The previous and subsequent chapters go into detail on TJNAF (CEBAF) because that is the project in which the author was most deeply involved and about which he is most knowledgeable. Many other accelerator laboratories have played, and continue to play, an important role in electron scattering studies of nuclei and nucleons. Worth highlighting from the early years are the Nuclear Physics Laboratory at the University of Illinois, where the betatron provided a tool to do the very first study of nuclear structure with electrons [Ly51, Il87], and the High Energy Physics Laboratory at Stanford (HEPL), where Hofstadter carried out his pioneering work on charge and magnetization densities [Ho56, Ho63]. Many other important facilities sprang from the work at HEPL, including those at Amsterdam, Darmstadt, Mainz, Saskatchewan, Tohuko, and the Saclay Laboratory, which played a particularly important role in the development of the field. The Stanford Linear Accelerator (SLAC), under Wolfgang Panofsky's inspired leadership, found its roots in HEPL, as did TJNAF. A prototype of the CEBAF superconducting accelerator was first constructed at HEPL. An excellent discussion of the early years of electron scattering is to be found in [I187].

It is the Bates Laboratory at M.I.T., where a variety of precision experiments truly demonstrated the power of electron scattering to study the nucleus, and the Stanford Linear Accelerator Center (SLAC), where high-energy experiments demonstrated the pointlike, asymptotically free, substructure of the nucleon and examined its weak neutral current, that are responsible for the role that electron scattering plays in nuclear and particle physics in the U.S. today.

The principal centers today for nuclear structure studies with electrons are TJNAF in Newport News, the Bates Laboratory at M.I.T. in Boston, and the Mainz Microtron (MAMI), in Germany.

A top priority for the field of nuclear physics in the U.S. since the late 1970's, the Thomas Jefferson National Accelerator Facility (TJNAF) was approved for construction by Congress in 1987. This project was originally called CEBAF, the Continuous Electron Beam Accelerator Facility. It came into operation in Newport News, Virginia, in 1994. The first physics results were reported at the Particles and Nuclei International Conference (PANIC) held at the College of William and Mary in Williamsburg in 1996. The experimental program at TJNAF is now fully underway, and one can look forward to a steady output of significant experimental results providing insight into the structure of hadronic matter well into the 21st century.

Some of the new experimental results from TJNAF have already been referred to in this book, and we discuss more of the anticipated program in the next chapter. In order to fully understand the future opportunities this facility provides, we present a brief overview of the existing accelerator and major experimental equipment.

There is no single feature that makes TJNAF unique; each of the characteristics has been achieved previously at one location or another. Rather, it is the combination of properties that makes TJNAF (CEBAF) the world's most powerful microscope for looking at the nucleus. A schematic of the accelerator complex at TJNAF is shown in Fig. 29.1.

The accelerator itself is in the form of a racetrack 10m underground. The basic accelerating structure is a superconducting niobium cavity fed with microwave power at a frequency of v = 1497 MHz.

We currently possess three levels of understanding of the nucleus within the following frameworks [Wa95]:

(I) Traditional, Non-Relativistic, Many-Body Systems [Fe71]. This approach uses static two-body potentials fit to two-nucleon scattering and bound-state data. These potentials are then inserted in the non-relativistic Schrödinger equation, and that equation is then solved in some approximation; with few-nucleon systems and large-scale computing capabilities, the equations can now be solved exactly. Electroweak currents constructed from the properties of free nucleons are then used to probe the nuclear system. Although this approach has had a great many successes [Bl52, Ma55, Bo69, Fe71, de74, Pr75, Fe91, Wa95], it is clearly inadequate for a more detailed understanding of the nuclear system.

(II) Relativistic Many-Body Systems. A more appropriate set of degrees of freedom for nuclear physics consists of the hadrons, the strongly interacting mesons and baryons. There are many arguments one can give for this. For example, the long-range part of the best modern two-nucleon potentials is given by meson exchange, predominantly π with (Jπ, T) = (0–, 1), σ(0+, 0), ρ(1–, 1) and ω(1–, 0) [La80, Ma89]. Furthermore, one of the successes of electromagnetic nuclear physics is the unambiguous demonstration of the existence of exchange currents, additional electromagnetic currents in the nucleus arising from the flow of charged mesons between nucleons. In addition, one daily sees copious production of mesons from nuclei in high-energy accelerators.

The only consistent theoretical framework we have for describing such a strongly-coupled, relativistic, interacting, many-body system is relativistic quantum field theory based on a local lagrangian density. It is convenient to refer to relativistic quantum field theory models of the nuclear system based on hadronic degrees of freedom as quantum hadrodynamics (QHD).

One of the primary goals of electron scattering experiments is to understand the internal structure of the nucleon, both its static and dynamic properties. Ultimately, electron scattering data will provide benchmarks against which the theoretical predictions of QCD can be compared.

Elastic scattering from the nucleon has been discussed in chapter 22. There are no discrete bound states of the nucleon as there are in nuclei, and thus excited states of the nucleon show up as resonances in particle production processes. This is analogous to the situation with giant resonances in nuclei which lie above particle emission threshold. Nucleon resonances are characterized by strong interaction widths, a typical value for which is given by the time it takes a light signal to travel a pion Compton wavelength, or Γ ≈ ħc/(ħ/mπc) ≈ mπc2 ≈ 135 MeV.

The first inelastic process on the nucleon occurs with the production of the lightest hadron, the pion. The coincidence cross section for the reaction N(e, e′ π)N follows immediately from the general analysis in chapter 13. The angular distribution in the C-M system for arbitrary nucleon helicities is given by Eq. (13.68). If the nucleon target is unpolarized and its final polarization unobserved, the angular distribution reduces to that given in Eqs. (13.71) and (F.9). The analysis of pion electroproduction starting from the covariant, gauge invariant S-matrix and reducing it to the contribution of multipoles leading to states of definite Jπ in the final π−N system is presented in detail in appendix H.