Outlook and Open Questions

The HERMES experiment at DESY will be underway during 1995. Analyses of the SLAC and CERN experiments indicated that the dependence of is exceedingly important in deducing its contribution to the sum rules. Thus, the new experiments E154-5 [Hu93] at SLAC and the HERMES experiments are aimed at studying the dependence of for the proton and neutron as well as providing improved data for the neutron. These studies should lead to an improved test of the Bjorken sum rule.

In the near term, the new experiments will focus on the neutron. SLAC E154 will provide new data for the neutron using the spin-exchange He target and the new 50-60 GeV, polarized electron beam soon to be available in End Station A. SMC also plans another year of data collection for the polarized deuterium target. HERMES will begin operation this year with an internal polarized He target. After E154, SLAC E155 is expected to collect data for the polarized and targets.

Measurements of the structure function in the high- region are particularly difficult to perform at the high energy physics facilities, because the combination of high energy and consequently small scattering angles conspire to reduce the longitudinal polarization of the virtual photon. The high current and 6 GeV polarized beam expected at CEBAF are well-suited to provide high accuracy measurements of the neutron spin structure function at high . Plans are being made to use the spin- exchange polarized He target and the high-current, high figure-of-merit polarized beam expected at CEBAF to make these measurements.

Another goal of the CERN, SLAC and DESY experiments is to provide data for the transverse spin structure funtion, . This structure function is expected to be very sensitive to higher order QCD corrections. Although this is a very difficult measurement because is expected to be small, some data may already be available from E143 and SMC. The new measurments plan to improve the accuracy of the past work.

In addition, the HERMES detector in conjunction with the high luminosity and duty factor available at HERA will permit pion identification so that the asymmetry in can be measured. Thus, a flavor-decomposition of the asymmetry should be achieved. Some information on this process may also be available from the SLAC E154 and E155 experiments.

A particularly exciting idea is to measure the asymmetry for the reaction in order to determine the polarization of the sea. Since the has an quark structure, the detection of the leading permits tagging sea quarks directly. The addition of a laser-driven H/D target in conjunction with a RICH detector will make the experiment feasible at HERA. The RICH detector is necessary to separate pions from kaons. The laser-driven H/D target groups at Illinois, Argonne and Erlangen are actively developing the target technology. Plans for implementing a RICH detector are being discussed now and it is likely that a RICH detector will be necessary for the HERMES experiment during this five-year period.

The polarization of the up quark sea can be probed directly in polarized Drell-Yan processes. Discussions are underway to determine whether these experiments could be performed at the Fermilab main injector or at RHIC.

In order to fully understand the composition of the proton spin, it is necessary to establish the spin fraction of the proton carried by the gluons and the orbital angular momentum of the quarks. The RHIC Spin Collaboration is planning to make measurements of the longitudinal asymmetry in direct photon production from a Compton-like process in polarized colliding beams of protons at RHIC. Other methods that are under investigation are to measure the asymmetry in semi-inclusive J/ production in deep inelastic polarized electron scattering from polarized protons at SLAC (E154) and at DESY (HERMES).

Limits on the contribution to the proton spin from the quark orbital angular momentum may be set by measuring the azimuthal asymmetry in the deep inelastic scattering experiments at DESY (HERMES). These experiments are made possible by the open geometry of the detector in the forward scattering direction. Another constraint on the quark angular momentum is expected to be set from the measurement of the C2 and E2 amplitudes in the excitation of the in experiments at MIT/Bates, CEBAF and LEGS. These experiments require the development of high-figure-of-merit polarized electron beams at both MIT/Bates and CEBAF.

The Hadron Muon Collaboration (HMC) is forming at CERN in order to develop plans for a new detector which will permit efficient detection of hadrons in coincidence with the scattered muons for semi-inclusive studies. These plans should be developed during this period.