The Strong Force: From Quarks to Hadrons and Nuclei

Abstract

The strong force, which binds quarks and gluons together in hadrons, is commonly attributed to the color charge, a property of quarks and gluons only, which is analogous to the electric charge responsible for the electromagnetic force. In analogy to the electromagnetic force which is thought to be mediated by the exchange of virtual photons, the strong force is thought to be mediated by the exchange of gluons. At femtometer distances the strong force is much stronger than the Coulombic repulsion and increases with distance, a behavior called colorconfinement. At shorter distances the strong force becomes weaker, a behavior known as asymptotic freedom. The residual strong force, which keeps protons and neutrons bound in nuclei, is typically a factor of 100 weaker than the strong force. It is thought to be analogous to the van der Waals forces in chemistry which originate from the Coulombic forces but are much weaker due to charge screening. A similar type of color charge screening is thought to make the residual strong interaction (energies of ∼ 5 MeV ) much weaker than the strong force itself (energies of ∼ 500 MeV ). A first disappointment for the newcomer is that there is no simple expression, such as Coulomb’s or Newton’s law, allowing for fast computation of the strong force between two particles. The current theory of the strong force is quantum chromodynamics (QCD) and one of its predictions is that for energies below ∼ 200 MeV, frequently termed QCD scale, the quark-gluon plasma condenses to form hadrons, i.e. baryons and mesons.