First seven years of physics with CEBAF
Jefferson Lab staff and users reviewed the first seven years of physics with the Continuous Electron Beam Accelerator Facility, CEBAF, during a symposium held in conjunction with the 2003 JLab User Group meeting
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Jefferson Lab staff and users reviewed the first seven years of physics with the Continuous Electron Beam Accelerator Facility, CEBAF, during a symposium held in conjunction with the 2003 JLab User Group meeting.
The unique design of this Department of Energy physics research facility's electron accelerator allows three experimental halls to be operated simultaneously with a total beam current of 200 uA (microamps) and a beam polarization of up to 80 percent. CEBAF, located in Newport News, Va., has a user community of more than a thousand scientists from 187 institutions in 20 countries. As of summer 2003, 81 nuclear physics experiments have been completed with substantial data taken on 23 more. From the data obtained in these experiments, more than 250 refereed journal articles and 146 doctoral degrees have been awarded. For the near future, there are over 60 experiments planned and 128 more Ph.D. theses in progress.
To recognize and review the accomplishments of Jefferson Lab while also looking toward the future, the JLab User Group Board of Directors organized the symposium which was held June 11 through 13 and dedicated to the memory of Nathan Isgur, Jefferson Lab's first Chief Scientist. The meeting was divided into eight physics topics: nucleon form factors, few-body physics, reactions involving nuclei, strangeness production, structure functions, parity violation, deep exclusive reactions, and hadron spectroscopy. Each topic was presented by one experimentalist and one theorist.
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The symposium began with presentations on the nucleon form factors given by Donal Day, University of Virginia, and John Ralston, University of Kansas. The nucleon form factors probe the electromagnetic structure of the proton and neutron. The presentations included discussion of the most referenced and surprising result from Jefferson Lab, that the proton's form factors do not follow an expected simple relation. While theorists have proposed different models to explain this result, the basic ingredient in almost all new models is the addition of relativistic effects.
From the discussion of the nucleon, the talks continued with presentations focusing on few-body systems, such as the deuteron and 3He, by Paul Ulmer, Old Dominion University, and Franz Gross, College of William and Mary. Typically for these experiments, the Jefferson Lab electron beam is used to knock out a proton from the few-body system or to probe it with elastic scattering. The expected yield of these experiments can be calculated exactly, assuming nucleons and mesons are the underlying particles. The presentations showed that even with beam energies of up to 5.7 GeV (billion electron volts), the electron scattering results are surprisingly well explained by the nucleon-meson models to distance scales of order 0.5 fm (femtometers). In contrast, experiments on deuteron photodisintegration, which probe even smaller distance scales, have revealed clear evidence of the limitations of the nucleon-meson models and of the onset of quark-gluon degrees of freedom.
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For reactions involving nuclei, i.e. many-body systems such as oxygen and carbon, statistical methods in the context of the nucleon-meson picture are used to calculate the expected yields of the quasi-elastic reaction. Larry Weinstein, Old Dominion University, presented a talk titled "So Where Are the Quarks?" in which he showed that the nucleon-meson model describes even the highest momentum transfer Jefferson Lab data, while Misak Sargsian, Florida International University, spoke mostly of the future, when the quark-gluon nature of matter should become evident from experiments with a 12 GeV electron beam.
Reinhard Schumacher, Carnegie Mellon University, and Steve Cotanch, North Carolina State University, presented reactions involving strangeness production which includes the production of particles such as kaons. The presenters showed [at the time] new Jefferson Lab data confirming the Theta+ particle as discovered by SPring-8 in Japan. This exotic particle is comprised of five quarks and has been dubbed the pentaquark. This had been described as the first observed nucleon resonance comprised of more than three valence quarks and has sparked international excitement. (A Jefferson Lab experiment to further study this new particle ended in spring 2004, and the data is currently being analyzed).
Keith Griffioen, College of William and Mary, and Wally Melnitchouk, Jefferson Lab presented structure function experiments, which provide information on the quark and gluon structure of the nucleon. While CEBAF's beam energy is relatively low for this type of experiment, the high luminosity that is available has allowed many high precision structure function results to be produced. An interesting feature of the Jefferson Lab data is that if one scales the smooth deep inelastic cross section results from high energy physics to the kinematics, the scaled results will pass through the average of the resonant peaks of the JLab data.
This effect, known as duality, may lead to a better understanding of how the underlying quarks and gluons link to the nucleon-meson models.
Krishna Kumar, University of Massachusetts, and Michael Ramsey-Musolf, California Institute of Technology, presented the parity violation experiments, where the strange quark distributions in the proton can be extracted by measuring the extremely small asymmetry in the elastic scattering of polarized electrons from an unpolarized proton target. One series of these experiments has already been completed at Jefferson Lab and several more are planned, including the G0 (G-Zero) experiment and HAPPEX-II (Hall A Proton Parity Experiment) experiments scheduled for next year.
Deep exclusive reactions, experiments done in deep inelastic kinematics but where the detection of multiple particles allows the final state of the system to be determined, were presented by Michel Garçon, SPhN/Saclay (France), and Andrei Belitsky, University of Maryland. Generalized parton distribution models, which should enable a complete description of the nucleon's quark and gluon distributions to be extracted from this type of data, were presented along with the results from the HERMES (Germany) and Jefferson Lab deeply virtual Compton scattering experiments. The results indicate that generalized parton distributions can be extracted from this type of data. Several high-precision experiments are planned for the coming years.
Steve Dytman, University of Pittsburgh, and Simon Capstick, Florida State University, presented the wealth of hadron spectroscopy data that is coming from Jefferson Lab. The analysis of the vast set of data produced by JLab on the nucleon resonances has only been partially completed but hints of new states are already emerging, while the work of doing a full partial-wave analysis of the data is getting underway.
Following the physics presentations, Larry Cardman, Jefferson Lab's Associate Director for Physics, presented the long-term outlook for the Lab. This talk focused primarily on upgrading CEBAF to a 12 GeV machine and building a fourth experimental hall. The higher energy would allow Jefferson Lab to continue its mission of mapping out the transition from the low-energy region where matter can be thought of as made of nucleons and mesons to the high-energy region that reveals the fundamental quark and gluon nature of matter.
Copies of these presentations can be found at: www.jlab.org/div_dept/physics_division/talks/Users_meeting_2003.html.
Scheduled for the 2004 JLab Users' Group Workshop and Annual Meeting, June 16-18, the program will highlight: The Next Seven Years: Workshops on the JLab near term physics program. The workshop will feature one plenary session on pentaquark physics, and double track parallel workshop sessions. Each parallel session will consist of an overview talk followed by a number of specialized presentations and discussions. For additional information, visit: http://www.jlab.org/ugm04/index.html
by Douglas Higinbotham
Staff Scientist