Finite Size Effects and the Mechanism of Nucleon-Nucleon Attraction

The kinetic energy of a particle confined in a finite volume decreases with increasing volume. In the covalent bonding, the delocalization of electrons exchanged between atoms of the molecule is equivalent to an increase in their localization volume compared to their localization volume in the individual atoms. This results in a decrease in their kinetic energies, which is at the origin of the attraction mechanism of the atoms in the molecule. The quantum mechanical approach to the covalent bonding (Valence Bond Picture) used to describe the delocalization of the two exchanged electrons in the hydrogen molecule is used to describe the delocalization of the two exchanged pions in the nucleon-nucleon system. In particular, the increase in the average localization radius of each pion in the nucleon-nucleon system compared to its average localization radius in an individual nucleon is calculated. This increase in average localization radius is correlated with the Lattice QCD data on the decrease in the residual mass of a pion confined in a finite volume with increasing volume. The decrease of the residual masses of delocalized pions in the nucleon-nucleon system compared with their residual masses in individual nucleons is at the origin of the mechanism of nucleon-nucleon attraction. An expression for the central nucleon-nucleon potential due to two-pion exchange is obtained.