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In [[particle physics]], a '''rho meson''' is a short-lived [[hadron]]ic particle that is an [[isospin]] [[Spin triplet|triplet]] whose three states are denoted as {{SubatomicParticle|Rho+}}, {{SubatomicParticle|Rho0}} and {{SubatomicParticle|Rho-}}.  After the [[pion]]s and [[kaon]]s, the rho [[meson]]s are the lightest strongly interacting particle with a mass of roughly {{val|770|ul=MeV}} for all three states.  There should be a small mass difference between the {{SubatomicParticle|Rho+}} and the {{SubatomicParticle|Rho0}} 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 {{val|0.7|u=MeV}}.

The rho mesons have a very short lifetime and their [[decay width]] is about {{val|145|u=MeV}} with the peculiar feature that the decay widths are not described by a [[Relativistic Breit–Wigner distribution|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 [[electron]]s or [[muon]]s which occurs with a branching ratio of {{val|5|e=-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 [[Trinification|De Rujula&ndash;Georgi&ndash;Glashow]] description of hadrons,<ref name=GeorgiRujulaGlashow>Rujula, Georgi, Glashow (1975) "Hadron Masses in Gauge Theory." [http://prd.aps.org/abstract/PRD/v12/i1/p147_1 Physical Review D12, p.147 ]</ref> 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''&nbsp;=&nbsp;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&ndash;Georgi&ndash;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 boson]]s of a [[spontaneously broken]] [[gauge symmetry]] whose local character is [[Induced gravity|emergent]] (arising from [[Quantum chromodynamics|QCD]]); Note that this broken gauge symmetry (sometimes called hidden local symmetry) is distinct from the  [[global symmetry|global]] [[chiral symmetry]] acting on the [[flavor (physics)|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]].<ref name=GeorgiVectorRealizationChiralSymPaper>H. Georgi. (1990) "Vector Realization of Chiral Symmetry." [http://inspirehep.net/record/282562 inSPIRE Record]</ref>

More recently, the point of view that the rho mesons are gauge bosons has been enhanced by a program known as [[AdS/QCD]], which is an application of [[AdS/CFT]] derived from [[string theory]].   In this description, there is a small extra dimension that is a slice of [[anti-de Sitter space]].  The global flavor symmetries are promoted to  five dimensional gauge symmetries that are broken at the boundary between space and isospin.   The rho mesons are lightest [[Kaluza&ndash;Klein]] resonances of the fifth dimension.    This program has the advantage that it is capable of making quantitative predictions for the interactions of the rho mesons.  These predictions are usually accurate to 10%.   There is some concern as to whether this five dimensional description is under perturbative control and this is currently under active research.  Conceptually, the AdS/QCD approach is very close in spirit to "The Vector Limit of Chiral Symmetry";  if one [[dimensional deconstruction|deconstructs]] the 5th dimension, one finds an effective field theory very similar to the one described in the "Vector Limit."

{| class="wikitable sortable" style="text-align: center;"
|+Rho mesons
|-
! class=unsortable|Particle name
! Particle <br>symbol
! Antiparticle <br>symbol
! class=unsortable|Quark<br>content<ref name=PDGQuarkmodel>C. Amsler ''et al.'' (2008): [http://pdg.lbl.gov/2008/reviews/quarkmodrpp.pdf Quark Model]</ref>
! [[Rest mass]] ([[electron volt|MeV]]/[[speed of light|c]]<sup>2</sup>)
! width="50"|[[Isospin|I]]<sup>[[G parity|G]]</sup>
! width="50"|[[Total angular momentum|J]]<sup>[[Parity (physics)|P]][[C parity|C]]</sup>
! width="50"|[[strangeness|S]]
! width="50"|[[charm (quantum number)|C]]
! width="50"|[[bottomness|B']]
! [[Mean lifetime]] ([[second|s]])
! class=unsortable|Commonly decays to<br>
(>5% of decays)
|-
| Charged rho meson<ref name=PDGRho>C. Amsler ''et al.'' (2008): [http://pdg.lbl.gov/2008/listings/m009.pdf Particle listings – {{SubatomicParticle|rho}}]</ref>
|align="center"| {{SubatomicParticle|Rho+}}(770)
|align="center"| {{SubatomicParticle|Rho-}}(770)
|align="center"| {{SubatomicParticle|link=yes|Up quark}}{{SubatomicParticle|link=yes|Down antiquark}}
|align="center"| {{val|775.4|0.4}}
|align="center"| 1<sup>+<sup>
|align="center"| 1<sup>&minus;</sup>
|align="center"| 0
|align="center"| 0
|align="center"| 0
|align="center"| {{val|p=~|4.5|e=-24}}<sup>{{ref|Lifetime2|[a]}}</sup><sup>{{ref|Lifetime3|[b]}}</sup>
|align="center"| {{nowrap|{{SubatomicParticle|link=yes|Pion+-}} + {{SubatomicParticle|link=yes|Pion0}}}}
|-
| Neutral rho meson<ref name=PDGRho/>
|align="center"| {{SubatomicParticle|Rho0}}(770)
|align="center"| Self
|align="center"| <math>\mathrm{\tfrac{u\bar{u}-d\bar{d}}{\sqrt 2}}\,</math>
|align="center"| {{val|775.49|0.34}}
|align="center"| 1<sup>+<sup>
|align="center"| 1<sup>&minus;&minus;</sup>
|align="center"| 0
|align="center"| 0
|align="center"| 0
|align="center"| {{val|p=~|4.5|e=-24}}<sup>{{ref|Lifetime2|[a]}}</sup><sup>{{ref|Lifetime3|[b]}}</sup>
|align="center"| {{nowrap|{{SubatomicParticle|link=yes|Pion+}} + {{SubatomicParticle|link=yes|Pion-}}}}
|-
|}
<sup>[a]</sup> {{note|Lifetime2}} PDG reports the resonance width (Γ). Here the conversion τ&nbsp;=&nbsp;{{frac|ħ|&Gamma;}} is given instead. <br>
<sup>[b]</sup> {{note|Lifetime3}} The exact value depends on the method used. See the given reference for detail.

== References ==

{{reflist}}

{{Particles}}

[[Category:Mesons]]