resonance (particle physics)

{{Short description|Concept in scattering theory}}

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 {{val||e=-23|u=seconds}} or less. It is also used to describe particles in intermediate steps of a decay, so-called virtual particles.{{cite web|url=http://www.phy.duke.edu/~kolena/modern/dudley.html |title=What is a Resonance Particle? |last1=Dudley|first1=Chris |website=phy.duke.edu |access-date=24 April 2017}}

The width of the resonance (Γ) is related to the mean lifetime (τ) of the particle (or its excited state) by the relation

:$\backslash Gamma=\backslash frac\{\backslash hbar\}\{\backslash tau\}$

where $\{\backslash hbar\}=\backslash frac\{h\}\{2\backslash pi\}$ 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 {{val|2.6033|e=-8|u=s}}.K.A. Olive et al. (Particle Data Group) (2016): [http://pdg.lbl.gov/2016/listings/rpp2016-list-pi-plus-minus.pdf Particle listings – {{Subatomic particle|Pion+-}}] Therefore, its resonance width is very small, about {{val|2.528|e=-8|u=eV}} or about 6.11 MHz. Pions are generally not considered as "resonances". The charged rho meson has a very short lifetime, about {{val|4.41|e=-24|u=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.K.A. Olive et al. (Particle Data Group) (2016): [http://pdg.lbl.gov/2016/listings/rpp2016-list-rho-770.pdf Particle listings – {{Subatomic particle|rho}}]