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The Spin Structure of the Nucleon

G. K. Mallot

TL;DR

The paper reviews the nucleon spin structure in light of the EMC result that the quark spins do not saturate the proton's spin. It synthesizes polarized DIS data on $g_1$ and $g_2$ from SMC, SLAC, and Hermes, and combines them with QCD analyses of polarized parton distributions, highlighting a small quark spin contribution and a poorly constrained gluon polarization. It discusses sum rules (Ellis–Jaffe, Bjorken) and the role of extrapolations to $x\to0$ in determining the first moments, as well as twist-3 effects in $g_2$ and insights from semi-inclusive measurements that enable flavour separation and access to transversity $h_1$. The review emphasizes that, despite significant progress, the spin puzzle remains unresolved and calls for direct measurements of $\Delta g$ and orbital angular momentum, with upcoming experiments (COMPASS, RHIC) and possible polarized beams (HERA) offering crucial opportunities.

Abstract

The present status of the nucleon's spin structure is reviewed with emphasis on new experimental results.

The Spin Structure of the Nucleon

TL;DR

The paper reviews the nucleon spin structure in light of the EMC result that the quark spins do not saturate the proton's spin. It synthesizes polarized DIS data on and from SMC, SLAC, and Hermes, and combines them with QCD analyses of polarized parton distributions, highlighting a small quark spin contribution and a poorly constrained gluon polarization. It discusses sum rules (Ellis–Jaffe, Bjorken) and the role of extrapolations to in determining the first moments, as well as twist-3 effects in and insights from semi-inclusive measurements that enable flavour separation and access to transversity . The review emphasizes that, despite significant progress, the spin puzzle remains unresolved and calls for direct measurements of and orbital angular momentum, with upcoming experiments (COMPASS, RHIC) and possible polarized beams (HERA) offering crucial opportunities.

Abstract

The present status of the nucleon's spin structure is reviewed with emphasis on new experimental results.

Paper Structure

This paper contains 7 sections, 13 equations, 15 figures, 1 table.

Table of Contents

  1. Introduction
  2. Experiments and status of structure function data
  3. QCD analyses of $g_1$ data
  4. Sum rules and moments
  5. Status of $A_2$ measurements
  6. Semi-inclusive data
  7. Summary and Outlook

Figures (15)

  • Figure 1: Kinematic ranges of the individual experiments. The double logarithmic scale emphasises the new data in the small-$x$, small-$Q^2$ region from a special SMC trigger.
  • Figure 2: Proton (left) and deuteron structure functions $g_1(x,Q^2)$ for $Q^2>1~{\mathrm GeV}^2$. The proton data from E155 are preliminary. Also shown are a phenomenological fit to the data (dashed) and the E154 QCD-fit (solid) E154_97b
  • Figure 3: The structure function $g_1(x,5~{\mathrm GeV}^2)$ for the proton (left, top), the deuteron (left, bottom) and the neutron (top) E155_web. The E155 proton data are preliminary. Also shown is the systematic error of the E155 data.
  • Figure 4: Asymmetries, $A_1(x)$, for the proton (left) and the deuteron (right) from the SMC low-$x$ and standard triggers SMC_99a. The small-$x$ trigger extends to $x=6\cdot10^{-5}$.
  • Figure 5: Structure functions, $x\cdot g_1(x)$, for the proton (left) and the deuteron (right) from the SMC SMC_99a. The value of $Q^2$ is below 1 GeV$^2$ for $x<0.003$.
  • ...and 10 more figures