Colloquium: The shape of hadrons
Date
2012Source
Reviews of Modern PhysicsVolume
84Issue
3Pages
1231-1251Google Scholar check
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This Colloquium addresses the issue of the shape of hadrons and, in particular, that of the proton. The concept of shape in the microcosm is critically examined. Special attention is devoted to properly define the meaning of shape for bound-state systems of near massless quarks. The ideas that lead to the expectation of nonsphericity in the shape of hadrons, the calculations that predict it, and the experimental information obtained from recent high-precision measurements are examined. Particular emphasis is given to the study of the electromagnetic transition between the nucleon and its first excited state, the Δ(1232) resonance. The experimental evidence is critically examined and compared with lattice calculations, as well as with effective-field theories and phenomenological models. © 2012 American Physical Society.