a 1000 replica fit, which can then be compressed to around a 100 replicas PDF set while maintaining good accuracy of most relevant statistical estimators. This is done with the pyCompressor library, a python compression code that extracts, from an initial PDF set of replicas, the subset that most truthfully reproduces the underlying probability distribution of the prior. pyCompressor is an updated python version of compressor, which was used in previous releases.

Other codes

Hoppet (‘Higher Order Perturbative Parton Evolution Toolkit’) is an alternative PDF evolution code which is capable of evolving unpolarised PDFs.

Pineline

The full theory production pipeline used in the NNPDF framework is extensively described in the Pineline documentation.