Strongly Interacting Fermions and Their Universal Behavior

Every observed physical phenomenon can be explained by the four fundamental forces, and the strong interaction is the most complicated one, explaining how particles interact with one another.

Fermions are particles with half-integer spin, divided into two groups: quarks and leptons. The quarks make up protons and neutrons, which are composite fermions themselves. Leptons include the electron and similarly, heavier particles (muon and tauon) and neutrino.

A team of theorists at the University of Queensland Centre for Quantum-Atom Optics in Australia and at Renmin University of China in Beijing, discovered a single equation that can describe universal behavior in strongly interacting fermions.

The strong interaction only acts directly upon elementary particles, today understood to represent the interactions between quarks and gluons. Although different strongly interacting fermions and their laws are known, so far it's been hard to describe the universal behavior through a single equation, since most quantum many-body systems (such as molecules in a volume of water) are very complicated and require different theories to be worked out for every specific type of atom or particle.

The team experimented with ultracold Fermi gases, like potassium-40 and lithium-6, to better understand interactions among fermions in high-temperature superconductors and other complex systems such as supernovae.

The scientists postulated the concept of "universal thermodynamic regime," which says that when the force between fermions is strong enough all fermion species should behave in essentially the same way.

They said that this must be true for all masses, densities or interaction details, as long as the forces operate over a short range compared to the space between the particles.

Recently, this overall picture of universality is rapidly gaining widespread acceptance and the results of this new study could be potentially applied to understanding matter made of quarks, such as protons.