Nucleon spin structure

Nucleon spin structure describes the partonic structure of nucleon (proton and neutron) intrinsic angular momentum (spin). The key question is how nucleon spin 1/2 is carried by its constituents, say partons (quarks and gluons). It was originally expected before the 1980s that quarks carry all of the nucleon spin, but later experiments contradict this expectation. In the late 1980s, the European Muon Collaboration conducted experiments that suggested the spin carried by quarks is not sufficient to justify the total spin of the nucleons. This problem of where the missing spin lies is sometimes referred to as the proton spin crisis.

From a modern point of view, with the known fact that quarks only carry half the total momentum of the nucleon, it seemed not quite a surprise for us today to realized that quarks only carry a small fraction of nucleon spin. But it did astonished particle physicists at that time. Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) at CERN, by HERMES at DESY, by COMPASS, by JLab, by RHIC and others. Global analysis of data from all major experiments confirmed the original discovery by European Muon Collaboration and showed that quark spin did contribute about 30% to the total spin of nucleons. A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried by either gluon spin, or by gluon and quark orbital angular momentum. This fact is expressed by the sum rule,

$\frac{1}{2} =\frac{1}{2} \Sigma_q + \Sigma_g + L_q + L_g.$

The gluon spin compoments Σ_g is being measured by many experiments, and quark and gluon angular momentum will be measured by measuring so-called Generalized Parton Distributions (GPD) through deeply virtual compton scattering (DVCS), which is conducted mainly at JLab.

External links

• Polarized colliders may prove to be the key in mapping out proton spin structure
• Dr. Deshpande research webpage
• Spin Muon Collaboration
• HERMES
• COMPASS
• Spin Structure of the Nucleon - Status and Recent Results