An important goal of Jefferson Lab is to provide a detailed, three-dimensional picture of the nucleon in terms of its quark and gluon constituents, and to understand how this complex structure leads to its well known properties such as mass, spin and magnetic moment. A promising theoretical framework for this task is provided by generalized parton distributions (GPDs), which are hybrids of the usual form factors and parton distributions, but in addition include correlations between states of different longitudinal and transverse momenta.
The Electroweak Standard Model (SM) has to date been enormously successful. The search for a fundamental description of nature which goes beyond the SM is driven by two complementary experimental strategies. The first is to build increasingly energetic colliders, such as the Large Hadron Collider (LHC) at CERN, to excite matter into a new form.
Protons and neutrons are complex bound states of quarks and gluons, held together by the strong interactions of quantum chromo dynamics (QCD). Their structure may be modified inside of the dense environment of a nucleus, and such modification of hadron properties in the nuclear environment is of fundamental importance in understanding QCD. Measurements of deep inelastic scattering in nuclei show that the quark distributions in heavy nuclei are not simply the sum of the quark distributions of the constituent protons and neutron, as one might expect for a weakly bound system.
One of the many success stories of JLab's resonance physics program has been what we have learned about the Delta resonance, which is the lowest energy quantum excitation of the nucleon. There are several ways the nucleon can be electromagnetically excited to the Delta. One, denoted M1, or magnetic dipole moment, gives us information about the distribution of the quarks' electric current within the nucleon and Delta. Another, denoted E2, or electric quadrupole moment, describes the deviation from sphericity of the quarks' electric charge distribution.
The newly upgraded Jefferson Lab CEBAF Accelerator opens door to strong force studies.
The Science
Scientists have been rigorously commissioning the experimental equipment to prepare for a new era of nuclear physics experiments at the newly upgraded Continuous Electron Beam Accelerator Facility (CEBAF) at Jefferson Lab in Newport News, Va. These activities have already led to the first scientific result, which demonstrates the feasibility of detecting a potential new form of matter.
The strength of the strong force is set by the value of its coupling αs. At small distances, much smaller than a fermi (1 fermi = 10-15m, about the size of a proton), αs is small and the strong force can be studied with the standard methods of perturbation theory. This discovery by David J. Gross, H. David Politzer and Frank Wilczek was acknowledged by the 2004 Nobel Prize in Physics.
Jefferson Lab has become the acknowledged world leader in the development of an innovative use of electron linear accelerators (linacs) in light sources and, potentially, particle colliders: the energy-recovering linac, or ERL. Jefferson Lab built the first ERL with high average current to drive the first kilowatt-scale free-electron laser.
Generalized parton distributions (GPDs) unify the concepts of the nucleon elastic form factors (measured in elastic eN scattering) and parton distributions (measured in inclusive deep-inelastic eN scattering). GPDs describe the form factors for the emission and absorption of a quark/gluon by a fast-moving nucleon, depending on the quark's longitudinal momentum fraction, and the invariant momentum transfer to the nucleon, t.
For the first time, Jefferson Lab’s Continuous Electron Beam Accelerator Facility delivered beams for experiments simultaneously to all four of its experimental halls: Halls A, B and C, as well as its newest hall, Hall D.
Jefferson Lab continues to develop innovative solutions to problems shared by the accelerator community. A common challenge in accelerators based on superconducting radiofrequency technology (SRF) is the presence of additional radiofrequency waves inside the accelerator cavities, in addition to the primary frequency needed for accelerating particles. These so-called higher-order modes (HOMs) can seriously degrade beam quality.
