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 seconds or less. It is also used to describe particles in intermediate steps of a decay, so-called virtual particles. The width of the resonance (Γ) is related to the mean lifetime (τ) of the particle (or its excited state) by the relation where <Math>{\hbar}=\frac{h}{2\pi} and h is the Planck constant. 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 2.603 s. Therefore, its resonance width is very small, about 2.528 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 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.