Spin-dependent parton distributions of the nucleon

Contributor: S. Pate.

The polarized distributions $(\Delta u(x) + \Delta\bar{u}(x))$ and $(\Delta d(x) + \Delta\bar{d}(x))$ of the nucleon have been measured at HERMES, and will be published soon. With the particle identification upgrade now complete, HERMES will additionally measure $(\Delta s(x) + \Delta\bar{s}(x))$ over the next two years, while improving the precision of the $(\Delta u(x) + \Delta\bar{u}(x))$ and $(\Delta d(x) + \Delta\bar{d}(x))$ values. Further, HERMES will perform an integral measurement of $\Delta G/G$ to $\pm30$% over the next two years. In a complimentary spin program, the Relativistic Heavy Ion Collider (RHIC) will provide a measurement of the distribution $\Delta G/G(x)$ by approximately 2002, and later measure the polarized quark and anti-quark distributions.

Items identified as needing theoretical study include:

• Lattice calculations, including sea quarks, addressing the spin problem.
• Phenomenological approaches to the nucleon spin problem.
• Comparison of the estimates of the gluon distribution $\Delta G_{NLO}(x)$ from the next-to-leading order analysis of the HERMES $\Deltaq(x)$ with the expected RHIC measurement $\Delta G_{EXP}(x)$.
• Outline of the theoretical uncertainties to the determination of $\Delta G$ from the experimental asymmetries proposed.