Mesons were originally predicted as carriers of the force that bind Proton s and Neutron s 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 Interaction s are mediated by meson exchange.)
Yukawa was awarded Nobel Prize in Physics for this. He originally named it 'mesotron', but was famously corrected by Werner Heisenberg (whose father was a professor in Greek at University of Munich) that there is no 'tr' in the Greek word 'mesos'.
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, depending on whether the meson has a flavor or not.
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:
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Notes:
- Note that some combinations are forbidden: 0− −, 0+ −, 1− +, 2+ −, 3− +...
:† First row form isospin triplets: ''π''−, ''π''0, ''π''+ etc.
:
‡ Second row contains pairs of elements: φ is supposed to be a state, and ω a state. on the other cases it is not known the exact composition so a prime is used to distinguish the two forms.
:
• For historical reasons, 13''S''1 form of ''ψ'' is called ''J''/ψ
- --- The Bottomonium state symbol is a capital upsilon (may be rendered as a capital Y depending of the font/browser)
The 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:
- In this scheme, particles with ''J''''P'' = 0− are known as ''pseudoscalars'', and mesons with ''J''''P'' = 1− are called ''vectors''. For particles other than those, the number ''J'' is added as a subindex: ''a''0, ''a''1, ''χ''''c''1, etc.
- For most of ''ψ'', ''Υ'' and ''χ'' states is common to include the spectroscopic information: ''Υ''(1S), ''Υ''(2S). The first number is the Principal Quantum Number , and the letter is the spectroscopic notation for ''L''. Multiplicity is omitted since is implied by the symbol, and ''J'' appears as a subindex when needed: ''χ''''b''2(1P). If the spectroscopic information is not available, the mass is used instead: ''Υ''(9460)
- The naming scheme does not differentiate between "pure" quark states and Gluonium states, so gluonium states follow the same naming scheme.
- However, exotic mesons with "forbidden" quantum numbers ''J''''PC'' = 0− −, 0+ −, 1− +, 2+ −, 3− +... would use the same convention as the meson with identical ''J''''P'' numbers, but adding a ''J'' subindex. A meson with isospin 0 and ''J''''PC'' = 1− + would be denoted ''ω''1.
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.
:Note the fact that for ''s'' and ''b'' quarks we get an antiparticle symbol. This is because it is adopted the convention that flavor charge and
Electric Charge must agree in sign. This is also true for the third component of
Isospin : quark up has positive ''I''
3 and charge, quark down has negative charge and ''I''
3. The effect of that is: any flavor of a charged meson has the same sign than the meson's electric charge.
2. If the second quark has also flavor (it is ''u'' or ''d'') then the identity of that second quark is given by a subindex (''s'', ''c'' or ''b'', and in theory ''t'').
- " superindex if the meson is in the normal spin-parity series, i.e. ''J''''P'' = 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:
:
† ''J'' is omitted for 0− and 1−
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).
