An improved global analysis of nuclear parton distribution functions including RHIC data

TL;DR

This work advances the global analysis of nuclear parton distribution functions by incorporating RHIC d+Au high-$p_T$ hadron data into a leading-order DGLAP framework. It introduces a refined $\chi^2$ treatment with normalization uncertainties and weights, enabling a more faithful utilization of diverse data to constrain gluon shadowing. The resulting EPS08 nPDFs exhibit significantly stronger gluon shadowing at small $x$ than prior analyses, driven in large part by BRAHMS forward-rapidity measurements, while still remaining compatible with DIS and DY data. These results provide a more constrained, publicly available set of nPDFs for interpreting hard processes in nuclear environments and highlight the value of including RHIC data in global analyses, with planned extensions to NLO and additional datasets.

Abstract

We present an improved leading-order global DGLAP analysis of nuclear parton distribution functions (nPDFs), supplementing the traditionally used data from deep inelastic lepton-nucleus scattering and Drell-Yan dilepton production in proton-nucleus collisions, with inclusive high-$p_T$ hadron production data measured at RHIC in d+Au collisions. With the help of an extended definition of the $χ^2$ function, we now can more efficiently exploit the constraints the different data sets offer, for gluon shadowing in particular, and account for the overall data normalization uncertainties during the automated $χ^2$ minimization. The very good simultaneous fit to the nuclear hard process data used demonstrates the feasibility of a universal set of nPDFs, but also limitations become visible. The high-$p_T$ forward-rapidity hadron data of BRAHMS add a new crucial constraint into the analysis by offering a direct probe for the nuclear gluon distributions -- a sector in the nPDFs which has traditionally been very badly constrained. We obtain a strikingly stronger gluon shadowing than what has been estimated in previous global analyses. The obtained nPDFs are released as a parametrization called EPS08.

Figures (14)

• Figure 1: An illustration of the smoothly matched fit functions $R_i^A(x)$ and the role of the parameters $x_a^A$, $y_a^A$, $x_e^A$ and $\Delta_e^A$. The superscripts $A$ have been suppressed in the figure.
• Figure 2: The nuclear modification factors $R_V^A$, $R_S^A$ and $R_G^A$ for C, Ca, Sn, and Pb at $Q_0^2 = 1.69 \, {\rm GeV}^2$. The DIS ratio $R_{F_2}^A$ is shown for comparison.
• Figure 3: The computed ratio $R_{F_2}^A(x,Q^2)$ vs. $R_{F_2}^{\mathrm C}(x,Q^2)$ compared with the NMC data Arneodo:1996rv. The open symbols are the data points with statistical and systematic errors added in quadrature, the filled ones are the corresponding results from this analysis.
• Figure 4: The calculated ratio $R_{F_2}^A(x,Q^2)$ compared with the NMC 95 (squares) Arneodo:1995cs and the reanalysed NMC 95 (circles) data Amaudruz:1995tq.
• Figure 5: The computed ratio $R_{F_2}^A(x,Q^2)$ vs. $R_{F_2}^{\mathrm Li}(x,Q^2)$ (filled circles) compared with the NMC data Amaudruz:1995tq (open circles).