A type of particle. Mesons and baryons are the two types of hadron. All hadrons are (experimental data seems to confirm) made up of quarks, as independently theorized by Murray Gell-Mann and George Zweig in 1964. Mesons are made up of one quark and one antiquark. Some types of meson are pion, kaon, rho, B-zero and eta-c. There are 140 types of meson.

Meson (from the Greek 'middle' - its a middle weight). The meson is a class of particles that is, in general, heavier than the lepton but lighter than those of the baryon. The meson is a member of the hadron class of particles and is composed of quarks. However, unlike the baryons (of which protons and neutrons belong), the meson is composed of two quarks (instead of three), one of which is always an antiquark.

The meson was predicted in 1935 by Hideki Yukawa (who theorized that it was responsible for the force holding the nucleus together). The meson was first detected in 1947 by C. F. Powell in secondary effects of cosmic rays (collisions between a cosmic ray and an atom in the atmosphere). The particle that Yukawa predicted was the pion.

All mesons consist of a quark bound to an anti-quark. The pion (π +), is composed of an up quark and a down anti-quark. The anti-pion (π-), is composed of an up quark is composed of an up anti-quark and a down quark. The pion also happens to be the lightest of the meson family of particles. There is also the neutral pion (π0) which is composed of a a strange combination of (uu + dd)/sqrt(2) - no, I don't understand it at all. A curious fact about the neutral pion is that it is its own anti-particle.

The very nature of the meson being composed of a quark and an anti-quark make it unstable. A charged pion decays with an average lifetime of 2.55 * 10-8 seconds into a muon of the same charge and a neutrino or anti-neutrino. A neutral pion will decay in 2.55 * 10-15 seconds, typically into a pair of photons, but will occasionally decay into a positron-electron pair and a photon.

There are on the order of 140 different types of mesons. Some of them are more stable than other - the kaon (K-) is made of a strange quark and an anti-up quark and is much heavier and long lived than other mesons. Currently, there is much research in the B-zero (B0) meson - it contains an anti-bottom quark and thus is much heaver than other mesons (about 50 times more massive than a pion and 10 times heavier than a kaon).

The pion has been used to predict the range of the the strong force. Using the uncertainty principle the and making the assumption that it is an exchange force (involves sending things back and forth) and involves neutral pions the equation:

            h
R = cDt = -----
           2mc
The range of 0.73 * 10-15m results, or about 0.6 times the radius of a proton.

Because the pion is composed of an up and anti down quark (anti-matter has the same mass as normal matter to within testable limits (a few billionth of a part)) it would be expected that the pion have a mass on the order of 2/3 of the mass of a proton (composed of two up quarks and a down quark) However, the mass of the pion was only 1/6 of the mass of a proton thus showing that the mass of hadrons depend on the dynamics of the particle and not just the components of it.

A meson is a particle that consists of a quark and an antiquark. Mesons form a subset of hadrons

The following is a (growing) list of mesons. If you spot an error, feel free to /msg me. If you want to add some, /msg me too, rather than creating a new writeup.

Notes:

  • All rest masses are in MeV/c2.
  • "Makeup" refers to the constituent quarks (u=up, d=down, s=strange, c=charm, t=top, b=bottom).
  • An underlined symbol denotes an antiparticle, which is normally written with a bar over the symbol.

Pion Positive (π+)
Makeup: ud
Rest Mass: 139.6
Charge: +1
Common Decay Modes: π+ -> μ+ + νμ

Pion Zero (π0)
Makeup: (uu+dd)/√2
Rest Mass: 135.0
Charge: 0
Common Decay Modes: π0 -> 2γ

Pion Negative (π-)
Makeup: ud
Rest Mass: 139.6
Charge: -1
Common Decay Modes: π- -> μ- + νμ

Kaon Positive (K+)
Makeup: us
Rest Mass: 493.7
Charge: +1
Common Decay Modes: K+ -> μ+ + νμ, K+ -> π+ + π0

Kaon Negative (K-)
Makeup: us
Rest Mass: 493.7
Charge: -1
Common Decay Modes: K- -> μ- + νμ, K- -> π- + π0

Kaon Zero Short (K0S)
Makeup: (symmetric mix of ds and ds)
Rest Mass: 497.7
Charge: 0
Common Decay Modes: K0S -> π+ + π-, K0S -> 2π0
(this meson is weird - someone should do a good writeup in Kaon about it)

Kaon Zero Long (K0L)
Makeup: (antisymmetric mix of ds and ds)
Rest Mass: 497.7
Charge: 0
Common Decay Modes: K0L -> π+ + e- + νe
(this meson is weird - someone should do a good writeup in Kaon about it)

Eta Zero (η0)
Makeup: (uu + dd - 2ss)/√6
Rest Mass: 548.8
Charge: 0
Common Decay Modes: η0 -> 2γ, η0 -> 3μ
(this meson is weird - someone should do a good writeup in Eta about it)

Rho Positive (ρ+)
Makeup: ud
Rest Mass: 770
Charge: +1
Common Decay Modes: ρ+ -> π+
(this meson is considered to be an "excited" pion - the spins of the consituent quarks are "aligned")

Rho Negative (ρ-)
Makeup: ud
Rest Mass: 770
Charge: -1
Common Decay Modes: ρ- -> π-
(this meson is considered to be an "excited" pion - the spins of the consituent quarks are "aligned")

Phi (Φ)
Makeup: ss
Rest Mass: 1020
Charge: 0
Common Decay Modes: Φ -> K+ + K-, Φ -> K0 + K0

D Positive (D+)
Makeup: cd
Rest Mass: 1869.4
Charge: +1
Common Decay Modes: D+ -> K + ?, D+ -> e + ?

D Zero (D0)
Makeup: cu
Rest Mass: 1864.6
Charge: 0
Common Decay Modes: D0 -> K + ?, D0 -> μ + ?, D0 -> e + ?

D Negative (D+)
Makeup: cd
Rest Mass: 1869.4
Charge: -1
Common Decay Modes: D- -> K + ?, D- -> e + ?

D Positive Strange (D+S)
Makeup: cs
Rest Mass: 1969
Charge: +1
Common Decay Modes: D+S -> K + ?

D Negative Strange (D-S)
Makeup: cs
Rest Mass: 1969
Charge: -1
Common Decay Modes: D-S -> K + ?

J/Psi (J/Ψ)
Makeup: cc
Rest Mass: 3096.9
Charge: 0
Common Decay Modes: J/Ψ -> e+ + e-, J/Ψ -> μ+ + μ-, J/Ψ -> τ+ + τ-

B Positive (B+)
Makeup: ub
Rest Mass: 5279
Charge: +1
Common Decay Modes: B+ -> D0 + ?

B Zero (B0)
Makeup: db
Rest Mass: 5279
Charge: 0
Common Decay Modes: B0 -> D0 + ?

B Negative (B-)
Makeup: ub
Rest Mass: 5279
Charge: -1
Common Decay Modes: B- -> D0 + ?

Upsilon (Y)
Makeup: bb
Rest Mass: 9460.4
Charge: 0
Common Decay Modes: Y -> -> e+ + e-, Y -> μ+ + μ-, Y -> τ+ + τ-

Mes"on (?), n. [NL., fr. Gr. middle, neut. of , a., middle.] Anat.

The mesial plane dividing the body of an animal into similar right and left halves. The line in which it meets the dorsal surface has been called the dorsimeson, and the corresponding ventral edge the ventrimeson.

B. G. Wilder.

 

© Webster 1913.

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