The Spin Structure of the Nucleon

Xiangdong Ji,Yong Zhao

doi:10.1142/s2010194516600016

Abstract

We justify the physical meaning of the spin and orbital angular momentum of free partons in the infinite momentum frame, and discuss the relationship between the Jaffe-Manohar and Ji’s sum rules for proton spin. The parton orbital angular momentum in the Jaffe-Manohar sum rule can be measured through twist-three GPD’s in hard scattering processes such as deeply virtual Compton scattering. Furthermore, we propose that the paton orbital angular momentum as well as the gluon helicity can be calculated in lattice QCD through a large momentum effective theory approach, and provide all the one-loop matching conditions for the proton spin content in perturbative QCD.

Highlights

It is an important goal in hadron physics to understand the spin structure of the nucleon
We propose that the paton orbital angular momentum as well as the gluon helicity can be calculated in lattice QCD through a large momentum effective theory approach, and provide all the one-loop matching conditions for the proton spin content in perturbative QCD
In 1987, the European Muon Collaboration (EMC) at CERN discovered that the quark spin only accounts for a very small portion of the longitudinal proton spin,[1, 2] which is far less than expected from people’s understanding at that time

Summary

Introduction

It is an important goal in hadron physics to understand the spin structure of the nucleon. The EMC result has inspired generations of hadron physics programs. This is an Open Access article published by World Scientific Publishing Company. Our current understanding of the proton spin structure is that the quark spin contributes about one third,[4] while there is strong evidence that the gluon polarization shares a considerable positive amount;[5] the rest is believed to be distributed between the quark and gluon orbital angular momentum (OAM). What is a physical sum rule for the proton spin? How to probe the OAM of the quark and gluon partons? How to calculate the gluon helicity and parton OAM in lattice QCD?

The Jaffe-Manohar and Ji’s Sum Rules for Proton Spin

Probing the Parton Orbital Angular Momentum

Lattice Calculation of the Proton Spin Content