Experimental Hall B
Jefferson Lab has four experimental halls. The smallest of these is Hall B, measuring roughly 98 ft in diameter and 65 ft from floor to ceiling. From 1995 to 2012, the heart of the Hall B physics program involved the use of a particle detector system known as “CLAS,” an acronym that stands for the CEBAF Large Acceptance Spectrometer. This magnetic spectrometer was based on a superconducting toroid and measured the trajectories of charged particles created in interactions of the beam with a fixed target to determine their momenta. The system was designed for use with beams of electrons or photons with energies up 6 GeV and covered nearly the full angular range about the beam-target interaction region. The full detector system was instrumented with specialized subsystems that together allowed for the full reconstruction and characterization of nuclear reactions with multiple particles in the final state.
The Hall B physics program with CLAS covered a broad range of topics. The major research programs included experiments to measure the spectrum of excited states of the nucleon to understand nuclear structure and quark confinement, to perform three-dimensional imaging of the quark structure of the nucleon, and to characterize nucleon-nucleon correlations in matter. Each of these programs was designed to address fundamental questions regarding the structure of matter. Other major programs in Hall B were carried out that did not rely on the CLAS detector. These included the PRAD experiment that measured the charge radius of the proton and the HPS experiment that began a search for so-called heavy photons that may be linked to dark matter annihilation or decay.
The research program using data collected with the CLAS detector has resulted in more than 200 papers published by the team of roughly 200 physicists that make up the CLAS Collaboration. The members of this collaboration represent more than 60 institutions in nearly 20 countries spanning 6 continents. This wealth of published data has resulted in hundreds of additional papers by phenomenologists and theorists, as work progresses to use the experimental data collected to test, develop, and hone our understanding of strongly interacting particles in nature. As well, an important function of the scientific program at Jefferson Lab is the training of the next generation of physicists. Based on data collected with CLAS, more than 170 graduate students have earned their Ph.D.s to date, with an additional 40 more still in progress.
In the period from 2012 to 2017, Jefferson Lab underwent a significant facility upgrade that doubled the maximum energy of its electron beam from 6 GeV to 12 GeV. As part of this energy upgrade, CLAS was replaced with the new CLAS12 spectrometer. The infrastructure and the detector subsystems for CLAS12 were mainly funded by the U.S. Department of Energy, however, significant contributions were also provided by our domestic and foreign collaborators.
The CLAS12 spectrometer was designed with similar principles and capabilities as the original CLAS spectrometer, but with two new superconducting magnets (a toroid and a solenoid) and new detectors to allow study of nuclear reactions with electron beams at energies up to 11 GeV (the maximum possible for Hall B). The new CLAS12 spectrometer allows for the detection and reconstruction of charged and neutral particles created in the beam interactions using a variety of nuclear targets. As was the case for CLAS, the CLAS12 detector, also a large acceptance system, covers nearly the full angular range surrounding the beam-target interaction region.
At the end of 2017, the final elements of the detector subsystems for CLAS12 were installed and tested. The completed CLAS12 detector then began a two-month period of commissioning and testing to fully understand and characterize its performance. Beginning in the spring of 2018, the physics program in Hall B will officially commence. The existing approved physics program with CLAS12 already consists of more than 35 experiments that will take roughly 10 years to complete. The broad physics program with CLAS12 will employ electron beam energies of 6.6 GeV, 8.8 GeV, and 11 GeV with both polarized beams and targets. The program includes nucleon spectrum and structure studies, precision measurements of the quark distributions within the nucleon, and searches for exotic forms of matter. With new opportunities will come new ideas to pursue our scientific mission in new and exciting ways, and the physics program in Hall B will continue to grow. The promise of the research program in Hall B will allow for fundamental investigations into the nature of matter that will continue to focus and shape our understanding of the universe from the subatomic scale to the scale of the universe.
For more detailed information about Hall B and its experimental program, click here.