Nucleon Structure and then−pMass Difference

The possibility of understanding the $n-p$ mass difference by means of electromagnetic interactions among the particles of a structured nucleon is pointed out. The success of this approach depends on the same general features that seem to be necessary in understanding the nucleon moments, namely, the enhancement of the annihilation of a ${\pi }^{+}$, ${\pi }^{-}$ pair. Thus, further confirmation is provided for the hypothesis of a pion-pion attraction. Information on the spatial extent of the pion cloud is obtained, namely that $\rho \approx 2.4$ nucleon Compton wavelengths, where ${\rho }^{-3\mathrm{}}$ is related to the average of the inverse cube of the distance between the pions and the core.