In particle physics, a fermion is any particle that follows Fermi–Dirac statistics. These particles obey the Pauli exclusion principle, which states that no two identical fermions can occupy the same quantum state simultaneously. Fermions include all quarks and leptons, as well as any composite particle made of an odd number of these, such as all baryons and many atoms and nuclei.
Fermions differ from bosons, which follow Bose–Einstein statistics. A key difference between fermions and bosons is that exchange symmetry (the permutation symmetry of identical particles) does not work in the same way for them: swapping two identical fermions changes the overall wave function by a minus sign (this is called the Fermi–Dirac statistics), while with bosons it does not change at all. This behavior gives rise to subtle effects like superconductivity and superfluidity, which are exploited in many technologies today. It also leads to thePauli exclusion principle, which prevents fermions from occupying exactly the same quantum state; this underlies much of chemistry’s periodic table of elements. Finally, it implies fundamental limits on how closely fermions can be packed together—a fact exploited in nuclear engineering to prevent undesirable fusion reactions from occurring inside stars or artificial reactors (as distinct from controlled fusion).