Quark-Hadron Duality

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Results of the JLab experiments are shown for the F2 (top) and FL (bottom) structure functions, which describe different aspects of proton structure. Data from previous CERN (NMC) and SLAC experiments are also indicated. The peaks in the JLab data reflect scattering off a composite hadron, whereas the curves reflect scattering off point-like quarks in relative isolation.

Some three decades after the inception of Quantum Chromodynamics (QCD) as the accepted theory for strong interactions, mysteries remain. At high energies, the property of QCD known as asymptotic freedom, in which quarks interact very weakly at short distances, allows for an efficient description of observables in terms of quarks and gluons — or partons. In contrast, at low energies the effects of confinement impose a more efficient description in terms of collective degrees of freedom, the physical mesons and baryons — or hadrons.

Despite this apparent dichotomy, scientists have observed a striking similarity between data measured at high and low energies. This is referred to as "quark-hadron duality." Data accumulated in Hall C at JLab have shown that this quark-hadron duality phenomenon occurs at much lower values of momentum transfer, in more observables, and in far less limited regions of energy than hitherto believed. These studies provide vital clues to the long-standing challenge of QCD to describe the forces at large distances, comparable to the size of hadrons (~1 fm).

W. Melnitchouk, R. Ent and C.E. Keppel, Physics Reports 406 (2005) 127
F. E. Close, Physics World, October 2000, 28
D. Higinbotham, CERN Courier, December 2004