LHAPDF6: parton density access in the LHC precision era

TLDR

LHAPDF has long provided standardized parton density function access, but Run 1 LHC revealed fundamental design limits that hindered physics studies and risked incorrect results. This paper introduces LHAPDF 6, a complete re‑engineering of the PDFLIB/LHAPDF paradigm to eliminate concurrent‑set limits, reduce memory, improve CPU performance, and fix multi‑set metadata bugs. LHAPDF 6’s new design, currently limited to interpolated PDFs, centralises numerical routines and employs a cascading metadata system that decouples software releases from new PDF data and supports arbitrary parton content. Over 200 PDF sets have been migrated to the new format, LHAPDF 6 is supported by many Monte Carlo generators and other programs, and is recommended for LHC Run 2 and beyond.

Abstract

The Fortran LHAPDF library has been a long-term workhorse in particle physics, providing standardised access to parton density functions for experimental and phenomenological purposes alike, following on from the venerable PDFLIB package. During Run 1 of the LHC, however, several fundamental limitations in LHAPDF’s design have became deeply problematic, restricting the usability of the library for important physics-study procedures and providing dangerous avenues by which to silently obtain incorrect results. In this paper we present the LHAPDF 6 library, a ground-up re-engineering of the PDFLIB/LHAPDF paradigm for PDF access which removes all limits on use of concurrent PDF sets, massively reduces static memory requirements, offers improved CPU performance, and fixes fundamental bugs in multi-set access to PDF metadata. The new design, restricted for now to interpolated PDFs, uses centralised numerical routines and a powerful cascading metadata system to decouple software releases from provision of new PDF data and allow completely general parton content. More than 200 PDF sets have been migrated from LHAPDF 5 to the new universal data format, via a stringent quality control procedure. LHAPDF 6 is supported by many Monte Carlo generators and other physics programs, in some cases via a full set of compatibility routines, and is recommended for the demanding PDF access needs of LHC Run 2 and beyond.