EPS09

EPS09
Name	EPS09
Developer	Eskola, Paukkunen, Salgado
Latest release	2009
Programming language	Fortran, C++
License	Proprietary/academic
Domain	High-energy physics

EPS09

EPS09 is a global nuclear parton distribution function (nPDF) set developed to quantify modifications of parton distributions in bound nucleons relative to free protons and neutrons. It provides parametrizations and uncertainty estimates used in perturbative quantum chromodynamics calculations for processes measured at facilities such as CERN, Brookhaven National Laboratory, and Fermilab. The analysis was produced by a collaboration of researchers based at institutions including the University of Jyväskylä, Helsinki Institute of Physics, and the Universidad de Santiago de Compostela.

Overview

EPS09 delivers nuclear modification factors for quark and gluon parton distribution functions across Bjorken x and factorization scale Q^2, designed for use with standard proton PDF sets such as CTEQ and MSTW. The parameter set was constrained using deep inelastic scattering data from experiments like EMC (experiment), NMC, and BCDMS as well as Drell–Yan data from E772 and inclusive hadron production data from PHENIX and STAR at RHIC. The release included a central fit and a set of 30 error eigenvector sets to propagate experimental uncertainties into cross-section predictions for processes at LHC and future colliders.

Methodology

The EPS09 analysis fits nuclear modification functions R_i^A(x,Q^2) by performing a global χ^2 minimization using next-to-leading order perturbative Quantum chromodynamics evolution governed by the DGLAP equations. The fit framework relied on baseline free-nucleon PDFs such as CTEQ6.1M and incorporated heavy-flavor schemes employed by groups like ACOT and MSbar treatments used in global analyses by NNPDF and MSTW. Experimental systematic uncertainties from collaborations including SLAC, CERN NA10, and FNAL E866 were treated via covariance matrices and penalty terms following procedures common to analyses by the CTEQ collaboration and the HERA combined fits.

Parameterization and Uncertainties

EPS09 parameterizes nuclear effects—shadowing, antishadowing, EMC effect, and Fermi motion—through piecewise analytic functions for valence quarks, sea quarks, and gluons with A-dependent coefficients inspired by trends observed by European Muon Collaboration measurements and the NMC nuclear program. Uncertainties were estimated using the Hessian method producing 30 error sets analogous to error prescriptions by CTEQ and MSTW, allowing users to compute plus/minus uncertainty bands for observables measured by ATLAS, CMS, and ALICE. The treatment of normalization and correlated systematics paralleled approaches used by the PDF4LHC working group and by analyses from ZEUS and H1 at HERA.

Comparisons with Other Nuclear PDF Sets

EPS09 was compared against contemporaneous nPDF sets such as nDS, HKN07, and later efforts like EPPS16 and nCTEQ15. Relative to HKN07, EPS09 typically provided a stronger gluon shadowing at small x motivated by RHIC forward hadron suppression measured by BRAHMS. Compared with nCTEQ15, EPS09 used different baseline proton PDFs and distinct uncertainty treatments; cross-comparisons were conducted in phenomenological studies involving teams from CERN Theory Division and the Institute for Nuclear Theory. Subsequent global fits by collaborations including CTEQ-TEA and the EPPS group expanded datasets and implemented Bayesian reweighting methods analogous to techniques used by the NNPDF collaboration.

Applications in High-Energy Physics

EPS09 has been employed to predict nuclear modification factors R_AA and R_pA for jet, hadron, and heavy-quark production analyzed by ALICE, CMS, and ATLAS at LHC Run I and Run II. It informed baseline calculations for quarkonium suppression studies involving PHENIX and STAR, and served in modeling initial-state effects in heavy-ion simulations developed by groups at Oak Ridge National Laboratory and Lawrence Berkeley National Laboratory. Collider phenomenology applications included calculations of electroweak boson production in proton–nucleus collisions measured by LHCb and cross-section estimates relevant to fixed-target programs at CERN SPS.

Validation and Experimental Constraints

Validation of EPS09 used comparisons to inclusive DIS structure functions measured by SLAC E139 and nuclear Drell–Yan cross sections from E772 and E866/NuSea. RHIC forward-rapidity data from BRAHMS and midrapidity neutral pion yields from PHENIX provided key constraints on gluon modifications, while subsequent comparisons to LHC pPb data from CMS and ALICE tested small-x extrapolations. Tensions between datasets were discussed in the context of systematic uncertainties reported by collaborations such as NA60 and global analysis workshops organized by Les Houches and the LHCP conference series.

Software and Availability

EPS09 was distributed as grid files and Fortran routines compatible with common tools like LHAPDF and analysis frameworks used by experimental collaborations including ROOT and RIVET. Installation guides referenced repositories maintained by groups at Helsinki Institute of Physics and the University of Jyväskylä, and users typically interfaced EPS09 error sets with Monte Carlo generators such as PYTHIA and HERWIG for simulation studies. Later efforts recommended migration to newer nPDF sets in LHAPDF6 format for compatibility with contemporary analysis pipelines.

Category:Parton distribution functions Category:High-energy physics