Impact-parameter dependent nuclear parton distribution functions: EPS09s and EKS98s and their applications in nuclear hard processes

Ilkka Helenius,Heli Honkanen,Kari J Eskola,Carlos A Salgado

doi:10.1007/jhep07(2012)073

Abstract

Abstract We determine the spatial (impact parameter) dependence of nuclear parton distribution functions (nPDFs) using the A-dependence of the spatially independent (averaged) global fits EPS09 and EKS98. We work under the assumption that the spatial dependence can be formulated as a power series of the nuclear thickness functions T A . To reproduce the A-dependence over the entire x range we need terms up to [T A ]4. As an outcome, we release two sets, EPS09s (LO, NLO, error sets) and EKS98s, of spatially dependent nPDFs for public use. We also discuss the implementation of these into the existing calculations. With our results, the centrality dependence of nuclear hard-process observables can be studied consistently with the globally fitted nPDFs for the first time. As an application, we first calculate the LO nuclear modification factor $ R_{{AA}}^{{1{\text{jet}}}} $ for primary partonic-jet production in different centrality classes in Au+Au collisions at RHIC and Pb+Pb collisions at LHC. Also the corresponding central-to-peripheral ratios $ R_{{CP}}^{{1{\text{jet}}}} $ are studied. We also calculate the LO and NLO nuclear modification factors for single inclusive neutral pion production, $ R_{\text{dAu}}^{{{\pi^0}}} $ , at mid- and forward rapidities in different centrality classes in d+Au collisions at RHIC. In particular, we show that our results are compatible with the PHENIX mid-rapidity data within the overall normalization uncertainties given by the experiment. Finally, we show our predictions for the corresponding modifications $ R_{\text{pPb}}^{{{\pi^0}}} $ in the forthcoming p+Pb collisions at LHC.

Highlights

The global nuclear parton distribution functions (nPDFs) fits have so far resulted in leading-order (LO) nPDF sets EKS98 [10], HKM [11] and HKN04 [12], and next-to-leading order (NLO) sets nDS [13], HKN07 [14], EPS09 [15], nCTEQ [16, 17], and DSZS [18]
And to the free proton case, with the error sets of EPS09, one can nowadays quantify how the uncertainties remaining in the nPDFs, illustrated in figure 1, are transmitted to the nuclear hard-process cross-sections
We do this in a manner which is for the first time fully consistent with the nPDFs from a global analysis

Summary

Introduction

I in the colliding particle A (and correspondingly for the flavor j in B). The PDFs are universal, process-independent functions of nonperturbative origin, whose evolution in the scale Q2 can, be obtained from the DGLAP equations [3,4,5,6] derived from perturbative QCD. A precise knowledge of the universal PDFs is vital for interpreting any hardprocess results at the present colliders BNL-RHIC and CERN-LHC. This holds as well for proton-proton collisions as for proton-nucleus and nucleus-nucleus collisions. The purpose of this study is to consider this problem by pinning down the spatial dependence of the nPDFs, i.e. the dependence of the nuclear modifications of the PDFs on the nucleon’s position in a nucleus We do this in a manner which is for the first time fully consistent with the nPDFs from a global analysis. The theoretical modeling of the J/Ψ production discussed recently in ref. [33] has motivated our study

Objectives

Results

Conclusion