Extracting the impact parameter dependence of the nPDFs from the EKS98 and EPS09 global fits

TL;DR

This paper develops spatially dependent nuclear parton distribution functions (nPDFs) from existing global fits, enabling consistent calculation of cross sections across different collision centralities, and validates these with RHIC data.

Contribution

It introduces a method to incorporate impact parameter dependence into nPDFs using a power series of the nuclear thickness function, extending previous impact parameter independent fits.

Findings

Successfully calculated nuclear modification factors in various centrality bins.

Achieved good agreement with RHIC experimental data.

Provided a new tool for centrality-dependent nPDF analyses.

Abstract

As all the globally fitted nuclear PDFs (nPDFs) have been so far impact parameter independent, it has not been possible to calculate the hard process cross sections in different centrality classes consistently with the global analyses. In \cite{Helenius:2012wd} we have offered a solution to this problem by determining two spatially dependent nPDF sets, \texttt{EPS09s} and \texttt{EKS98s}, using the $A$-systematics of the earlier global fits EPS09 and EKS98 and an assumption that the spatial dependence can be written as a power series of the nuclear thickness function. For a data comparison, we have calculated the nuclear modification factor of inclusive neutral pion production in d+Au collisions at RHIC in four centrality bins at midrapidity and compared these to a PHENIX measurement. In addition, we have also performed a similar calculation for inclusive photon production in d+Au collisions at RHIC.