Meson
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2008-07-13T19:11:52Z
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{{Cleanup|date|date=May 2008}}
{{Infobox Particle
| bgcolour =
| name = Meson
| image = [[Image:Noneto mesônico de spin 0.png|200px]]
| caption = Mesons of spin 0 form a [[nonet]]
| num_types = ~140 ([[List of mesons|List]])
| composition = [[Composite particle|Composite]] - [[Quark]]s and [[Antiparticle|antiquark]]s
| family = [[Hadron]]
| group =
| generation =
| interaction = [[Strong interaction|Strong]]
| particle =
| antiparticle =
| status =
| theorized = [[Hideki Yukawa]] (1935)
| discovered = 1947
| symbol =
| mass =
| decay_time =
| decay_particle =
| electric_charge =
| color_charge =
| spin = Integer
| num_spin_states =
}}
[[Image:Noneto mesônico de spin 1.png|thumb|Mesons of spin 1 form a nonet]]
In [[particle physics]], a '''meson''' is a [[strong interaction|strongly interacting]] [[boson]]—that is, a [[hadron]] with integer [[spin (physics)|spin]]. In the [[Standard Model]], mesons are composite (non-elementary) particles composed of an even number of [[quark]]s and [[Antiparticle|antiquark]]s. All known mesons are believed to consist of a quark-antiquark pair—the so-called [[valence quark]]s—plus a "sea" of virtual quark-antiquark pairs and virtual [[gluon]]s. Searches for [[exotic meson]]s that have different constituents are ongoing. The [[valence quark]]s may exist in a superposition of [[flavor (particle physics)|flavor]] states; for example, the neutral [[pion]] is neither an up-antiup pair nor a down-antidown pair, but an equal [[quantum superposition|superposition]] of both. [[Pseudoscalar meson]]s (spin 0), where the quark and antiquark have opposite spin, have the lowest rest energy. Next lowest in rest energy are [[vector meson]]s (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 [[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 hydrodynamics]] for a discussion of modern theories in which [[nucleon-nucleon interaction]]s are mediated by meson exchange.)
==History==
In 1949 [[Hideki Yukawa]] was awarded the [[Nobel Prize]] in [[Physics]] for predicting the existence of the meson. He called the particle the meson, from ''mesos'', Greek for ''intermediate'', because its mass was between that of the electron and proton. He had originally named it the 'mesotron', but was corrected by [[Werner Heisenberg]] (whose father was a professor in Greek at the University of Munich), who pointed out that there is no 'tr' in the Greek word 'mesos'.
==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 [[Flavour (particle physics)|flavor]]: flavorless mesons and flavored mesons.
===Flavorless 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 quantum number|spin]] ''S'' and total [[azimuthal quantum number|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. These two quantum numbers determine the [[parity]] ''P'' and the [[C parity|charge-conjugation parity]] ''C'' of the meson:
:''P'' = (−1)<sup>''L''+1</sup>
:''C'' = (−1)<sup>''L''+''S''</sup>
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]] <sup>2''S''+1</sup>''L''<sub>''J''</sub> (a letter code is used instead of the actual value of ''L'', see the [[spectroscopic notation]]) and the symbol ''J''<sup>''PC''</sup> (here only the sign is used for ''P'' and ''C'').
The different possibilities and the corresponding meson symbols are given in the following table:
<table border="0" cellspacing="0" cellpadding="0">
<tr>
<th width="150px" style="border-top: 6px double windowtext"> </th>
<th width="70px" style="border-right: 2px solid windowtext; border-top: 6px double windowtext" align="right">''J<sup>PC</sup>'' = </th>
<th width="100px" style="border-top: 6px double windowtext" align="center">
(0, 2…)<sup>− +</sup></th>
<th width="100px" style="border-top: 6px double windowtext" align="center">
(1, 3…)<sup>+ −</sup></th>
<th width="100px" style="border-top: 6px double windowtext" align="center">
(1,2…)<sup>− −</sup></th>
<th width="100px" style="border-top: 6px double windowtext" align="center">
(0, 1…)<sup>+ +</sup></th>
</tr>
<tr>
<th style="border-bottom: 2px solid windowtext;" align="center">Quark composition</th>
<th style="border-right: 2px solid windowtext; border-bottom: 2px solid windowtext" align="right">
<sup>2''S''+1</sup>''L''<sub>''J''</sub> = <sup>[[#JPC|*]]</sup></th>
<th style="border-bottom: 2px solid windowtext" align="center">
<sup>1</sup>(''S'', ''D''…)<sub>''J''</sub></th>
<th style="border-bottom: 2px solid windowtext" align="center">
<sup>1</sup>(''P'', ''F''…)<sub>''J''</sub></th>
<th style="border-bottom: 2px solid windowtext" align="center">
<sup>3</sup>(''S'', ''D''…)<sub>''J''</sub></th>
<th style="border-bottom: 2px solid windowtext" align="center">
<sup>3</sup>(''P'', ''F''…)<sub>''J''</sub></th>
</tr>
<tr>
<td>
<math>u \bar d\mbox{, }u \bar u - d\bar d\mbox{, }d\bar u</math> <sup>[[#I|†]]</sup></td>
<td style="border-right: 2px solid windowtext" align="center">
[[isospin|''I'']] = 1</td>
<td align="center">π</td>
<td align="center">''b''</td>
<td align="center">ρ</td>
<td align="center">''a''</td>
</tr>
<tr>
<td><math>u \bar u + d \bar d \mbox{, }s \bar s</math> <sup>[[#pairs|‡]]</sup></td>
<td style="border-right: 2px solid windowtext" align="center">
''I'' = 0</td>
<td align="center">''η'', ''η''’</td>
<td align="center">''h'', ''h’''</td>
<td align="center"><math>\phi\,\!</math>, ''ω''</td>
<td align="center">''f'', ''f''’</td>
</tr>
<tr>
<td><math>c \bar c</math></td>
<td style="border-right: 2px solid windowtext" align="center">
''I'' = 0</td>
<td style="border: medium none" align="center">''η<sub>c</sub>''</td>
<td align="center">''h''<sub>c</sub></td>
<td align="center">''ψ'' <sup>[[#Jpsi|•]]</sup></td>
<td align="center">''χ<sub>c</sub>''</td>
</tr>
<tr>
<td style="border-bottom: 6px double windowtext"><math>b \bar b</math></td>
<td style="border-bottom: 6px double windowtext; border-right: 2px solid windowtext" align="center">
''I'' = 0</td>
<td style="border-bottom: 6px double windowtext;" align="center">
''η<sub>b</sub>''</td>
<td style="border-bottom: 6px double windowtext;" align="center">''h<sub>b</sub>''</td>
<td style="border-bottom: 6px double windowtext;" align="center">''Υ'' <sup>[[#Upsilon|**]]</sup></td>
<td style="border-bottom: 6px double windowtext;" align="center">
''χ<sub>b</sub>''</td>
</tr>
</table>
<small>Notes:</small>
:<span id="jpc"><sup>*</sup></span> <small>Note that some combinations are forbidden: 0<sup>− −</sup>, 0<sup>+ −</sup>, 1<sup>− +</sup>, 2<sup>+ −</sup>, 3<sup>− +</sup>...</small>
:<span id="I"><sup>†</sup></span> <small>First row form isospin triplets: ''π''<sup>−</sup>, ''π''<sup>0</sup>, ''π''<sup>+</sup> etc.</small>
:<span id="pairs"><sup>‡</sup></span> <small>Second row contains pairs of elements: φ is supposed to be a <math>s\bar s</math> state, and ω a <math>u \bar u + d \bar d</math> state. In the other cases, the exact composition is not known, so a prime is used to distinguish the two forms.</small>
:<span id="Jpsi"><sup>•</sup></span> <small>For historical reasons, 1<sup>3</sup>''S''<sub>1</sub> form of ''ψ'' is called [[J/Psi particle|''J''/ψ]]</small>
:<span id="Upsilon"><sup>**</sup></span> <small>The [[bottomonium]] state symbol is a capital upsilon (may be rendered as a capital Y depending of the font/browser)</small>
The '''normal spin-parity series''' is formed by those mesons where ''P''=(−1)<sup>''J''</sup>. In the normal series, ''S'' = 1 so ''PC'' = +1 (i.e., ''P'' = ''C''). This corresponds to some of the triplet states (triplet states appear in the last two columns).
[[Image:Eta-decay.png|thumb|right|260px|[[Feynman diagram]] of one mode in which the eta particle can decay into 3 [[pion]]s by [[gluon]] emission.]]
Since some of these symbols can refer to more than one particle, some extra rules are added:
* In this scheme, particles with ''J''<sup>''P''</sup> = 0<sup>−</sup> are known as ''pseudoscalars'', and mesons with ''J''<sup>''P''</sup> = 1<sup>−</sup> are called ''vectors''. For particles other than those, the number ''J'' is added as a subindex: ''a''<sub>0</sub>, ''a''<sub>1</sub>, ''χ''<sub>''c''1</sub>, etc.
* For most of ''ψ'', ''Υ'' and ''χ'' states it is common to include 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 it is implied by the symbol, and ''J'' appears as a subindex when needed: ''χ''<sub>''b''2</sub>(1P). If spectroscopic information is not available, 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''<sup>''PC''</sup> = 0<sup>− −</sup>, 0<sup>+ −</sup>, 1<sup>− +</sup>, 2<sup>+ −</sup>, 3<sup>− +</sup>... would use the same convention as the meson with identical ''J''<sup>''P''</sup> numbers, but adding a ''J'' subindex. A meson with isospin 0 and ''J''<sup>''PC''</sup> = 1<sup>− +</sup> would be denoted ''ω''<sub>1</sub>.
When the quantum numbers of a particle are unknown, it is designated with an ''X'' followed by its mass in parentheses.
===Flavored mesons===
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.
{| align="center" cellspacing="0"
|-
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext" | quark
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext; border-right: 2px solid windowtext;" | symbol
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext" | quark
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext" | symbol
|-
| align="center" | ''c''
| align="center" style="border-right: 2px solid windowtext;" | ''D''
| align="center" | ''t''
| align="center" | ''T''
|-
| align="center" style="border-bottom: 2px solid windowtext" | ''s''
| align="center" style="border-bottom: 2px solid windowtext; border-right: 2px solid windowtext;" | <math>\bar K</math>
| align="center" style="border-bottom: 2px solid windowtext" | ''b''
| align="center" style="border-bottom: 2px solid windowtext" | <math>\bar B</math>
|}
:For ''s'' and ''b'' quarks we get an antiparticle symbol. This is because the adopted convention is 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''<sub>3</sub> and charge, quark down has negative charge and ''I''<sub>3</sub>. The effect of that is: any flavor of a charged meson has the same sign as the meson's electric charge.
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. ''J''<sup>''P''</sub> = 0<sup>+</sup>, 1<sup>−</sup>, 2<sup>+</sup>...
4. For mesons other than ''pseudoscalars'' (0<sup>−</sup>) and ''vectors'' (1<sup>−</sup>) the [[total angular momentum quantum number]] ''J'' is added as a subindex.
To sum it up, we have:
{| align="center" cellspacing="0"
|-
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext" | quark composition
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext; border-right: 2px solid windowtext;" | Isospin
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext" | ''J<sup>P</sup>'' = 0<sup>−</sup>, 1<sup>+</sup>, 2<sup>−</sup>...
! align="center" width="100px" style="border-top: 2px solid windowtext; border-bottom: 2px solid windowtext" | ''J<sup>P</sup>'' = 0<sup>+</sup>, 1<sup>−</sup>, 2<sup>+</sup>...
|-
| align="center" | <math>\bar su,\ \bar sd</math>
| align="center" style="border-right: 2px solid windowtext;" | 1/2
| align="center" | <math>K_J</math> <sup>[[#note|†]]</sup>
| align="center" | <math>K^*_J</math>
|-
| align="center" | <math>c \bar u,\ c\bar d</math>
| align="center" style="border-right: 2px solid windowtext;" | 1/2
| align="center" | <math>D_J</math>
| align="center" | <math>D^*_J</math>
|-
| align="center" | <math>c \bar s</math>
| align="center" style="border-right: 2px solid windowtext;" | 0
| align="center" | <math>D_{sJ}</math>
| align="center" | <math>D^*_{sJ}</math>
|-
| align="center" | <math>\bar bu,\ \bar bd</math>
| align="center" style="border-right: 2px solid windowtext;" | 1/2
| align="center" | <math>B_J</math>
| align="center" | <math>B^*_J</math>
|-
| align="center" | <math>\bar bs</math>
| align="center" style="border-right: 2px solid windowtext;" | 0
| align="center" | <math>B_{sJ}</math>
| align="center" | <math>B^*_{sJ}</math>
|-
| align="center" style="border-bottom: 2px solid windowtext" | <math>\bar bc</math>
| align="center" style="border-bottom: 2px solid windowtext; border-right: 2px solid windowtext;" | 0
| align="center" style="border-bottom: 2px solid windowtext" | <math>B_{cJ}</math>
| align="center" style="border-bottom: 2px solid windowtext" | <math>B^*_{cJ}</math>
|}
:<small><span id="note">†</span> ''J'' is omitted for 0<sup>−</sup> and 1<sup>−</sup></small>
In some cases, particles can mix between them. For example, the neutral kaon, <math>K^0\,(\bar sd)</math> and its antiparticle <math>\bar K^0\,(s\bar d)</math> can combine in a symmetric or antisymmetric manner, originating two new particles, the short-lived and the long-lived neutral kaons <math>K^0_S = \begin{matrix}{1 \over \sqrt 2}\end{matrix}(K^0-\bar K^0),\;K^0_L = \begin{matrix}{1 \over \sqrt 2}\end{matrix}(K^0 + \bar K^0)</math> (neglecting a small [[CP violation| CP-violating]] term).
==See also==
* [[List of mesons]]
* [[List of particles]]
* [[Quark model]]
{{particles}}
==External links==
* [http://hyperphysics.phy-astr.gsu.edu/hbase/particles/meson.html#c1 A table of some mesons and their properties]
* Authoritative information on particle properties is compiled by the ''Particle Data Group'' http://pdg.lbl.gov
*[http://arxiv.org/abs/hep-ph/0211411 hep-ph/0211411: The light scalar mesons within quark models]
* [http://pdg.lbl.gov/2004/reviews/namingrpp.pdf Naming scheme for hadrons] (a pdf file)
===Recent Findings===
* [http://www.fnal.gov/pub/presspass/press_releases/DZeroB_s.html What Happened to the Antimatter? Fermilab's DZero Experiment Finds Clues in Quick-Change Meson]
* [http://www.fnal.gov/pub/presspass/press_releases/CDF_meson.html CDF experiment's definitive observation of matter-antimatter oscillations in the Bs meson]
[[Category:Mesons| ]]
[[ar:ميزون]]
[[bs:Mezon]]
[[bg:Мезон]]
[[ca:Mesó]]
[[cs:Mezon]]
[[de:Meson]]
[[el:Μεσόνιο]]
[[es:Mesón (partícula)]]
[[eo:Mezono]]
[[fa:مزون]]
[[fr:Méson]]
[[ko:중간자]]
[[hr:Mezon]]
[[is:Miðeind]]
[[it:Mesone]]
[[he:מזון (חלקיק)]]
[[ka:მეზონი]]
[[lv:Mezoni]]
[[lt:Mezonas]]
[[hu:Mezon]]
[[nl:Meson]]
[[ja:中間子]]
[[no:Meson]]
[[nds:Meson]]
[[pl:Mezony]]
[[pt:Méson]]
[[ro:Mezon]]
[[ru:Мезон]]
[[sk:Mezón]]
[[fi:Mesoni]]
[[sv:Meson]]
[[vi:Meson]]
[[tr:Mezon]]
[[uk:Мезони]]
[[zh:介子]]