With completion of a nearly decadelong upgrade of the Thomas Jefferson National Accelerator Facility’s electron accelerator, researchers now have the capability to analyze quark confinement in detail and to obtain three-dimensional views of the structure of protons and neutrons.
The $338 million upgrade of the lab’s Continuous Electron Beam Accelerator Facility (CEBAF), which began in 2008 and concluded last month, includes a doubling of its energy to 12 GeV and the addition of a fourth experimental hall. The accelerator will be capable of delivering simultaneously a 12 GeV beam to the new hall, which will be devoted to studying quark confinement, and 11 GeV beams to the three other halls. The overall scientific output should increase by 25% when the machine is running, says Allison Lung, director of the upgrade project and chief planning officer for the Newport News, Virginia, lab.
The accelerator’s electron beams are steered into gaseous, solid, or liquid targets that can be as light as hydrogen or as heavy as gold. Some of the electrons penetrate deep into nuclei and collide with quarks. Spectrometers measure the energy and trajectory of particles created by the collisions. Among electron accelerators, CEBAF’s combination of high luminosity and high polarization in continuous-wave electron beams is unique in the world, says Lung.
Although the improvements to CEBAF’s tandem linear accelerators were completed in 2014, the Department of Energy did not declare the entire upgrade project to be finished until the commissioning of the fourth experimental hall last month. The major components of the upgrade were 10 superconducting RF cavities, or cryomodules, that will impart the same amount of energy to the electron beams as did the machine’s original 40 units.
Lung says that 12 GeV is the “must-have energy” to convincingly prove the existence of exotic particles dominated by gluons, which exchange the strong force between quarks. (It’s also the highest achievable energy within the existing accelerator ring.) The energy is sufficient to perform 3D tomography of the structure of protons and neutrons and to quantitatively investigate the mechanism behind quark confinement for the first time. Although quarks are assumed not to exist in isolation, free quarks are not excluded by the standard model, Lung notes.
In its original 6 GeV configuration, CEBAF allowed researchers to study the forces that bind nucleons together. Physicists took precision measurements of parity violation in electron scattering, setting tight upper constraints on the contribution of strange quarks. And they tested the standard model’s completeness with high-precision measurements at low energies, which had the potential to reveal signatures of new forms of matter.
The lab expects to operate CEBAF for 12 weeks during fiscal year 2018, says Lung. That’s two weeks longer than was specified in the Trump administration’s budget request but less than half the 30 weeks of experimental time that was typical for the machine prior to the upgrade. Maintenance normally requires 20 weeks or more annually. But refurbishments to the lab’s aging infrastructure, particularly to its cryogenic plant, are expected to necessitate a longer downtime this year.
The laboratory’s user community has grown from 1200 in the pre-upgrade era to more than 1500 today. Thus far, 78 experiments have been approved for running on the upgraded CEBAF. Of those, four experiments have already been completed, having run during the accelerator’s commissioning and ramp-up.
CEBAF is one of three major DOE nuclear-physics experimental facilities. The others are the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in New York and the Argonne Tandem Linear Accelerator System in Illinois. Construction of a fourth, the Facility for Rare Isotope Beams, is expected to be completed in 2022 at Michigan State University.
Editor’s note, 19 October: The article was clarified to more accurately describe the CEBAF research on quarks.
