At the beginning of the five year period, attention was drawn to the issue
of the ground state neutral weak currents of the nucleon by the nucleon
spin crisis described in the previous section. Kaplan and
Manohar [Ka88] noted the connection between the Ellis-Jaffe sum rule
matrix elements and those measured in elastic neutrino scattering - the
axial neutral weak currents of the nucleon. As a result of these
investigations it became clear that the vector neutral weak currents of
the nucleon were essentially unconstrained by data and that there existed
the possibility of separating the contributions of ,
, and
quarks to the ordinary charge and magnetic form factors of the proton.
The contributions of the
quarks are of particular interest as they
provide the first direct information in terms of the QCD degrees of
freedom on the quark sea at large distance scales.
Within the same time frame, the results from the first parity-violating
electron scattering measurements of elastic scattering became available.
Measurments of parity violation in elastic electron scattering from
C were performed [So90] at MIT/Bates, and in quasielastic
electron scattering from
Be at Mainz [He89]. Through these and
other pioneering efforts, the necessary technological know-how has been
developed to allow a new generation of parity-violating electron scattering
experiments to be undertaken.
The first activities directed at these issues are: 1) the SAMPLE experiment [SA89] at MIT/Bates (including both hydrogen and deuterium targets) which will measure the contributions of the individual flavors to the proton magnetic moment, and 2) the use of the LSND [Lu90] at LAMPF to observe quasi-elastic neutrino scattering and measure more carefully the flavor singlet axial current at low momentum transfer. The LSND experiment has taken data which are presently being analyzed. The SAMPLE target and detector are installed and the experiment is expected to take data this year.