Figure 2. The basic elements (degrees of freedom) that atomic nuclei are made of
depend on the energy of the experimental probe and the distance scale. The building
blocks of quantum chromodynamics are quarks and gluons, which are lurking inside
mesons and baryons. In low-energy nuclear physics experiments, nuclei can be well
described in terms of individual protons and neutrons, their densities and currents, or—
for certain nuclear excitations—collective coordinates describing rotations and
vibrations of the nucleus as a whole. Major theoretical approaches to the nuclear
many-body problem (lattice QCD, ab initio models, configuration interaction
techniques, nuclear density functional theory, and collective model) are marked at
different resolution scales (adopted from [5]