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In particle physics, a meson is a strongly interacting boson, that is, it is a hadron with integral spin. In the Standard Model, mesons are composite (non-elementary) particles composed of an even number of quarks and antiquarks. All known mesons are believed to consist of a quark-antiquark pair — the so-called valence quarks — plus a "sea" of virtual quark-antiquark pairs and virtual gluons. Searches for exotic mesons that have different constituents are ongoing. The valence quarks may exist in a superposition of flavor states; for example, the neutral pion is neither an up-antiup pair nor a down-antidown pair, but an equal superposition of both. Pseudoscalar mesons (spin 0) have the lowest rest energy, where the quark and antiquark have opposite spin, and then the vector mesons (spin 1), where the quark and antiquark have parallel spin. Both come in higher-energy versions where the spin is augmented by orbital angular momentum. All mesons are unstable.
Mesons were originally predicted as carriers of the force that binds protons and neutrons together. When first discovered, the muon was identified with this family from its similar mass and was named "mu meson", however it did not show a strong attraction to nuclear matter and is actually a lepton. The pion was the first true meson to be discovered. (The current picture of intranuclear forces is quite complicated; see quantum hadrodynamics for a discussion of modern theories in which nucleon-nucleon interactions are mediated by meson exchange.)
In 1949 Hideki Yukawa was awarded the Nobel Prize in Physics for predicting the existence of the meson. He originally named it 'mesotron', but was corrected by Werner Heisenberg (whose father was a professor in Greek at University of Munich), who pointed out that there is no 'tr' in the Greek word 'mesos'.
Additional recommended knowledge
Naming of the mesons
The name of a meson is devised so that its main properties can be inferred. Conversely, given a meson's properties, its name is clearly determined. The naming conventions fall in two categories based on flavor, flavorless mesons and flavored mesons.
Flavorless mesons are mesons whose flavor quantum numbers are all equal to zero. This means that these quarks are quarkonium states (quark-antiquark pairs of the same flavor) or a linear superposition of such states.
The name of a flavorless meson is determined by its total spin S and total orbital angular momentum L. As a meson is composed of two quarks with s = 1/2, the total spin can only be S = 1 (parallel spins) or S = 0 (anti-parallel spins). The orbital quantum number L is due to the revolution of one quark around the other. Usually higher orbital angular momenta translate into a higher mass for the meson. These two quantum numbers determine the parity P and the charge-conjugation parity C of the meson:
Also, L and S add together to form a total angular momentum quantum number J, whose values range from |L−S| to L+S in one-unit steps. The different possibilities are summarized with the use of the term symbol 2S+1LJ (a letter code is used instead of the actual value of L, see the spectroscopic notation) and the symbol JPC (here only the sign is used for P and C).
The different possibilities and the corresponding meson symbol are given in the following table:
The normal spin-parity series is formed by those mesons were P=(−1)J. In the normal series, S = 1 so PC = +1 (i.e., P = C). This corresponds to some of the triplet states (triplet states appear on the last two columns).
Since some of these symbols can refer to more than one particle, some extra rules are added:
When the quantum numbers of a particle are unknown, it is designated with an X followed by its mass in parentheses.
For flavored mesons, the naming scheme is a little simpler.
1. The meson name is given by the heaviest of the two quarks. From more to less massive, the order is: t > b > c > s > d > u. However, u and d do not carry any flavor, so they do not influence the naming scheme. Quark t never forms hadrons, but a symbol for t-containing mesons is reserved anyway.
2. If the second quark has also flavor (it is not u or d) then the identity of that second quark is given by a subindex (s, c or b, and in theory t).
3. Add a "*" superindex if the meson is in the normal spin-parity series, i.e. JP = 0+, 1−, 2+...
4. For mesons other than pseudoscalars (0−) and vectors (1−) the total angular momentum quantum number J is added as a subindex.
To sum it up, we have:
In some cases, particles can mix between them. For example, the neutral kaon, and its antiparticle can combine in a symmetric or antisymmetric manner, originating two new particles, the short-lived and the long-lived neutral kaons (neglecting a small CP-violating term).
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Meson". A list of authors is available in Wikipedia.