particle physics Particle physics or high-energy physics is the study of Elementary particle, fundamental particles and fundamental interaction, forces that constitute matter and radiation. The field also studies combinations of elementary particles up to the s ...

, a rho meson is a short-lived

hadron In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong nuclear force. Pronounced , the name is derived . They are analogous to molecules, which are held together by the electri ...

ic particle that is an

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triplet whose three states are denoted as , and . Along with

pion In particle physics, a pion (, ) or pi meson, denoted with the Greek alphabet, Greek letter pi (letter), pi (), is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the ...

s and omega mesons, the rho meson carries the nuclear force within the

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. After the pions and kaons, the rho

meson In particle physics, a meson () is a type of hadronic subatomic particle composed of an equal number of quarks and antiquarks, usually one of each, bound together by the strong interaction. Because mesons are composed of quark subparticles, the ...

s are the lightest strongly interacting particle, with a mass of for all three states.There should be a small mass difference between the and the 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 . The rho mesons have a very short lifetime and their decay width is about 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

electron The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...

s or muons which occurs with a branching ratio of . This decay of the neutral rho to leptons can be interpreted as a mixing between the

photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are massless particles that can ...

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 A neutrino ( ; denoted by the Greek letter ) is an elementary particle that interacts via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass is so small ('' -ino'') that i ...

; however, this has never been observed.

History

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

Composition

The rho mesons can be interpreted as a bound state of a

quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nucleus, atomic nuclei ...

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

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chiral symmetry acting on the

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. 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.

Notes

References

{{Authority control Mesons Subatomic particles with spin 1