In particle physics, a resonance is the peak located around a certain energy found in differential cross sections of scattering experiments. These peaks are associated with subatomic particles, which include a variety of bosons, quarks and hadrons (such as nucleons, delta baryons or upsilon mesons) and their excitations. In common usage, "resonance" only describes particles with very short lifetimes, mostly high-energy hadrons existing for 10⁻²³ seconds or less. It is also used to describe particles in intermediate steps of a decay, so-called virtual particles.^[1]

The width of the resonance (Γ) is related to the mean lifetime (τ) of the particle (or its excited state) by the relation

${\displaystyle \Gamma ={\frac {\hbar }{\tau ))}$

where h is the Planck constant and ${\displaystyle {\hbar }={\frac {h}{2\pi ))}$.

Thus, the lifetime of a particle is the direct inverse of the particle's resonance width. For example, the charged pion has the second-longest lifetime of any meson, at 2.6033×10⁻⁸ s.^[2] Therefore, its resonance width is very small, about 2.528×10⁻⁸ eV or about 6.11 MHz. Pions are generally not considered as "resonances". The charged rho meson has a very short lifetime, about 4.41×10⁻²⁴ s. Correspondingly, its resonance width is very large, at 149.1 MeV or about 36 ZHz. This amounts to nearly one-fifth of the particle's rest mass.^[3]