Continuous Electron Beam Accelerator Facility (CEBAF)
Description
The Continuous Electron Beam Accelerator Facility (CEBAF), at Thomas Jefferson National Accelerator Facility (TJNAF), provides high quality beams of polarized electrons that allow scientists to extract information on the quark and gluon structure of protons and neutrons. Until 2012, CEBAF operated as a pair of superconducting radio frequency linear accelerators (linacs) in a “racetrack” configuration, capable of delivering 6 GeV (Giga electron-Volt) electron beams simultaneously to three experimental halls. In 2012, the facility began a major upgrade project to double the maximum energy to 12 GeV, and add new experimental apparatus. Primary goals of the 12 GeV CEBAF Upgrade include doubling the accelerating voltages of the linacs by adding ten new high‐performance, superconducting radiofrequency (SRF) cryomodules (CMs), doubling the capacity of the existing cryogenics cooling plant, and adding eight superconducting magnets. The upgrade also includes the construction of a new experimental hall (Hall D) for dedicated research on exotic mesons produced by energetic photons incident on a target.
Science
Much of the nature of the visible matter in the universe is determined by the theory known as quantum chromodynamics, or QCD. QCD describes the interactions among its fundamental constituents—quarks and gluons—as they exist in composite particles known as hadrons, in the quark-gluon plasma, and inside nuclei. Confinement is a fundamental property of QCD which dictates that quarks and gluons normally exist only inside composite particles, making it impossible to study them in isolation. Among other predictions, QCD theorizes that the proton mass is dynamically generated by an intense field of gluons that interact amongst themselves and with the quarks. The nuclei of atoms comprise 99% of the visible mass in the universe which is conjectured to result via this mechanism.
The 12 GeV CEBAF Upgrade will provide unprecedented capability to illuminate the nature of QCD and the origin of confinement via a quantitative understanding of the internal structure of nucleons. For the first time, higher energy beams made possible by the upgrade will enable observation of heretofore unobserved exotic mesons providing a direct means to study excitations of the gluon field responsible for confinement and the dynamic generation of mass. Information needed to reconstruct tomographic images of the internal three-dimensional spatial and motional distributions of quarks, antiquarks and gluons within nucleons will also be provided, revealing in greater detail than ever before, the underlying quark-gluon dynamics. These results will occasion a major advance in our understanding of the structure and properties of nuclei based on first principles of QCD.
Additionally, in a kinematic regime complementary to RHIC, the scattering of polarized electrons off polarized targets will elucidate how the spins of the proton and the neutron are assembled from the spins and motions of their internal quark, antiquark, and gluon constituents. Short-range forces between nucleons, recently recognized as being fundamentally important for correctly predicting the structure of light nuclei, will also be quantified. Sensitive tests for evidence of particles and forces beyond the “Standard Model” of all known elementary particles and their interactions will be carried out via high statistics searches for minute violations of a fundamental symmetry of nature called “parity”. Parity-violating electron scattering “PVES” experiments will also determine with precision the thickness of the neutron "skin" outside the dense cores of heavy nuclei, illuminating the properties of the matter in neutron stars.
