LHAPDF6: parton density access in the LHC precision era

• Published in: The European physical journal. C, Particles and fields Vol. 75; no. 3; pp. 1 - 20
• Main Authors: Buckley, Andy, Ferrando, James, Lloyd, Stephen, Nordström, Karl, Page, Ben, Rüfenacht, Martin, Schönherr, Marek, Watt, Graeme
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 20.03.2015; Springer
• Subjects: Astronomy; Astrophysics and Cosmology; Density; Design engineering; Elementary Particles; Hadrons; Heavy Ions; Libraries; Measurement Science and Instrumentation; Metadata; Monte Carlo methods; Nuclear Energy; Nuclear Physics; Partons; Physics; Physics and Astronomy; Portable document format; Quantum Field Theories; Quantum Field Theory; Routines; Special Article - Tools for Experiment and Theory; String Theory; Parton Density; Interpolation Grid; Parton Density Function; Fixed Flavour Number Scheme; Index File
• ISSN: 1434-6044; 1434-6052
• DOI: 10.1140/epjc/s10052-015-3318-8

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.