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Jefferson Lab was created in 1984 and started operating in about 1996. 2011 is an appropriate time to try to take a look at the results that have appeared, what has been learned, and what has been exciting for our scientific community.

Rather than attempt to construct a coherent view with a single author or at least a small number, we have, instead, invited small groups of people who have been intimately involved in the work itself to make contributions. These people are accelerator experts, experimentalists and theorists, staff and users. We have, in the main, sought reviews of the actual sub-fields. The primary exception is the first paper, which sets the scene as it was, in one person's view, at the beginning of Jefferson Lab.

In reviewing the material as it appeared, I was impressed by the breadth of the material. Major advances are documented from form factors to structure functions, from spectroscopy to physics beyond the standard model of nuclear and particle physics. Recognition of the part played by spin, the helicities of the beams, the polarizations of the targets, and the polarizations of final state particles, is inescapable. Access to the weak interaction amplitudes through measurements of the parity violating asymmetries has led to quantification of the strange content of the nucleon and the neutron radius of lead, and to measurements of the electroweak mixing angle.

Lattice QCD calculations flourished and are setting the platform for understanding of the spectroscopy of baryons and mesons. But the star of the game was the accelerator. Its performance enabled the physics and also the use of the technology to generate a powerful free electron laser. These important pieces of Jefferson Lab physics are given their place.

As the third Director of Jefferson Lab, and on behalf of the other physicists and others presently associated with the lab, I would like to express my admiration and gratitude for the efforts of the directors, chief scientists, associate directors, physicists, engineers, technicians and administrators who made it all possible. In sum, we should celebrate the science that Jefferson Lab has realized in this, its first long decade of physics.

Hugh Montgomery, Director

All papers published in this volume of Journal of Physics: Conference Series have been peer reviewed through processes administered by the proceedings Editors. Reviews were conducted by expert referees to the professional and scientific standards expected of a proceedings journal published by IOP Publishing.

This chapter is a personal account of the initial planning and competition for a new laboratory, which eventually became known as the Thomas Jefferson National Accelerator Facility, with the official nickname "Jefferson Lab." The period covered starts as far back as 1964, with the introduction of quarks, and extends up to the late 1980s after the initial team was assembled, the superconducting design was in place, and construction was well underway. I describe some of the major experiments that were proposed to justify the laboratory, reflect on the present status of those initially proposed experiments, and very briefly outline some of the new ideas that emerged after the laboratory was constructed. The science is presented in a simple manner intended for a lay audience, with some of the ideas illustrated by cartoons that were often used in popular lectures given during this period.

The charge and magnetization distributions of the proton and neutron are encoded in their elastic electromagnetic form factors, which can be measured in elastic electron–nucleon scattering. By measuring the form factors, we probe the spatial distribution of the proton charge and magnetization, providing the most direct connection to the spatial distribution of quarks inside the proton. For decades, the form factors were probed through measurements of unpolarized elastic electron scattering, but by the 1980s, progress slowed dramatically due to the intrinsic limitations of the unpolarized measurements. Early measurements at several laboratories demonstrated the feasibility and power of measurements using polarization degrees of freedom to probe the spatial structure of the nucleon. A program of polarization measurements at Jefferson Lab led to a renaissance in the field of study, and significant new insight into the structure of matter.

The simplest models might describe the nucleon as 3 light quarks, but this description would be incomplete without inclusion of the sea of glue and q pairs which binds it. Early indications of a particularly large contribution from strange quarks in this sea to the spin and mass of the nucleon motivated an experimental program examining the role of these strange quarks in the nucleon vector form factors. The strangeness form factors can be extracted from the well-studied electromagnetic structure of the nucleon using parity-violation in electron-nuclear scattering to isolate the effect of the weak interaction. With high luminosity and polarization, and a very stable beam due to its superconducting RF cavities, CEBAF at Jefferson Lab is a precision instrument uniquely well suited to the challenge of measurements of the small parity-violating asymmetries. The techniques and results of the two major Jefferson Lab experimental efforts in parity-violation studies, HAPPEX and G0, as well as efforts to describe the strange form factors in QCD, will be reviewed.

Over the past decade measurements of unpoiarized structure functions with unprecedented precision have significantly advanced our knowledge of nucleon structure. These have for the first time allowed quantitative tests of the phenomenon of quark-hadron duality, and provided a deeper understanding of the transition from hadron to quark degrees of freedom in inclusive scattering. Dedicated Rosenbluth-separation experiments have yielded high-precision transverse and longitudinal structure functions in regions previously unexplored, and new techniques have enabled the first glimpses of the structure of the free neutron, without contamination from nuclear effects.

Spin-dependent observables have been a powerful tool to probe the internal structure of the nucleon and to understand the dynamics of the strong interaction. Experiments involving spin degrees of freedom have often brought out surprises and puzzles. The so-called "spin crisis" in the 1980s revealed the limitation of naive quark-parton models and led to intensive worldwide efforts, both experimental and theoretical, to understand the nucleon spin structure. With high intensity and high polarization of both the electron beam and targets, Jefferson Lab has the world's highest polarized luminosity and the best figure-of-merit for precision spin structure measurements. It has made a strong impact in this subfield of research. This chapter will highlight Jefferson Lab's unique contributions in the measurements of valence quark spin distributions, in the moments of spin structure functions at low to intermediate Q², and in the transverse spin structure.

The goal of the comprehensive program in deeply virtual exclusive scattering at Jefferson Lab is to create transverse spatial images of quarks and gluons as a function of their longitudinal momentum fraction in the proton, the neutron, and in nuclei. These functions are the generalized parton distributions (GPDs) of the target nucleus. Cross section measurements of the deeply virtual Compton scattering (DVCS) reaction ep → epγ in Hall A support the QCD factorization of the scattering amplitude for Q² ≥ 2 GeV². Quasi-free neutron-DVCS measurements on the deuteron indicate sensitivity to the quark angular momentum sum rule. Fully exclusive H(e,e'pγ) measurements have been made in a wide kinematic range in CLAS with polarized beam, and with both unpolarized and longitudinally polarized targets. Existing models are qualitatively consistent with the Jefferson Lab data, but there is a clear need for less constrained models. Deeply virtual vector meson production is studied in CLAS. The 12 GeV upgrade will be essential for for these channels. The ρ and ω channels reactions offer the prospect of flavor sensitivity to the quark GPDs, while the ϕ-production channel is dominated by the gluon distribution.

Lattice gauge theory provides our only means of performing ab initio calculations in the nonperturbative regime. It has thus become an increasingly important component of the Jefferson Lab physics program. In this paper, we describe the contributions of lattice QCD to our understanding of hadronic and nuclear physics, focusing on the structure of hadrons, the calculation of the spectrum and properties of resonances, and finally on deriving an understanding of the QCD origin of nuclear forces.

We discuss the results on the fundamental degrees of freedom underlying the nucleon excitation spectrum and how they evolve as the resonance transitions are investigated with increasingly better space-time resolution of the electromagnetic probe. Improved photocouplings for a number of resonant states, those for the N(1720)P₁₃ being significantly changed, have been determined and entered into the 2008 edition of the RPP. Strong sensitivity to the N(1900)P₁₃ state, listed now as a 2-star state in the same edition of RPP, has been observed in KΛ and KΣ photoproduction. None of the earlier observations of a Θ⁺₅(1540) was confirmed in a series of three Jefferson Lab high statistics dedicated measurements, and stringent upper limits on production cross sections were placed in several channels. For the four lowest excited states, the Δ(1232)P₃₃, N(1440)P₁₁, N(1520)D₁₃, and N(1535)S₁₁, the transition amplitudes have been measured in a wide range in photon virtuality Q². The amplitudes for the Δ(1232) show the importance of the pion-cloud contribution and do not show any sign of approaching the pQCD regime for Q² < 7 GeV². For the Roper resonance, N(1440)P₁₁, the data provide strong evidence for this state as a predominantly radial excitation of the nucleon as a 3-quark ground state. For the N(1535)S₁₁, comparison of the results extracted from π and η photo- and electroproduction data allowed one to specify the branching ratios of this state to the πN and ηN channels; they entered into the 2010 edition of the RPP. Measured for the first time, the longitudinal transition amplitude for the N(1535)S₁₁ became a challenge for quark models and can be indicative of large meson-cloud contributions or alternative representations of this state. The N(1520)D₁₃ clearly shows the rapid changeover from helicity-3/2 dominance at the real photon point to helicity-1/2 dominance at Q² > 0.5 GeV² confirming a long-standing prediction of the constituent quark model. The search for undiscovered but predicted states continues to be pursued with a vigorous experimental program. While recent data from Jefferson Lab and elsewhere provide intriguing hints of new states, final conclusions will have to wait for the results of the broad experimental effort currently underway with CLAS, and subsequent analyses involving the EBAC at Jefferson Lab.

We review the Jefferson Lab program concerning the interplay between hadronic and underlying quark degrees of freedom in exclusive reactions. Much of the program was initially based on predictions from perturbative QCD (pQCD) concerning scaling of reaction cross sections, helicity conservation, and asymptotic behavior of form factors. Although much of the data do not follow simple pQCD expectations, some observables scale better than expected. Generally, the underlying dynamics are best understood with nonperturbative quark models, but the elastic deuteron form factors provide an example of the success of hadronic models to high momentum transfer.

One of Jefferson Lab's original missions was to further our understanding of the short-distance structure of nuclei, in particular, to understand what happens when two or more nucleons within a nucleus have strongly overlapping wave-functions – a phenomena commonly referred to as short-range correlations. Herein, we review the results of the (e, e'), (e, e'p) and (e, e'pN) reactions that have been used at Jefferson Lab to probe this short-distance structure as well as provide an outlook for future experiments.

Chiral symmetry is one of the most fundamental symmetries in QCD. It is closely connected to hadron properties in the nuclear medium via the reduction of the quark condensate < q >, manifesting the partial restoration of chiral symmetry. To better understand this important issue, a number of Jefferson Lab experiments over the past decade have focused on understanding properties of mesons and nucleons in the nuclear medium, often benefiting from the high polarization and luminosity of the CEBAF accelerator. In particular, a novel, accurate, polarization transfer measurement technique revealed for the first time a strong indication that the bound proton electromagnetic form factors in ⁴He may be modified compared to those in the vacuum. Second, the photoproduction of vector mesons on various nuclei has been measured via their decay to e⁺ e⁻ to study possible in-medium effects on the properties of the ρ meson. In this experiment, no significant mass shift and some broadening consistent with expected collisional broadening for the ρ meson has been observed, providing tight constraints on model calculations. Finally, processes involving in-medium parton propagation have been studied. The medium modifications of the quark fragmentation functions have been extracted with much higher statistical accuracy than previously possible.

Jefferson Lab has now demonstrated ablility to test the fundamental symmetries of nature, and thereby probe for new physics beyond the Standard Model. Here we review the tremendous advances in precision parity-violation measurements with CEBAF that enable searches for new physics. This has been demonstrated with a determination of the weak charge of the proton, which is found to be in agreement with the prediction of the standard electroweak theory, and at a precision that rules out relevant new physics to the TeV scale. We also review the planned future experiments which aim to further test the electroweak theory at Jefferson Lab, including a further improvement on the proton weak charge, an ultra-precise Møller measurement, and a probe of the axial quark charges in PVDIS.

A program of hypernuclear spectroscopy experiments encompassing many hypernuclei has been undertaken in both Halls A and C using complimentary approaches. Spectra with sub-MeV resolution have been obtained for Li, B, and N in Hall A, while results from Hall C include He, B, and Al with new data still under analysis for He, Li, Be, B and V. High resolution and high precision in the determination of the single Λ binding energy at various shell levels has been the key success of these experiments using the (e,e'K⁺) reaction to produce Λ hypernuclei.

The development of superconducting radio frequency (SRF) accelerator technology for CEBAF's nuclear physics program set the stage for its application to a number of other efforts. We describe below the development of the a major advance in Free Electron Laser (FEL) Technology based on SRF linacs. The Jefferson Lab efforts achieved three orders of magnitude increase in the power delivered by FELs and firmly established the viability of energy recovering linac technology. We describe the details of the physics and engineering challenges addressed to accomplish this effort and then discuss some of the applications performed using the light source. We conclude with a look at the planned directions for the program in the future.

In the past decade, nuclear physics users of Jefferson Lab's Continuous Electron Beam Accelerator Facility (CEBAF) have benefited from accelerator physics advances and machine improvements. As of early 2011, CEBAF operates routinely at 6 GeV, with a 12 GeV upgrade underway. This article reports highlights of CEBAF's scientific and technological evolution in the areas of cryomodule refurbishment, RF control, polarized source development, beam transport for parity experiments, magnets and hysteresis handling, beam breakup, and helium refrigerator operational optimization.