Mesons (from Greek: τὸ μέσον (tó méson) "the middle one") are unstable subatomic particles . Made up of a quark - anti- quark pair, they form one of the two groups of hadrons . Mesons differ from the second group of hadrons, the baryons , in their integral spin ; they are therefore bosons .
Mesons arise in high-energy particle collisions (e.g. in cosmic rays or in experiments in high-energy physics ) and decay in fractions of a second. They are classified according to the type of quarks they contain, their spin and their parity . By means of their quarks, mesons take part in the strong and weak interaction as well as gravity ; Electrically charged mesons are also subject to electromagnetic interaction .
In the literature, mesons are sometimes only understood to mean bosons made up of a quark and an antiquark, and sometimes all bosons with a strong interaction. In the latter case, the hypothetical glueballs and tetraquarks would also count among the mesons, which are then also referred to as exotic mesons .
The quark model allows a consistent description of all observed mesons as the binding state of a quark with the antiparticle of a quark (antiquark). As composite particles, mesons are therefore not fundamental elementary particles .
Mesons have an integer (total) spin, the lightest J = 0 ( scalar or pseudoscalar mesons ) or J = 1 ( vector mesons or pseudo- vector mesons ). In the quark model, this can be explained by the fact that the two quarks that form a meson each have a spin of 1/2 and their spins can be antiparallel or parallel. In addition, mesons can also have internal excitation states, which are described by an orbital angular momentum> 0, as well as radial excitations. This increases their energy so that they have different properties (spin, decay products, ...) than the mesons in the ground state .
All mesons are unstable. They decay into lighter hadrons (mostly other, lighter mesons) and / or into leptons . Mesons without charge and flavor quantum numbers can also decay into photons electromagnetically .
Spin and parity
|Pseudoscalar meson||0 - +||0||-||+||0||0|
|Vector meson||1 −−||1||-||-||1|
|Scalar meson||0 ++||0||+||+||1||1|
|Pseudo- vector meson||1 + -||1||+||-||0|
|Tensor meson||2 ++||2||+||+||1|
- the total angular momentum (spin) J
- the parity P
- the charge conjugation C (only defined for mesons without charge and flavor)
These quantum numbers from spin and orbital angular momentum of quark and antiquark can be explained:
- The orbital angular momentum can assume all integer values: L = 0, 1, 2, ...
- The quark spins s = ½ couple to the total spin S = 0 or S = 1
- For the total angular momentum J is considered due to the orbit coupling spin then
- J = L for S = 0
- J = 1 for S = 1 and L = 0
- J = L - 1, L, L + 1 for S = 1 and L> 0
- For parity, the quarks, as fermions and antifermions, have opposite intrinsic parities: P = (−1) L + 1
- For the charge conjugation where: C = (-1) L + S .
The following combinations are therefore not implemented: J PC = 0 −− , (odd J) - + , (even J) + - . Should one discover “exotic mesons” with such quantum numbers, they would have to be composed differently ( tetraquarks , glueballs , ...).
Since there are six different quark flavors , you can expect 6 × 6 = 36 different flavor-anti-flavor combinations (if you count meson and antimesone only once in total). Theoretically, this results in 36 mesons for each combination of spin orientation (parallel, antiparallel), orbital angular momentum and radial excitation.
In practice there are clear restrictions: Meson states with higher energy are more difficult to generate, more short-lived and more difficult to separate spectroscopically . Therefore the number of known mesons is limited.
Quantum mechanics complicates this picture . The three lighter quarks u, d and s do not differ too much in their masses. Therefore, in certain cases, they form superimposed states of several quark-antiquark pairs: the neutral pion (π-meson), for example, is a mixture of a u u - with ad d -state (antiquarks are swept over). The 3 × 3 = 9 mesons from the three lighter quarks must therefore be treated in their entirety. If one considers the lowest states (orbital angular momentum L = 0; no radial excitation), depending on the total spin, nonets are formed from pseudoscalar mesons (J P = 0 - ) and vector mesons (J P = 1 - ). Three of these mesons each have charge Q = 0 and strangeness S = 0 and are quantum mechanical mixtures of u u , d d and s s .
At higher energies, more mesons appear, which can be interpreted as more highly excited quark-antiquark states. However, the assignment is not always easy and unambiguous, especially since quantum mechanical mixtures can occur here as well.
The masses of the heavy c and b quarks differ significantly from those of the u, d and s quarks and from each other, so the mesons can be viewed separately here. The t-quark, in turn, is extremely heavy and decays before it can form bound states with other quarks.
Based on the observed properties of atomic nuclei , Hideki Yukawa postulated a type of particle in 1935 that was supposed to mediate the attraction between protons and neutrons in the atomic nucleus. This Yukawa interaction leads to an attractive Yukawa potential , which depends on the mass of the exchange particle. Because the predicted mass was between the masses of the electron and the proton, he named it 'middle', 'in the middle', 'middle' after the Greek word μέσος mésos . Following the discovery of the first meson, the pion , in 1947 by Cecil Powell , Yukawa was awarded the Nobel Prize in Physics in 1949 .
The previously discovered muon , whose mass is also between electron and proton mass, was initially mistaken for the Yukawa particle and was called the mu-meson . However, later experiments showed that the muon is not subject to the strong interaction. Only gradually did the word meaning of meson change into the current definition given above.
In the decades that followed, further mesons were discovered, some of whose masses are above that of the proton. Their naming remained unsystematic until a comprehensive theory (quark model, quantum chromodynamics ) was formulated that explains the relationships between the mesons. The names that have been in use since 1988 are used below.
Mesons without a flavor quantum number
Mesons without a flavor quantum number either consist only of u and d quarks or are states of a quark and its own antiquark, a so-called quarkonium (s s , c c , b b ). In order for isospin triplets to have uniform names, the charged mesons from light quarks (u d , d u ) are also considered to be “without a flavor quantum number” in the sense of this nomenclature. The naming scheme was established in this form in 1986.
At the end of 2017, the naming scheme was expanded to include exotic mesons such as tetraquark or gluonium states. The naming scheme is primarily based on the quantum numbers (J, P, C and I). In this way, particles can also be precisely identified if their internal structure is not yet known or has not yet been precisely researched. New symbols (Π, R, W, Z) have been defined for the isospin-1 states with hidden charm or bottom flavor. However, the existence of the states Π and W has not yet been proven. Since the top quark is so heavy that it disintegrates too quickly to form bound states, structures like t t are no longer given a separate name.
The naming is as follows:
|J PC||2S + 1 L J||Isospin = 1||Isospin = 0||Isospin = 1|
|u d , d u ,
(u u −d d )
(u u + d d ) with s s
|c c||b b||c c||b b|
|1 −− , 2 −− , 3 −− , ...||3 (L straight) J||ρ||ω, φ||ψ 2)||Υ||R c||R b 1)|
|0 - + , 2 - + , 4 - + , ...||1 (L straight) J||π||η, η '||η c||η b||Π c 1)||Π b 1)|
|0 ++ , 1 ++ , 2 ++ , ...||3 (L odd) J||a||f, f '||χ c||χ b||W c 1)||W b 1)|
|1 + - , 3 + - , 5 + - , ...||1 (L odd) J||b||h, h '||h c||h b||Z c||Z b|
- To distinguish between mesons with the same quantum numbers, the mass in MeV / c 2 is put in brackets after the symbol.
- For mesons formed from light quarks (d, u, s), the spin J is given as the lower index, except for pseudoscalar and vector mesons, e.g. B. a 0 (980).
- For the quarkonia formed from heavy quarks (c, b), the spectroscopic designation is given, if known - e.g. B. ψ (2S), as well as J as a further index - z. B. χ c1 (1P). (For details, see Quarkonium .) Otherwise, the mass is also given here - e.g. B. ψ (3770).
- For the lowest state, the information on mass or spectroscopic state can be omitted - i.e. φ = φ (1020) and η c = η c (1S).
- The symbol X is used for unknown quantum numbers.
Mesons with a flavor quantum number
Mesons with a flavor quantum number are quark-antiquark combinations in which one (anti-) quark is an s, c or b and the other is not its antiparticle. Top quark states are no longer included in the naming scheme.
The following nomenclature applies to these mesons:
|down||(*)||(*)||K 0||D -||B 0|
|up||(*)||(*)||K +||D 0||B +|
|strange||K 0||K -||(*)||D s -||B s 0|
|charm||D +||D 0||D s +||(*)||B c +|
|bottom||B 0||B -||B s 0||B c -||(*)|
Mesons are highlighted in green, antimesons in yellow
- The code letter of the meson is based on the heavier (anti-) quark: depending on whether this is an s, c or b, the meson is called K, D or B.
- If the lighter (anti-) quark is not a u or d, it is also given as a lower index. Example: The combination c s is a D s meson.
- The electrical charge Q is given as the upper index.
- If the heavier (anti-) quark is positively charged (ie is an s , c or b ), it is by convention a meson; otherwise ( i.e. if the heavier (anti-) quark is an s, c or b) it is an antimeson. Example: the K 0 has the composition s d; the K 0 the composition s d . Electrically neutral antimesons are marked with a slash; this is not necessary for the electrically charged ones , since according to this convention positively charged q q combinations are always mesons and negatively charged q q combinations are always antimesons.
- Mesons with an even total spin and positive parity ( J P = 0 + , 2 + , ...) or an odd total spin and negative parity ( J P = 1 - , 3 - , ...) are also marked with a *. With these mesons, the spins of both quarks are parallel.
- For further differentiation, the mass (in MeV / c 2 ) is given in brackets. This can be omitted for the lightest mesons (ground state).
List of some mesons
Currently ( Particle Data Group , compilation from 2019) 139 mesons are known; there are indications for a further 74 mesons (We do not regard the other entries as established) . The following list gives a selection of the most important mesons (long-lived, ground states):
( MeV / c²)
( s )
|Pseudoscalar mesons made up of d-, u- and s-quarks|
|Pion||π + , π -||u d , u d||139.6||2.6 · 10 −8|
|Pion||π 0||(u u - d d )||135.0||8.5 · 10 −17|
|Kaon||K + , K -||u s , s u||493.7||1.2 · 10 −8|
|Kaon||K 0 , K 0||d s , s d||497.6||K S 1) : 9.0 · 10 −11
K L : 5.1 · 10 −8
|η meson||η||(u u + d d - 2 s s )||547.9||5 · 10 −19|
|η′-meson||η ′||(u u + d d + s s )||957.8||3 · 10 −21|
|Vector mesons from d-, u- and s-quarks|
|ρ meson||ρ + , ρ -||u d , u d||770||4 · 10 −24|
|ρ meson||ρ 0||(u u - d d )||775.5||4 · 10 −24|
|Kaon||K * + , K * -||u s , s u||891.8||1.3 · 10 −23|
|Kaon||K * 0 , K * 0||d s , s d||895.6||1.3 · 10 −23|
|ω meson||ω||(u u + d d )||782.6||7 · 10 −23|
|φ meson||φ||s s||1019.5||2 · 10 −22|
|Mesons with c and / or b quarks|
|D meson||D + , D -||c d , c d||1869.6||10.4 · 10 −13|
|D meson||D 0 , D 0||c u , c u||1864.8||4.1 · 10 −13|
|D s -Meson||D s + , D s -||c s , c s||1968.3||5.0 · 10 −13|
|J / ψ meson||J / ψ||c c||3096.9||8 · 10 −19|
|B meson||B + , B -||u b , u b||5279.3||1.6 · 10 −12|
|B meson||B 0 , B 0||b d, b d||5279.6||1.5 · 10 −12|
|Υ meson||Υ||b b||9460.3||1.3 · 10 −20|
Antiquarks and antiparticles are shown overlined .
- Particle Data Group . Measurement values of elementary particles
- WE Burcham, M. Jobes (1995)
- Hideki Yukawa : On the Interaction of Elementary Particles. I . In: Proc. Phys.-Math. Soc. Japan . tape 17 , 1935, pp. 48-57 .
- Wilhelm Gemoll : Greek-German school and hand dictionary. Munich / Vienna 1965.
- CMG Lattes, H. Muirhead, GPS Occhialini, CF Powell: Processes Involving Charged Mesons . In: Nature . tape 159 , 1947, pp. 694-697 , doi : 10.1038 / 159694a0 .
- Particle Data Group: Naming scheme for hadrons (Revised in 2017). (PDF; 86 KB) Accessed February 17, 2018 (English).
- M. Tanabashi et al. (Particle Data Group): Phys. Rev. D 98, 030001 (2018) and 2019 update (cutoff: Jan 15, 2019)