Resonance (particle physics)

In <a href="/facts/Particle_physics/YgIuyj2t">particle physics</a>, a resonance is the peak located around a certain energy found in <a href="/facts/Differential_cross_section/lo014h2D">differential cross sections</a> of <a href="/facts/Scattering/nMou7lEH">scattering experiments</a>. These peaks are associated with <a href="/facts/Subatomic_particle/tMEzcTWQ">subatomic particles</a>, which include a variety of <a href="/facts/Boson/XtPFcjb5">bosons</a>, <a href="/facts/Quark/hwuuVkV5">quarks</a> and <a href="/facts/Hadron/C0sd1zty">hadrons</a> (such as <a href="/facts/Nucleon/6fguU9Xk">nucleons</a>, <a href="/facts/Delta_baryon/xwNEsEKp">delta baryons</a> or <a href="/facts/Upsilon_meson/Ap9wz1cs">upsilon mesons</a>) and their <a href="/facts/Excited_state/czcLYNnr">excitations</a>. In common usage, "resonance" only describes particles with very short <a href="/facts/Mean_lifetime/wLB1zYDP">lifetimes</a>, mostly high-energy hadrons existing for 10−23 <a href="/facts/Second/jtFxnJXx">seconds</a> or less. It is also used to describe particles in intermediate steps of a decay, so-called <a href="/facts/Virtual_particle/CfHFkVL3">virtual particles</a>.
The <a href="/facts/Resonance_width/NNjGLb5b">width of the resonance</a> (Γ) is related to the <a href="/facts/Mean_lifetime/wLB1zYDP">mean lifetime</a> (τ) of the particle (or its excited state) by the relation

Γ
        =
        
          
            ℏ
            τ
          
        
      
    
    {\displaystyle \Gamma ={\frac {\hbar }{\tau }}}

where 
 
 
 
 
 ℏ
 
 =
 
 
 h
 
 2
 π
 
 
 
 
 
 {\displaystyle {\hbar }={\frac {h}{2\pi }}}
 
 and h is the <a href="/facts/Planck_constant/qGHAe3qq">Planck constant</a>.
Thus, the lifetime of a particle is the direct <a href="/facts/Multiplicative_inverse/yczdYyCw">inverse</a> of the particle's resonance width. For example, the charged <a href="/facts/Pion/x5HamzG3">pion</a> has the second-longest lifetime of any meson, at 2.6033×10−8 s. Therefore, its resonance width is very small, about 2.528×10−8 <a href="/facts/Electronvolt/YMT1fqX9">eV</a> or about 6.11 <a href="/facts/Megahertz/6Wshj5qm">MHz</a>. Pions are generally not considered as "resonances". The charged <a href="/facts/Rho_meson/55vNvssd">rho meson</a> has a very short lifetime, about 4.41×10−24 s. Correspondingly, its resonance width is very large, at 149.1 <a href="/facts/Megaelectronvolt/YMT1fqX9">MeV</a> or about 36 <a href="/facts/Zettahertz/6Wshj5qm">ZHz</a>. This amounts to nearly one-fifth of the particle's <a href="/facts/Rest_mass/aRloR0nW">rest mass</a>.

Resonance (particle physics) open-in-new

Resonance (particle physics)