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University of Maryland Department of Physics 1117 John S. Toll Physics Building College Park , MD 20742


xxxxxSeptember 2006 - Issue 49


Quantum Chromodynamics
By: Paulo Bedaque

All the forces we find in Nature are ultimately due to four basic ones. Out of those four, two are very familiar: the gravitational force (responsible for the fall of Newton's apple and the motion of the planets) and the electromagnetic force (the one that moves magnets, electric motors and also the ultimate cause of all chemical and biological processes). The remaining two forces are only noticed in the microscopic realm of atomic nuclei. They are the “weak” and the “strong” nuclear forces. While the weak force is responsible for the relatively slow beta decay of certain nuclei, the strong force is the cause for the very existence of protons, neutrons and their binding in atomic nuclei, as well as the existence of a whole host of particles like hyperons, pions, and kaons. Without the strong nuclear force there would be no nuclei and, consequently, atoms and the Universe as we know it.

The theory behind the strong force was essentially figured out in the early 1970s. It turns out that all hadrons are made of other particles, called quarks and gluons, whose interactions with each other is described by a theory called Quantum Chromodynamics (QCD), named after the property of “color”, a kind of charge, similar to the electric charge, that is carried by quarks and gluons. Despite the fact that the basic laws of QCD have been known for years, its study is today in a peculiar situation. The reason we believe the theory is essentially correct is because we can use approximate methods (perturbation theory) to compute some rare high energy events and compare those calculations with the results of experiments, a feat that was worth a Nobel prize in 2004 for Gross, Wilczek and Politzer. However, we cannot as easily compute the consequences of the QCD laws for low energy phenomena, among them the binding of nuclei that originated the concept of strong forces to begin with. The mystery of nuclear forces remains unresolved.

The work of Dr. Paulo Bedaque concentrates in deriving the implications of QCD to the low energy phenomena where perturbation theory does not work. They include the origin of the nuclear forces keeping nuclei together and the fate of matter when compressed beyond the density found in nuclei, as it is supposed to occur in neutron stars. The methods used by Dr. Bedaque include analytical approaches like effective field theories and, more recently, a direct, numerical attack on the problem using lattice field theory. In lattice field theory one simulates the random quantum fluctuations of the system by random numbers generated by a computer. Physical results are attained by the averages over these random numbers, and the more computer power available, the more precise the results are. This has led Dr. Bedaque recently to use some of the most powerful parallel computers available in the world. But computer power by itself will not be enough to unravel the mystery of nuclear forces. Innovative ideas involving the simulation of unphysical theories which are easier to simulate, but rigorously connected to the real World QCD, are being pursued. Some rough results about the nuclear forces are beginning now to appear.

In addition to these QCD studies, close analogies between high density nuclear matter and the recently achieved systems of cold atoms were observed, leading Dr. Bedaque to pursue some questions of interest to the physics of cold atoms. The cross fertilization between these fields promises to be very fruitful in the future, with experiments performed in atomic traps playing the role of “quantum analogic computers” simulating other strongly interacting systems.

Dr. Bedaque is an assistant professor of Physics for the University of Maryland . He is a member of the Theoretical Quarks, Hadrons and Nuclei research group. Feel free to contact him at, .