HERAPDF
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| HERAPDF | |
|---|---|
| Name | HERAPDF |
| Caption | Combined data from H1 and ZEUS |
| Field | High-energy physics |
| Originating institution | DESY |
| Country | Germany |
| First release | 2009 |
HERAPDF is a family of parton distribution function sets produced from a combined analysis of deep inelastic scattering data collected at the Hadron-Elektron-Ringanlage experimental facility by the H1 and ZEUS collaborations. The results were developed within collaborations involving DESY, the CERN theory community, and groups from Oxford University, University of Hamburg, University of Liverpool, University of Manchester, and MPI for Physics. HERAPDF has been used in precision predictions for processes at the Large Hadron Collider, comparisons with results from the Tevatron experiments CDF and DØ, and global PDF benchmarking with groups such as CTEQ, MSTW, and NNPDF.
Introduction
HERAPDF combines inclusive neutral-current and charged-current deep inelastic scattering measurements from the HERA collider running phases HERA I and HERA II, integrating inputs from the H1 and ZEUS detectors. The analysis framework interfaces with theoretical calculations based on the Dokshitzer–Gribov–Lipatov–Altarelli–Parisi evolution equations, perturbative Quantum Chromodynamics at next-to-leading order and next-to-next-to-leading order, and heavy-flavor schemes developed in part by groups at NNPDF, CTEQ, and MSTW. The resulting PDF sets provide determinations of quark and gluon distributions used by phenomenologists at CERN, experimentalists at ATLAS and CMS, and model builders working on beyond-the-Standard-Model searches related to Supersymmetry and Grand Unified Theory scenarios.
Development and Methodology
The methodology for HERAPDF analyses was developed through coordination among experimental collaborations such as H1 and ZEUS and theory groups including Alekhin, Martin, and Thorne teams, adopting systematic treatment of correlated uncertainties influenced by methods used by CTEQ and NNPDF. Fits employ χ² minimization techniques similar to those used by the MSTW collaboration, and utilize variable-flavor-number schemes inspired by the ACOT and Thorne–Roberts prescriptions. The fitting workflow integrates perturbative coefficient functions computed in schemes validated against calculations by S. Moch, J. Vermaseren, and A. Vogt, and benefits from analyses performed by groups at DESY and CERN phenomenology workshops.
Data Sets and Experimental Inputs
HERAPDF relies primarily on combined inclusive DIS cross sections from H1 and ZEUS covering a wide kinematic range in Bjorken-x and Q², incorporating low-x measurements complementary to fixed-target experiments such as NMC, BCDMS, and SLAC data. Heavy-flavor production inputs include charm and beauty measurements from H1 and ZEUS, and jet production constraints from HERA analyses cross-checked with jet results from CDF and DØ at the Tevatron. Electroweak parameter choices reference inputs from Particle Data Group, LEP electroweak fits, and mass determinations reported by ATLAS and CMS.
PDF Fits and Variants
HERAPDF produced multiple families of fits, including HERAPDF1.0, HERAPDF1.5, and later HERAPDF2.0 variants, each differing by perturbative order, heavy-flavor scheme, and parametrization choices. Variants include fits at next-to-leading order comparable to those released by CTEQ, next-to-next-to-leading order parallels to MSTW releases, and alternative parametrizations influenced by studies from NNPDF and ABM (Alekhin-Blumlein-Moch) groups. The ensembles provide central sets and uncertainty eigenvector sets allowing propagation of experimental and model uncertainties into predictions for processes studied by ATLAS, CMS, LHCb, and legacy analyses at CDF and DØ.
Validation and Uncertainties
Validation of HERAPDF fits was performed through comparisons with global fits by CTEQ, MSTW, NNPDF, and ABM collaborations, and through benchmarking against precision observables measured at Tevatron and LHC experiments. Uncertainties are decomposed into experimental, model, and parametrization components, following procedures similar to those advocated by the PDF4LHC working group and discussed at Les Houches and Moriond workshops. Sensitivity studies examined impacts of heavy-quark mass variations, αs(MZ) choices consistent with determinations by PDG, and kinematic cuts informed by analyses from H1 and ZEUS.
Applications and Impact
HERAPDF sets have been applied widely to cross-section predictions for Higgs production processes studied by ATLAS and CMS, electroweak precision measurements by LHCb, and searches for new resonances explored by ATLAS and CMS. The HERAPDF program influenced global PDF combinations, contributed to reduction of uncertainties in gluon distributions at low x relevant for small-x physics pursued at HERA and future facilities like the Electron–Ion Collider and LHeC, and informed parton-level inputs used in Monte Carlo tuning for generators such as PYTHIA, Herwig, and Sherpa.
Computational Tools and Availability
HERAPDF fits and grids are distributed in formats compatible with tools developed by collaborations including LHAPDF and are accessed by analysis frameworks used by ATLAS and CMS. The computational pipeline leverages codes for DGLAP evolution such as those by QCDNUM authors and Mellin-space calculations by teams including M. Botje and A. Vogt, and integrates with fitting frameworks employed by Professor and generator validation suites used at CERN. Researchers reproduce and extend HERAPDF studies using publicly available combined HERA data sets, analysis notes from H1 and ZEUS, and computational resources at institutions like DESY, CERN, Fermilab, and major universities.