LHAPDF

LHAPDF is the standard tool for evaluating parton distribution functions (PDFs) in high-energy physics. PDFs encode the flavour and momentum structure of composite particles, such as protons, pions and nuclei; most cross section calculations are based on parton-level matrix elements which must be connected to the real interacting particles, hence PDFs are an essential ingredient of phenomenological and experimental studies at hadron and heavy-ion colliders (e.g. LHC, HERA, Tevatron, EIC, FCC) and in cosmic-ray physics.

PDFs themselves are fitted to a range of data by various collaborations. LHAPDF provides the definitive community library of such fits, in a standard data-format, as well as C++ and Python interfaces for evaluating them. Written as a general purpose C++ interpolator for estimating PDFs from discretised data files, it has also found more general uses, such as for fragmentation functions (essentially the inverse of PDFs).