View source for Physics:Rho meson

{{Short description|Short-lived hadronic particle that is an isospin triplet}}
{{Infobox particle
| name = Rho meson
| image = 
| caption = 
| num_types = 3
| composition = 
| statistics = [[Physics:Bosonic|Bosonic]]
| group = Mesons
| interaction = [[Physics:Strong interaction|Strong]], [[Physics:Weak interaction|Weak]], Gravitational and Electromagnetic
| antiparticle= {{plainlist|
*{{SubatomicParticle|Rho+}}:  {{SubatomicParticle|Rho-}}
*{{SubatomicParticle|Rho0}}:  self}}
| theorized   = 
| discovered  = 
| symbol      = {{SubatomicParticle|Rho+}}, {{SubatomicParticle|Rho0}}, and {{SubatomicParticle|Rho-}}
| mass        = {{val|p=~|770|ul=MeV/c2}}<ref name="pdg">{{cite journal| title=Review of Particle Physics | last1=Zyla | first1=P. A. | last2=Barnett | first2=R. M. | display-authors=1 | collaboration = Particle Data Group | journal=Progress of Theoretical and Experimental Physics | volume = 2020 | page = 083C01 | year = 2020 | issue=8 | doi=10.1093/ptep/ptaa104| doi-access=free | hdl=11585/772320 | hdl-access=free }}</ref><ref name=Lifetime3 group=lower-alpha />
| mean_lifetime = {{val|p=~|4.5|e=-24|u=s}}<ref name=Lifetime2 group=lower-alpha /><ref name=Lifetime3 group=lower-alpha />
| decay_particle = {{plainlist|
*{{SubatomicParticle|Rho+-}}: {{SubatomicParticle|Pion+-}} + {{SubatomicParticle|Pion0}}
*{{SubatomicParticle|Rho0}}: {{SubatomicParticle|Pion+}} + {{SubatomicParticle|Pion-}}
}}
| electric_charge = {{plainlist|
*{{SubatomicParticle|Rho+-}}: {{+-}}1 [[Physics:Elementary charge|e]] 
*{{SubatomicParticle|Rho0}}: 0 e}}
| color_charge=  0
| spin        =  1
| strangeness = 
| charm = 
| bottomness = 
| topness = 
| isospin = {{plainlist|
*{{SubatomicParticle|Rho+-}}: {{+-}}1
*{{SubatomicParticle|Rho0}}: 0}}
| hypercharge = 0
| parity = -1
| c_parity = -1
}}
In [[Physics:Particle physics|particle physics]], a '''rho meson''' is a short-lived [[Physics:Hadron|hadron]]ic particle that is an [[Physics:Isospin|isospin]] triplet whose three states are denoted as {{SubatomicParticle|Rho+}}, {{SubatomicParticle|Rho0}} and {{SubatomicParticle|Rho-}}. Along with [[Physics:Pion|pion]]s and [[Physics:Omega meson|omega meson]]s, the rho meson carries the [[Physics:Nuclear force|nuclear force]] within the [[Physics:Atomic nucleus|atomic nucleus]]. After the pions and [[Physics:Kaon|kaon]]s, the rho [[Physics:Meson|meson]]s are the lightest strongly interacting particle, with a mass of {{val|775.45|0.04|ul=MeV}} for all three states.<ref group=lower-alpha>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 [[Physics:Meson#Isospin breaking|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}}.</ref>

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%.<ref group=lower-alpha>Neutral rho mesons can decay to a pair of [[Physics:Electron|electron]]s or [[Physics:Muon|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 [[Physics:Photon|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 [[Physics:Neutrino|neutrino]]; however, this has never been observed.</ref>

==History==
After several false starts, the ρ meson and the ω meson were discovered at [[Organization:Lawrence Berkeley National Laboratory|Lawrence Berkeley Laboratory]] in 1961.<ref name=Maglich:1976cpa>{{cite journal | last=Maglich | first=B. | title=Discovery of omega meson-first neutral vector meson: one researcher's personal account - Discovery story | journal=Adv. Exp. Phys. | volume=5 | pages=79 | year=1976}}</ref>

==Composition==
The rho mesons can be interpreted<ref name=GeorgiRujulaGlashow>{{cite journal | last1=De Rújula | first1=A. | last2=Georgi | first2=Howard | last3=Glashow | first3=S. L. | title=Hadron masses in a gauge theory | journal=Physical Review D | publisher=American Physical Society (APS) | volume=12 | issue=1 | date=1975-07-01 | issn=0556-2821 | doi=10.1103/physrevd.12.147 | pages=147–162| bibcode=1975PhRvD..12..147D }}</ref>  as a bound state of a [[Physics:Quark|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 [[Physics:Vector meson|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 [[Physics:Chiral symmetry breaking|chiral symmetry breaking]].

The rho mesons can be thought of as the [[Physics:Gauge boson|gauge boson]]s of a spontaneously broken gauge symmetry whose local character is [[Induced gravity|emergent]] (arising from [[Physics:Quantum chromodynamics|QCD]]); Note that this broken gauge symmetry (sometimes called hidden local symmetry) is distinct from the [[Physics:Global symmetry|global]] chiral symmetry acting on the flavors. This was described by [[Biography:Howard Georgi|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 [[Physics:Non-linear sigma model|non-linear sigma model]].<ref name=GeorgiVectorRealizationChiralSymPaper>{{cite journal | last=Georgi | first=Howard | title=Vector realization of chiral symmetry | journal=Nuclear Physics B | publisher=Elsevier BV | volume=331 | issue=2 | year=1990 | issn=0550-3213 | doi=10.1016/0550-3213(90)90210-5 | pages=311–330| bibcode=1990NuPhB.331..311G }}</ref>



{| 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>
! [[Physics:Mass|Mass]] (MeV/[[Physics:Speed of light|''c'']]<sup>2</sup>)<ref name=Lifetime3 group=lower-alpha/>
! width="50" | [[Physics:Isospin|I]]<sup>G</sup>
! width="50" | [[Physics:Total angular momentum|J]]<sup>[[Physics:Parity|P]][[Physics:C parity|C]]</sup>
! width="50" | [[Physics:Strangeness|S]]
! width="50" | [[Physics:Charm (quantum number)|C]]
! width="50" | [[Physics:Bottomness|B']]
! Mean lifetime ([[Second|s]])<ref name=Lifetime3 group=lower-alpha>The exact value depends on the method used. See the given reference for detail. In the table, data used is from [[Physics:Tau (particle)|tau]] decays and [[Physics:Electron–positron annihilation|electron–positron annihilation]].</ref>
! 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>
| {{SubatomicParticle|Rho+}}(770)
| {{SubatomicParticle|Rho-}}(770)
| {{SubatomicParticle|link=yes|Up quark}}{{SubatomicParticle|link=yes|Down antiquark}}
| {{val|775.11|0.34}}
| 1<sup>+</sup>
| 1<sup>&minus;</sup>
| 0
| 0
| 0
| {{val|4.415|0.024|e=-24}}<ref name=Lifetime2 group=lower-alpha>PDG reports the resonance width (Γ). Here the conversion τ&nbsp;=&nbsp;{{frac|ħ|&Gamma;}} is given instead.</ref>
| {{nowrap|{{SubatomicParticle|link=yes|Pion+-}} + {{SubatomicParticle|link=yes|Pion0}}}}
|-
| Neutral rho meson<ref name=PDGRho/>
| {{SubatomicParticle|Rho0}}(770)
| Self
| <math>\mathrm{\tfrac{u\bar{u}-d\bar{d}}{\sqrt 2}}\,</math>
| {{val|775.26|0.25}}
| 1<sup>+</sup>
| 1<sup>&minus;&minus;</sup>
| 0
| 0
| 0
| {{val|4.453|0.027|e=-24}}<ref name=Lifetime2 group=lower-alpha />
| {{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. --->

==Notes==
{{notelist|group=lower-alpha}}

==References==
{{Reflist}}

{{Particles}}

[[Category:Mesons]]
[[Category:Subatomic particles with spin 1]]

{{Sourceattribution|Rho meson}}