Current Knowledge of the spin structure of the nucleon

Spin is one of the most fundamental properties of the elementary particles such as a mass and intrinsic symmetries. Since static quark model succeeded in the explanation of such properties of the baryons, the spin of the proton has been believed to be carried by the quarks for long years.

The surprising results of the polarized muon scattering off the polarized proton target reported by the EMC collaboration [3] have stimulated both experimental and theoretical works to elucidate the spin structure of the proton. The fraction of the proton spin carried by quark, , was amazingly small comparing to the canonical expectation 0.600.12 [4]. Post-EMC experiments have confirmed the EMC results. Now precise data not only from proton but also from deuteron and He are available. It is worth mentioning that theoretical improvements have played very important role in the understanding of the experimental data. The Bjorken sum rule has been confirmed to be valid. The violation of Ellis-Jaffe sum rules for proton and neutron has been also confirmed. The measurement of for multi-photon production in pp collision [5] has rejected large gluon polarization as Altarelli-Stirling [6]. The current best guess on the quark contribution to the proton spin is:

Current knowledge on the quark polarization in the proton is summarized in Table ii.

  
Table ii: Summary of the experimental results and some theoretical predictions on the quark polarization in the proton.

The problems of the spin structure of the nucleon are;

• the size of gluon polarization inside the nucleon, ,
• the size of the sea-quark polarization, ,
• the validity of currently assumed flavor SU(3), and
• small-x extrapolation.

Conventional polarized deep-inelastic scattering experiments are not sensitive to these problems, since

• photon does not couple to gluon directly,
• photon cannot distinguish anti-quark from quark,
• photon is not sensitive to the flavor but charge, and
• current experiments are all fixed target experiment.

The spin physics program at RHIC will provide answers to the questions listed above. The helicity distribution of the gluon can be measured via the measurement of for high- prompt photon production, and heavy flavor productions. The anti-quark distribution can be separated in weak boson production. Since the flavors are almost fixed in W productions, flavor decomposition of the spin dependent quark distribution is also possible.