Keith Griffioen
Professor of Physics
Department of Physics
College of William and Mary
P.O. Box 8795
Williamsburg, VA 23187-8795
Office Phone: 757-221-3537
Fax: 757-221-3540
Email: griff at physics

Ever since the discovery of quarks, high-energy electron scattering experiments have provided increasingly more accurate data on how these quarks combine to form nucleons. (neutrons and protons). Our present knowledge is extensive but incomplete. We do not yet know precisely the size of the proton, where its spin comes from, and just how three quarks with masses of a few MeV, and the massless gluons that hold them together, can make a proton with a mass of 938 MeV. The strong nuclear force is interesting and challenging because there are three different "charges", dubbed "red", "green", and "blue," instead of the single charge for the electric force. Consequently, the math is extremely difficult to solve, but the resulting proton is intriguing, complex, and surprising. My experimental research, which is primarily done using the CLAS Spectrometer at Jefferson Lab, involves three-dimensional imaging of the proton and neutron to understand how the quarks and gluons are distributed in space, in momentum, in spin orientation, and in quark flavor (up, down, and strange). Such a program involves a number of different electron-scattering and photon absorption experiments with various combinations of spin polarized beams and targets. The computational techniques for solving quantum chromodynmaics, the theory of the strong interaction, have become accurate enough so that theory and experiment can now become joint partners in understanding the proton. We hope to discover exciting surprises about the proton and neutron's internal structure in the coming years.

The MAMI electron accelerator at the University of Mainz provides beams of photons and electrons at high intensity but much lower beam energies than at Jefferson Lab. Therefore, targeted, short experiments using MAMI play a unique role in understanding the proton's structure. A measurement of the deuteron charge radius has been completed, and precision tests of the Standard Model are being developed.

From the rapid orbits of outer stars in galaxies and the strength of gravitational lensing we can quantify the dark matter in the universe, even as we have no clue as to what it might be. Many experiments are currently searching for dark matter. If dark matter interacts with its own force, the force-carrier may be like the photon we know, except massive. The HPS experiment at Jefferson Lab's Hall B, is currently searching for the dark photon.

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