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−. After the pions and kaons, the rho mesons are the lightest strongly interacting particle with a mass of roughly 770 MeV for all three states. There should be a small mass difference between the ρ+ and the ρ0 that can be attributed to the electromagnetic self-energy of the particle as well as a small effect due to isospin breaking arising from the light quark masses; however, the current experimental limit is that this mass difference is less than 0.7 MeV.

The rho mesons have a very short lifetime and their decay width is about 145 MeV with the peculiar feature that the decay widths are not described by 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%. Neutral rho mesons can decay to a pair of electrons or muons which occurs with a branching ratio of 5×10⁻⁵. This decay of the neutral rho to leptons can be interpreted as a mixing between the photon and rho. In principle the charged rho mesons mix with the weak vector bosons and can lead to decay to an electron or muon plus a neutrino; however, this has never been observed.

In the De Rujula–Georgi–Glashow description of hadrons, 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. This mass difference between the pions and rho mesons is attributed to a large hyperfine interaction between the quark and anti-quark. The main 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.

Rho mesons

Particle name	Particle symbol	Antiparticle symbol	Quark content	Rest mass (MeV/c2)	IG	JPC	S	C	B'	Mean lifetime (s)	Commonly decays to (>5% of decays)
Charged rho meson	ρ+(770)	ρ−(770)	ud	775.4±0.4	1+	1−	0	0	0	~4.5×10−24	π± + π0
Neutral rho meson	ρ0(770)	Self	$\mathrm{\tfrac{u\bar{u}-d\bar{d}}{\sqrt 2}}\,$	775.49±0.34	1+	1−−	0	0	0	~4.5×10−24	π+ + π−

^[a] ^ PDG reports the resonance width (Γ). Here the conversion τ = ​^ħ⁄_Γ is given instead.
^[b] ^ The exact value depends on the method used. See the given reference for detail.