The CEBAF II/ELIC Upgrade of CEBAF
A strong physics case has been established for constructing an extremely high luminosity (∼ 10 38 cm −2 sec −1 ), CEBAF-like accelerator with energies in the 20-30 GeV range. There have also been a series of studies investigating the scientific potential of an electron-light ion collider (ELIC) operating in the 20-65 GeV center-of-mass energy range. The facility at Jefferson Lab can be upgraded to provide either (or both) of these options in a straightforward manner. An energy upgrade of CEBAF
... o 25 GeV would support extensions of the CEBAF 12 GeV program to smaller x and higher Q 2 , and, in particular, support a program of deeply virtual meson production that would permit the flavor separation of the Generalized Parton Distributions that characterize the nucleon's properties. A high-luminosity electronlight ion collider (ELIC) in the center-of-mass energy range √ s of 20-65 GeV, would build on the physics insights obtained from the CEBAF 12 GeV upgrade, and expand on our understanding of the structure of the nucleon and nuclear binding. While questions remain on the details of the science program and on technical aspects of the facility design, we expect that the facility's research program will be absolutely central to the field of nuclear physics. In particular, such a facility will provide a unique tool to: • Complete our quantitative understanding of how quarks and gluons provide the binding and the spin of the nucleon. There are still glaring gaps in our knowledge of the QCD structure of the proton. How large is the role of gluon and quark angular momentum in the description of the proton's spin? Can we fully disentangle the contribution of up, down, and strange quarks to the proton's momentum and spin? The 12 GeV Upgrade concentrates on the moderate to high-x structure of the protonthe regime where the valence structure emerges. The ELIC facility would concentrate on the small-x regime (10 −4 < x < 10 −1 ) with high luminosity, reaching the regime where gluons become prevalent. The combination of precision measurements at moderate Q 2 and high-x (from the 12 GeV Upgrade) and data at small-x and relatively large Q 2 (∼ 10 GeV 2 ) (from ELIC), using techniques ranging from "deep inelastic scattering" to "deep exclusive scattering", will permit us to disentangle the exact quark-gluon structure of the nucleon, over a wide range of x.