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. In common usage, "resonance" only describes particles with very shortlifetimes, mostly high-energy hadrons existing for10−23 seconds or less. It is also used to describe particles in intermediate steps of a decay, so-calledvirtual particles.[1]
Thewidth of the resonance (Γ) is related to themean lifetime (τ) of the particle (or its excited state) by the relation
where andh is thePlanck constant.
Thus, the lifetime of a particle is the directinverse of the particle's resonance 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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