Rho meson

In particle physics, a rho meson is a short-lived hadronic particle that is an isospin triplet whose three states are denoted as ρ+, ρ0 and ρ−. Along with pions and omega mesons, the rho meson carries the nuclear force within the atomic nucleus. After the octet consisting of the pions, kaons, and eta meson, the rho mesons are the lightest strongly interacting particle, with a mass of about 775 MeV for all three states.

The rho mesons have a very short lifetime and their decay width is about 145 MeV; because that is large compared with the mass, the resonance width measurably deviates from a Breit–Wigner form. The principal decay route of the rho mesons is to a pair of pions with a branching rate of 99.9% (however, all neutral pions is forbidden).

History

After several false starts, the ρ meson and the ω meson were discovered at Lawrence Berkeley Laboratory in 1961.

Composition

The rho mesons can be interpreted as a bound state of a quark and an anti-quark and is an excited version of the pion. Unlike the pion, the rho meson has spin j = 1 (a vector meson) and a much higher value of the mass. They attribute this mass difference between the pions and rho mesons to a large hyperfine interaction between the quark and anti-quark, although an objection with the De Rujula–Georgi–Glashow description is that it attributes the lightness of the pions as an accident rather than a result of chiral symmetry breaking.

The rho mesons can be thought of as the gauge bosons of a spontaneously broken gauge symmetry whose local character is emergent (arising from QCD); Note that this broken gauge symmetry (sometimes called hidden local symmetry) is distinct from the global chiral symmetry acting on the flavors.