Scientific Achievements Since the 1989 Long Range Plan

The past five year period opened with the issue of the ``proton spin crisis'', where measurements of the spin structure function by the European Muon Collaboration at CERN [As88] suggested that the fraction of the proton's spin carried by the quarks is significantly smaller than expected. This conclusion was based on the assumption of SU(3) flavor symmetry and the validity of the Bjorken sum rule [Bj66]. With the EMC data and these assumptions, it was concluded that the strange sea quarks must have a polarization opposite to that of the up quarks in the proton. These startling results spurred additional experiments at CERN: SMC [An93], SLAC: E142 [An93a],SLAC: E143 [Ab95], and DESY: HERMES [HE90] primarily to test the Bjorken sum rule and to confirm the original EMC discovery. In addition, there has been rapid development in theoretical analysis of the spin structure measurements.

The primary observable in these deep inelastic scattering experiments is the asymmetry, , in polarized lepton scattering from a polarized proton, deuteron or He target. The SLAC E142 target provided the first measurement for , the spin structure function, for the neutron; while SMC and E143 provided improved results for the proton as well as data for the neutron. The data for are shown in Figure . The most striking conclusion from the early EMC experiment at CERN was that very little of the proton spin is carried by the quarks, and that it was likely that the strange quark sea is polarized.

Available results from SMC, E142 and E143 together with -dependent analyses indicate that the fraction of the proton spin carried by the quarks is approximately as summarized in Figure . This figure shows the average spin fraction deduced from both the proton and the neutron data. In addition, it was found that the Bjorken sum rule is good to approximately . The experimental tests of the Bjorken sum rule are summarized in Fig . It is noted also in the figure that the Ellis-Jaffe sum rule [Ja74] is violated. Based on this violation, one now expects that the strange sea has a polarization of approximately .

It is now believed that the valence quarks carry the expected spin fraction of the nucleons, but that the sea quarks do not. Thus, the low- region is very important for future studies. Issues raised by these studies are the dependence of and consequently that of the spin structure function, and the flavor composition of the spin structure. Also, there is the question of the low- extrapolation in determining the integral of . In addition, it is expected that the orbital angular momentum of the quarks and the spin of the gluons might carry the "missing" spin fraction of the proton. All these considerations are central to the new round of experiments at SLAC, CERN and DESY during the next five years.

The SLAC experiments made use of innovative technology in pursuing the spin structure function measurements. The E142 polarized He target was a large volume, double-celled, spin-exchange target which set the standard for highly-polarized dense targets for use in electron scattering experiments. Experiment E143 made use of state of the art highly polarized NH3 and ND3 targets which could handle relatively high beam currents (100 nA). In addition, E143 was the first experiment to record data with a high electron polarization, near , from a strained GaAs crystal.

Although the DESY experiment is not underway, during this period, the DESY and HERMES groups established that longitudinally polarized electrons with a polarization of approximately can be achieved readily at HERA. The HERMES detector is near completion and the He target will be installed first to gain the much-needed data for the neutron. It is expected that approximately 4000 hours of beam time per year, in a parasitic mode, will be available to HERMES. The HERMES experiment is rather innovative technically in that it makes use of an internal polarized target concept. A laser-driven polarized He target, employing technology developed at MIT/Bates and Caltech for experiments at MIT/Bates including the future program of internal target spin physics and used to probe the ground state structure of He in the IUCF Cooler Ring [Mi95], will be used in HERMES. The laser-driven H/D targets being developed at Argonne and Illinois [Co92] also are expected to play an important part in the HERMES experiment.