Abstract
I investigate the behavior of spin-dependent parton distribution functions in the regions of small and large momentum fractions x. I present a systematic comparison between predictions for relevant observables obtained with various models of nucleon spin structure and a recent global analysis of spin-dependent distributions, NNPDFpol1.1. Together with its unpolarized counterpart, NNPDF2.3, they form a mutually consistent set of parton distributions. Because they include most of the available experimental information, and are determined with a minimally biased methodology, these are especially suited for such a study. I show how NNPDFpol1.1 can discriminate between different theoretical models, even though NNPDF uncertainties remain large near the endpoints x→0 and x→1, due to the lack of experimental information. I discuss how our knowledge of nucleon spin structure may be improved at small x by future measurements at an Electron–Ion Collider, and at large x by recent measurements at Jefferson Lab, also in view of its 12 GeV upgrade.
Highlights
The behavior of spin-dependent, or polarized, Parton Distribution Functions (PDFs) at small and large momentum fractions x has been recognized for a long time to be of particular physical interest [1, 2]
I have studied the behavior of polarized parton distributions in the regions of small and large momentum fractions, based on previous mutually consistent NNPDF determinations of polarized [16] and unpolarized
Among all PDF sets, these are the best suited in order for such a study to be effective: they include all the relevant experimental information which is presently available, and they are determined with a methodology devised to provide a minimally biased result
Summary
The behavior of spin-dependent, or polarized, Parton Distribution Functions (PDFs) at small and large momentum fractions x has been recognized for a long time to be of particular physical interest [1, 2]. An accurate knowledge of polarized PDFs over a broad range of x values is required to reduce the uncertainty with which the first moments of polarized distributions and structure functions can be determined. This is relevant for testing various sum rules [3,4,5,6] and potential SU(3) flavor-symmetry breaking [7], and for assessing quark and gluon contributions to the nucleon spin. These parton sets differ in the choice of data sets, details of the QCD analysis (such as the treatment of heavy quarks or higher-twist corrections) and the methodology used to determine PDFs, including the form of PDF parameterization and error propagation (for details, see e.g. Chap. 3 in Ref. [17])
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