Eur. Phys. J. C (2015) 75: 132
https://doi.org/10.1140/epjc/s10052-015-3318-8

Special Article - Tools for Experiment and Theory

LHAPDF6: parton density access in the LHC precision era

¹ School of Physics and Astronomy, University of Glasgow, Glasgow, UK
² School of Physics and Astronomy, University of Edinburgh, Edinburgh, UK
³ Departamento de Física Teórica y del Cosmos y CAFPE, Universidad de Granada, Granada, Spain
⁴ School of Informatics, University of Edinburgh, Edinburgh, UK
⁵ Physik-Institut, Universität Zürich, Zurich, Switzerland
⁶ Institute for Particle Physics Phenomenology, Durham University, Durham, UK

^* e-mail: andy.buckley@cern.ch

Received: 26 December 2014
Accepted: 10 February 2015
Published online: 20 March 2015

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.