Constraints on spin-dependent parton distributions at large x from global QCD analysis

P Jimenez-Delgado,H Avakian,W Melnitchouk

doi:10.1016/j.physletb.2014.09.049

P Jimenez-Delgado, H Avakian + Show 1 more

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https://doi.org/10.1016/j.physletb.2014.09.049

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Abstract

We investigate the behavior of spin-dependent parton distribution functions (PDFs) at large parton momentum fractions x in the context of global QCD analysis. We explore the constraints from existing deep-inelastic scattering data, and from theoretical expectations for the leading x→1 behavior based on hard gluon exchange in perturbative QCD. Systematic uncertainties from the dependence of the PDFs on the choice of parametrization are studied by considering functional forms motivated by orbital angular momentum arguments. Finally, we quantify the reduction in the PDF uncertainties that may be expected from future high-x data from Jefferson Lab at 12 GeV.

Highlights

A new global next-to-leading order (NLO) analysis [1] of spin-dependent parton distribution functions (PDFs) was performed by the JAM (Jefferson Lab Angular Momentum) Collaboration [2], in which particular attention was paid to the valence quarkdominated region at high parton momentum fractions x and low four-momentum transfersQ2
Reductions in the spin-dependent PDF errors such as these, combined with dramatic reductions expected for the uncertainty on the unpolarized d quark distribution [29], should at the very least allow one to discriminate between a ∆d/d ratio that remains negative, as in simple quark models, and one that approaches ∼ 1 in the x → 1 limit, as predicted by perturbative QCD (pQCD) arguments. Beyond this there may be additional constraints on the x → 1 behavior of spin-dependent PDFs from an electron-ion collider [30,31,32], if the spectator tagging technique [33] in semi-inclusive DIS from the deuteron or 3He can be extended to polarized beams and targets. The aim of this analysis was to investigate whether existing data from polarized lepton– nucleon DIS is able to provide any constraints on the x → 1 behavior of spin-dependent PDFs in the context of a global QCD analysis
Using the recent JAM fit as a baseline, we showed that demanding the polarized to unpolarized PDF ratios ∆q+/q+ to approach unity at x = 1 results in good fits to the available data, even though the resulting changes to the ∆d+ PDF are significant in the intermediate-x region

Summary

INTRODUCTION

A new global next-to-leading order (NLO) analysis [1] of spin-dependent parton distribution functions (PDFs) was performed by the JAM (Jefferson Lab Angular Momentum) Collaboration [2], in which particular attention was paid to the valence quarkdominated region at high parton momentum fractions x and low four-momentum transfers. The ∆d+ distribution (defined as ∆d+ ≡ ∆d + ∆d) was found to have a significantly larger magnitude at x 0.2 than in previous global analyses, driven partly by a large and positive twist-4 correction to the neutron g1 structure function. The goal of the present work is to examine the behavior of spin-dependent PDFs in the x ∼ 1 region in the context of a global QCD analysis, including the effects of the x → 1 constraints in Eq (1) and of the log2(1 − x) terms inspired by pQCD. IV we draw some conclusions of the present analysis and outline steps for future work

PARTON DISTRIBUTIONS AT LARGE x

CONSTRAINTS FROM FUTURE DATA

CONCLUSIONS