Inparticle physics, aresonance is the peak located around a certain energy found indifferential cross sections ofscattering experiments. These peaks are associated withsubatomic particles, which include a variety ofbosons,quarks andhadrons (such asnucleons,delta baryons orupsilon mesons) and theirexcitations. Resonances can be explained as excited states of the reacting particles, or asvirtual particles in intermediate steps of the reaction with very shortlifetimes (10⁻²³ seconds or less).^[1]

Thewidth of the resonance (Γ) is related to themean lifetime (τ) of the particle or excited state by the relation

${\displaystyle \mathrm{\Gamma }=\frac{\hslash }{\tau }}$

where${\displaystyle \hslash =\frac{h}{2\pi }}$ andh is thePlanck constant.

Thus, the lifetime of the resonance is the directinverse of the resonance's width. For example, the chargedpion has the second-longest lifetime of any meson, at2.6033×10⁻⁸ s.^[2] Therefore, its resonance width is very small, about2.528×10⁻⁸ eV or about 6.11MHz. Pions are generally not considered as "resonances". The chargedrho meson has a very short lifetime, about4.41×10⁻²⁴ s. Correspondingly, its resonance width is very large, at 149.1MeV or about 36ZHz. This amounts to nearly one-fifth of the particle'srest mass.^[3]

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