Long Range Structure of the Nucleon

Many physicists have the following picture of the structure of the nucleon: In the inner region at “short range” reside quarks bound by gluons, in the outer region at “large distances” live mesons and in particular pions. The nucleon consists of constituents (i.e., “constituent quarks” and pions), as the atom consists of a nucleus and electrons, and the nucleus of protons and neutrons. The range of the inner “colored” region, frequently called “confinement radius,” is rather elusive. It is a model parameter in the old bag model or models for the nucleon resonances based on the constituent quarks. One can estimate it from experiment by identifying it with the “annihilation radius” of the antiproton–proton system. Only quark–antiquark annihilation in the overlap region of color can contribute to annihilation [2 Povh, B. and Walcher, Th. 1986. Comments Nucl. Part. Phys., 16: 85 [Google Scholar]]. It amounts to approximately 0.8 fm, in reasonable agreement with the mentioned model parameters. However, it cannot be easily identified with the root-mean-square (rms) radius of the electric charge of the proton, since the pion cloud will contribute to the charge distribution. A rough idea of the range of this contribution may be gotten by the Compton wave length of the pion which is of the order of 1.4 fm.