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Experimental Nuclear Physics Research

By Professor Wolfgang Korsch

I have always been fascinated by the “spins” of subatomic systems, specifically the spins of atomic nuclei and their constituents. Although a spin is an intrinsic property of a fundamental particle, like its mass or charge, Nature exhibits remarkable spin-related phenomena. For example, protons and neutrons are composed of a multitude of structureless, or point-like, particles called quarks and mediators of the strong force, called gluons. The total number of quarks and gluons is completely unknown at any instant since it can vary due to quantum fluctuations. Quarks are spin-1/2 particles and the spin of gluons is 1 (in units of hbar). Since these fundamental constituents are confined to a tiny volume, it implies that the particles are highly relativistic and therefore possess significant angular momenta which contribute to the spin of the nucleon as well.

Experimentally one can probe the internal spin structure of the nucleon by performing scattering experiments with highly energetic polarized electrons. Such experiments have been a central part of nuclear and particle physics for several decades, and together with theoretical guidance, it can be concluded that about 30% of the nucleon spin is carried by the quarks, about 20% can be associated with the gluons, and the remaining approximately 50% is due to orbital angular momentum. It should be noted that the uncertainties in the gluon spin, and orbital angular momentum contributions are still quite sizable. My latest focus has been on detailed studies of quark-gluon correlations in scattering experiments. Such correlations are closely related to the response of the gluon (color) field to the direction of the nucleon spin and are closely related to color polarizabilities, similar to the response of electric charges and currents to electromagnetic fields. This research is conducted at Jefferson Lab in Newport News, Virginia.

Another major research component is the search for a permanent electric dipole moment (EDM) of the neutron. A permanent EDM is a time-independent separation of charges along the direction of a particle’s spin. For example, in the case of the neutron, although electrically neutral, an EDM arises if a net separation of the electric charges inside the neutron is present. The conservation of fundamental symmetries of Nature — such as parity conservation or time-reversal invariance — forbids the existence of such permanent EDMs. However, since parity is not a perfect symmetry of Nature, tiny EDMs are predicted within the Standard Model of particle physics. However, measuring such a tiny contribution is far beyond the sensitivity of current experimental techniques. So, why bother? It turns out that the discovery of an EDM with anticipated technical improvements would be equivalent to the indirect discovery of new, fundamental particles which contribute to the violation of these symmetries. Indeed, these new ingredients could point to a solution of the puzzle of the cosmic excess of baryons over antibaryons. One of these new experiments is currently being constructed at Oak Ridge National Lab with the goal of improving the present limit of the neutron EDM by about two orders of magnitude. This is a multi-million dollar effort funded by the Department of Energy and the National Science Foundation. I join Drs. Crawford and Plaster in playing a major role in the experiment.

Although the execution of the above-mentioned efforts are multi-year projects and are often beyond the scope of typical Ph.D. theses, many systematic studies have to be performed which are suitable for doctoral research. Our team has developed and conducted highly sensitive experiments to study the interaction of electric and magnetic fields with spins and electric charges using magneto- and electro-optical methods. Graduates of the “Korsch group” receive broad scientific training in atomic, nuclear, particle, and low-temperature physics and have moved on to positions related to database management, improvement of atomic clocks and magnetometers, development of better systems for positron emission tomography (PET), or research on new materials for permanent magnets, to name a few.