[Paper Review] Global reanalysis of nPDFs

K. Eskola, V.J. Kolhinen+2 more|ArXiv.org|Jun 30, 2007

Particle physics theoretical and experimental studies9 references3 citations

TL;DR

This paper presents an automated global reanalysis of nuclear parton distribution functions (nPDFs) using DGLAP evolution, constrained by deep inelastic scattering and Drell-Yan data, achieving a $χ^2$/d.o.f. of 0.82 with 16 parameters. The study confirms the EKS98 parametrization remains consistent with modern data and explores the possibility of stronger gluon shadowing, as suggested by RHIC BRAHMS data, without degrading fit quality.

ABSTRACT

In this talk, we present the results from our recent global reanalysis of nuclear parton distribution functions (nPDFs), where the DGLAP-evolving nPDFs are constrained by nuclear hard process data from deep inelastic $l+A$ scattering (DIS) and the Drell-Yan (DY) process in $p+A$ collisions, and by sum rules. The main improvements over our earlier work {\em EKS98} are the automated $χ^2$ minimization, better controllable fit functions and possibility for error estimates. The obtained 16-parameter fit to N=514 datapoints is good, $χ^2/{ m d.o.f}=0.82$. Fit quality comparison and the error estimates obtained show that the old {\em EKS98} parametrization is fully consistent with the present automated reanalysis. Comparison with other global nPDF analyses is presented as well. Within the DGLAP framework we also discuss the possibility of incorporating a clearly stronger gluon shadowing, which is suggested by the RHIC BRAHMS data from d+Au collisions.

Motivation & Objective

• To perform an automated, systematic reanalysis of nuclear parton distribution functions (nPDFs) using $χ^2$ minimization and error estimation.
• To assess whether the EKS98 nPDF parametrization remains valid in light of modern data and improved fitting techniques.
• To investigate whether the data allow for stronger gluon shadowing than previously found, particularly in light of RHIC BRAHMS data showing forward suppression.
• To compare the quality and consistency of the new fit with existing global nPDF analyses (HKM, HKN, nDS) and evaluate the impact of different data sets.
• To explore the implications of enhanced gluon shadowing on the quark sector and the overall consistency with measured $\log Q^2$-slopes in nuclear structure functions.

Proposed method

• Employed a DGLAP-evolving nPDF framework with three nuclear modification functions: $R_V^A$ for valence quarks, $R_S^A$ for sea quarks, and $R_G^A$ for gluons.
• Used a 16-parameter fit function with piecewise exponential and polynomial segments to model $R_i^A(x)$, with matching at $x_a^A$ and $x_e^A$.
• Applied automated $\chi^2$ minimization over 514 data points from DIS and Drell-Yan processes, incorporating momentum and baryon number sum rules.
• Calculated uncertainties via error propagation and compared with EKS98 and other global analyses to assess consistency.
• Tested the sensitivity of p+Au inclusive hadron production cross sections to gluon shadowing by comparing EKS98 and enhanced shadowing scenarios.
• Used KKP fragmentation functions and set factorization scales to partonic and hadronic transverse momentum to compute LO pQCD cross sections for comparison with BRAHMS data.

Experimental results

Research questions

• RQ1 Can an automated $\chi^2$ minimization improve upon the EKS98 nPDF parametrization while maintaining consistency with experimental data?
• RQ2 How do the error estimates and fit quality of the new reanalysis compare with those of EKS98, HKM, HKN, and nDS?
• RQ3 Is there evidence in the DIS and Drell-Yan data for stronger gluon shadowing than previously determined?
• RQ4 Can the DGLAP framework accommodate significantly stronger gluon shadowing without degrading the fit to $\log Q^2$-slopes in $F_2^{\text{Sn}}/F_2^{\text{C}}$?
• RQ5 Would enhanced gluon shadowing induce compensatory changes in the quark sector that preserve agreement with measured $\log Q^2$-slopes?

Key findings

• The new automated reanalysis achieves a $χ^2$/d.o.f. of 0.82 with 16 parameters and 514 data points, indicating a high-quality fit.
• The EKS98 parametrization is fully consistent with the new results, as confirmed by direct comparison and similar $χ^2$/N values (0.809 vs. 0.798).
• The fit quality is comparable to the NLO nDS analysis, which reports the lowest $χ^2$/N (0.715) among existing nPDF analyses.
• Even with a strongly enhanced gluon shadowing profile (as in Fig. 5), the $χ^2$/N increases only slightly to 0.95, indicating that the data do not strongly constrain the gluon sector.
• The BRAHMS d+Au data suggest a need for stronger gluon shadowing, but the current data set does not yet rule out the modest shadowing found in global analyses.
• The study concludes that while stronger gluon shadowing is not ruled out, a full reanalysis including d+Au data is necessary to confirm its viability within the DGLAP framework.