Resonance (particle physics)
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 and their excitations. Resonances can be explained as excited states of the reacting particles, or as virtual particles in intermediate steps of the reaction with very short lifetimes.
The width of the resonance is related to the mean lifetime of the particle or excited state by the relation
where and h is the Planck constant.
Thus, the lifetime of the resonance is the direct inverse of the resonance's width. For example, the charged pion has the second-longest lifetime of any meson, at. Therefore, its resonance width is very small, about or about 6.11 MHz. Pions are generally not considered as "resonances". The charged rho meson has a very short lifetime, about. 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.