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. This was described by
Howard Georgi in a paper titled "The Vector Limit of Chiral Symmetry" where he ascribed much of the literature of hidden local symmetry to a
non-linear sigma model. [a] PDG reports the resonance width (Γ). Here the conversion τ = is given instead. [b] The exact value depends on the method used. See the given reference for detail. ---> ==Notes==