Inparticle physics, arho meson is a short-livedhadronic particle that is anisospintriplet whose three states are denoted asρ⁺
,ρ⁰
andρ⁻
. Along withpions andomega mesons, the rho meson carries thenuclear force within theatomic nucleus. After the octet consisting of the pions,kaons, andeta meson, the rhomesons are the lightest strongly interacting particle, with a mass of about775 MeV for all three states.^[c]

The rho mesons have a very short lifetime and theirdecay width is about145 MeV; because that is large compared with the mass, the resonance width measurably deviates from aBreit–Wigner form. The principal decay route of the rho mesons is to a pair of pions with abranching rate of 99.9% (however, all neutral pions is forbidden).^[d]

After several false starts, the ρ meson and the ω meson were discovered atLawrence Berkeley Laboratory in 1961.^[2]

The rho mesons can be interpreted^[3] as a bound state of aquark and an anti-quark and is an excited version of the pion. Unlike the pion, the rho meson has spinj = 1 (avector meson) and a much higher value of the mass. They^[who?] attribute this mass difference between the pions and rho mesons to a largehyperfine 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 ofchiral symmetry breaking.

The rho mesons can be thought of as thegauge bosons of aspontaneously brokengauge symmetry whose local character isemergent (arising fromQCD); Note that this broken gauge symmetry (sometimes called hidden local symmetry) is distinct from theglobalchiral symmetry acting on theflavors. This was described byHoward Georgi in a paper titled "The Vector Limit of Chiral Symmetry" where he ascribed much of the literature of hidden local symmetry to anon-linear sigma model.^[4]

Rho mesons

Particle name	Particle symbol	Antiparticle symbol	Quark content[5]	Mass (MeV/c2)[a]	IG	JPC	S	C	B'	Mean lifetime (s)[a]	Commonly decays to (>5% of decays)
Charged rho meson[6]	ρ+ (770)	ρ− (770)	ud	775.11±0.34	1+	1−	0	0	0	(4.415±0.024)×10−24[b]	π± +π0
Neutral rho meson[6]	ρ0 (770)	Self	${\displaystyle \frac{\mathrm{u}\overline{\mathrm{u}}-\mathrm{d}\overline{\mathrm{d}}}{\sqrt{2}}}$	775.26±0.25	1+	1−−	0	0	0	(4.453±0.027)×10−24[b]	π+ +π−

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