One of the best ways to study the internal structure of nuclei is to illuminate them with beams of very high-energy electrons and study the pattern of electrons that are scattered from the target material. Past experiments at SLAC using energies up to 25 GeV have indicated that nucleons (neutrons and protons) are composed of tightly bound systems of pointlike charged constituents, called quarks. The high-energy, high-intensity electron beams from the SLAC linac, together with the spectrometers in End Station A, are ideal tools for studying the properties of quarks in nuclei.
The End Station A experiments of the past have provided important and exciting results. As mentioned previously, they proved the existence of quarks in the nucleon. Today, a program continues in End Station A that augments these early experiments and uses new techniques to extend the work. In recent times, polarized beams and targets have made new experiments possible. Advances in detector technologies and in computing have opened new experimental approaches. Physicists are beginning to take advantage of these new developments.
Recent experiments in the End Station A use polarized beams and polarized targets to study the spin substructure of the proton and neutron. When electrons at high energy scatter from the innards of a proton or neutron, they interact with the quarks. When the quarks are polarized, the difference in spin-aligned scattering versus anti-aligned scattering can be used to evaluate the quark spin. Current experiments study three targets, the proton, the deuteron, and He 3 nuclei. The latter two can be used to extract information about the neutron. Studies of the spin structure of the proton and neutron are continuing in End Station A.
A future experiment being planned at this writing will investigate the interaction between high-energy electrons from the linac scattering on stationary electrons from a liquid hydrogen target. This interaction will use the polarized electron beam at high energy, 50 GeV, scattering from the unpolarized electrons in the hydrogen atom.
A change in the scattering when the incoming polarization is reversed
tells us about the weak interactions between electrons that scatter at
high energies. This process is purely “leptonic,” and has precise predictions
from theories about the nature of the forces involved. A precision measurement
of this scattering is a clean test of our theoretical understanding of
the electroweak forces, and potentially a window on new processes at high
energies.