Determination of nuclear parton distributions

TL;DR

This work develops leading-order nuclear parton distributions by fitting a large set of $F_2^A$ data across nuclei using weight functions that modify nucleon PDFs at $Q_0^2=1$ GeV$^2$. It introduces two functional forms (quadratic and cubic) for the $x$-dependence and an $A$-dependent factor $1-1/A^{1/3}$, constrained by charge, baryon-number, and momentum. The analysis yields reasonably good fits, with valence distributions well constrained, antiquark distributions at small $x$ moderately constrained, and gluons poorly constrained, and provides practical analytical expressions and a library for applying the nuclear PDFs. The results have implications for interpreting nuclear modification mechanisms and for refining nucleon PDFs using nuclear data, as well as for heavy-ion phenomenology via improved initial parton distributions.

Abstract

Parametrization of nuclear parton distributions is investigated in the leading order of alpha_s. The parton distributions are provided at Q^2=1 GeV^2 with a number of parameters, which are determined by a chi^2 analysis of the data on nuclear structure functions. Quadratic or cubic functional form is assumed for the initial distributions. Although valence quark distributions in the medium x region are relatively well determined, the small x distributions depend slightly on the assumed functional form. It is difficult to determine the antiquark distributions at medium x and gluon distributions. From the analysis, we propose parton distributions at Q^2=1 GeV^2 for nuclei from deuteron to heavy ones with the mass number A~208. They are provided either analytical expressions or computer subroutines for practical usage. Our studies should be important for understanding the physics mechanism of the nuclear modification and also for applications to heavy-ion reactions. This kind of nuclear parametrization should also affect existing parametrization studies in the nucleon because "nuclear" data are partially used for obtaining the optimum distributions in the "nucleon".

Figures (11)

• Figure 1: $A$ dependence of measured $F_2^A/F_2^D$ ratios at $x=0.5$.
• Figure 2: Kinematical range is shown by plotting $x$ and $Q^2$ points of the used data.
• Figure 3: Fitting results are compared with the helium data. The dashed and solid curves are for the quadratic and cubic types, respectively, at $Q^2=5$ GeV$^2$, whereas the data are taken at various $Q^2$ points.
• Figure 4: Comparison with the lithium data.
• Figure 8: Comparison with the aluminum data.