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Rho meson
Statistics	Bosonic
Family	Mesons
Interactions	Strong, Weak, Gravitational and Electromagnetic
Symbol	; ρ+; , ; ρ0; , and ; ρ−;
Antiparticle	; ρ+; : ; ρ−; ; ; ρ0; : self;
Types	3
Mass	~770 MeV/c2
Mean lifetime	~4.5×10−24 s
Decays into	; ρ±; : ; π±; + ; π0; ; ; ρ0; : ; π+; + ; π−; ;
Electric charge	; ρ±; : ±1 e; ; ρ0; : 0 e;
Color charge	0
Spin	1
Isospin	; ρ±; : ±1; ; ρ0; : 0;
Hypercharge	0
Parity	−1
C parity	−1

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 pions and kaons, the rho mesons are the lightest strongly interacting particle, with a mass of 775.45±0.04 MeV for all three states.^[c]

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%.^[d]

History

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

Composition

The rho mesons can be interpreted^[3] 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.^[4]

Rho mesons
Particle name	Particle symbol	Antiparticle symbol	Quark content^[5]	Mass (MeV/c²)^[a]	I^G	J^PC	S	C	B'	Mean lifetime (s)^[a]	Commonly decays to (>5% of decays)
Charged rho meson^[6]	ρ⁺ (770)	ρ⁻ (770)	u d	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	$\mathrm {\tfrac {u{\bar {u}}-d{\bar {d}}}{\sqrt {2}}} \,$	775.26±0.25	1⁺	1⁻⁻	0	0	0	(4.453±0.027)×10⁻²⁴^[b]	π⁺ + π⁻

Notes

^ ^a ^b ^c ^d The exact value depends on the method used. See the given reference for detail. In the table, data used is from tau decays and electron–positron annihilation.
^ ^a ^b ^c PDG reports the resonance width (Γ). Here the conversion τ = ħ⁄Γ is given instead.
^ 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.
^ 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.

References

^ Zyla, P. A.; et al. (Particle Data Group) (2020). "Review of Particle Physics". Progress of Theoretical and Experimental Physics. 2020 (8): 083C01. doi:10.1093/ptep/ptaa104. hdl:11585/772320.
^ Maglich, B. (1976). "Discovery of omega meson-first neutral vector meson: one researcher's personal account - Discovery story". Adv. Exp. Phys. 5: 79.
^ De Rújula, A.; Georgi, Howard; Glashow, S. L. (1975-07-01). "Hadron masses in a gauge theory". Physical Review D. 12 (1). American Physical Society (APS): 147–162. Bibcode:1975PhRvD..12..147D. doi:10.1103/physrevd.12.147. ISSN 0556-2821.
^ Georgi, Howard (1990). "Vector realization of chiral symmetry". Nuclear Physics B. 331 (2). Elsevier BV: 311–330. Bibcode:1990NuPhB.331..311G. doi:10.1016/0550-3213(90)90210-5. ISSN 0550-3213.
^ C. Amsler et al. (2008): Quark Model
^ ^a ^b C. Amsler et al. (2008): Particle listings –
ρ