SPIN STRUCTURE FUNCTIONS

Abstract

Key Words proton spin, deep inelastic scattering, quark-parton model, sum rules, quantum chromodynamics s Abstract We review the study of the internal spin structure of the proton and neutron. High-energy scattering of polarized leptons by polarized protons, neutrons, and deuterons provides a measurement of the nucleon spin structure functions. These structure functions give information on the polarized quark contributions to the spin of the proton and the neutron and allow tests of the quark-parton model and quantum chromodynamics. We discuss the formalism of deep inelastic scattering of polarized leptons on polarized nucleons, the past decade of experimental progress, and future programs to measure the polarized gluon contribution to the proton spin.

Exploring the polarization of gluons in the nucleon

“…The field of QCD spin physics has been driven by the hugely successful experimental program of polarized deeplyinelastic lepton-nucleon scattering (DIS) [1]. One of the most important results of this program has been the finding that the quark and anti-quark spins (summed over all flavors) provide only about a quarter of the nucleon's spin, ∆Σ ≈ 0.25 in the proton helicity sum rule [2,3,4] …”

“…Figure 1 shows a LO toy calculation of the Q 2 -evolution of the contributions to the proton spin in Eq. (1), assuming that at an initial scale Q 0 = 1 GeV we have ∆Σ = 0.25, ∆G = L q = 0.2, L g = −0.025. The rise of ∆G ∝ log(Q 2 ) or 1/α s (Q 2 ) (compensated by an opposite evolution of L g ) is an important prediction of QCD and awaits experimental testing.…”

High Precision Measurement of the Proton Elastic Form Factor Ratio at Low Q²

“…This result can be related to the failure of quarks spin to account for the total angular momentum of the proton and the expected importance of quark orbital angular momentum [184].…”

Measurement of the neutron (³He) spin structure functions at low Q²: A connection between the Bjorken and gerasimov-drell-hearn sum rule

“…The importance of QCD sum rules is that they are model-independent. The Bjorken sum rule relates the first moments of [10]. The errors for Γ p 1 − Γ n 1 are statistical, systematic, and theoretical.…”

“…In an attempt to "salvage" the sum rule, he chose to emphasize an inequality involving unpolarized cross sections instead [7]. Nowadays, with the aid of rapid technological advances in both polarized sources and targets, the Bjorken sum rule has been verified to better than 10% [10]. Experiments are always performed at finite Q 2 .…”