Meson

Mesons (from Greek: τὸ μέσον ( tó Méson ) " which located in the middle ", points to its medium-heavy mass back ) are unstable subatomic particles. Made up of a quark - antiquark pair, they form one of the two groups of hadrons. From the second Hadron Group, baryons, mesons is distinguished by its integer spin; they are thus bosons.

Mesons produced in high-energy particle collisions (eg in cosmic rays or in particle accelerator experiments) and disintegrated in a split second. They are classified according to the type of quarks contained, its spin and its parity. Through its quarks participate mesons in the strong and weak interaction and gravitation; electrically charged mesons are also subject to the electromagnetic interaction.

Properties

The quark model allows a consistent description of all observed mesons as bound state of a quark with the antiparticle of a quark ( antiquark ). As composite particles mesons are thus no fundamental elementary particles. Formal can be formed exactly 36 combinations of the six known quarks and the six corresponding anti-quarks; within these combinations you will find the antiparticle to a given meson, when the quark is replaced by its corresponding anti-quark and the antiquark by its quark.

Mesons are characterized by the quantum numbers:

• Coupling of the quark spins ( number or value ) or multiplicity ( number or value )
• Angular momentum and
• Total angular momentum (possible values ​​due to the spin-orbit coupling).

The total angular momentum is also called " spin of the meson ". Observable quantities are spinning this as well:

• Parity, since fermion and antifermion have opposite intrinsic parities, ie, and the product resulting from intrinsic parity and the parity of the spatial wave function. Further
• The C- parity.

Orbital angular momentum and quark spin coupling can be derived.

Mesons have an integer (total) spin, the lightest J = 0 ( scalar or pseudoscalar mesons ) or J = 1 ( vector meson or pseudo- vector mesons ). This can be explained by the fact that the two quarks that form a meson, each of which has a spin of 1/2 and their spins antiparallel or can be parallel in the quark model. In addition can also have inner mesons excitation states, which are described by an orbital angular momentum > 0, as well as radial suggestions. This increases their energy so that they have different properties ( spin, decay products, ...) as the mesons in the ground state.

All mesons are unstable. They decay into lighter hadrons (mostly other lighter mesons ) and / or leptons. Mesons without charge and flavor quantum numbers can also electromagnetically decay into photons.

Multiplets

Since there are six different quark flavors, one can 6 × 6 = 36 different flavor Anti flavor combinations expect (if you Meson and Antimeson counts only time in total ). From this theoretical each 36 mesons result (parallel, antiparallel) for each combination of spin orientation, orbital angular momentum and the radial excitation.

Mesonzustände with higher energy are more difficult to produce short-lived and difficult to separate spectroscopically: In practice, significant restrictions apply. Therefore, the number of known mesons is limited.

This is complicated by the quantum mechanics. The three lighter quarks u, d and s differ in their masses not too much. Therefore, they form in certain cases superposition states of several quark-antiquark pairs: the neutral pion ( π -meson ) is about a mixture of a - uu with a dd - state ( anti-quarks are overlined ). The 3 × 3 = 9 mesons from the three lighter quarks must therefore be treated in its entirety. Looking at the lowest states ( orbital angular momentum, no radial excitation ), thus forming depending on the spin coupling Nonet from pseudoskaleren mesons (JP = 0 -) and vector mesons (JP = 1 -). Three of these mesons have charge Q = 0 and strangeness S = 0 and are quantum mechanical mixtures of uu, dd and ss.

The masses of the heavy c and b quarks are significantly different, thus you can here consider separately the mesons. The t- quark in turn is extremely hard and decays before it can form bound states with other quarks.

Other mesons are known, which can be interpreted as higher excited quark-antiquark states. However, the assignment is not always simple and clear, especially since here can quantum mechanical mixtures occur again.

Naming

Based on observed properties of the atomic nuclei postulated Hideki Yukawa in 1935, a particle, which should convey the attraction between protons and neutrons in the nucleus ( Yukawa potential). Because the predicted mass of the particle was between the masses of the electron and the proton, he named it after the Greek word μέσος mesos, middle ',' in the middle ',' medium '. After the discovery of the first meson, the pion, in 1947 by Cecil Powell Yukawa was awarded in 1949 the Nobel Prize for Physics.

The previously discovered muon whose mass is also between electron and proton mass, was initially thought to be the Yukawa particle and was called My- meson. Later experiments showed, however, that the muon is not subject to the strong interaction. Only gradually did the word meaning Meson transformed into today's above mentioned definition.

In the following decades other mesons were discovered, their masses partially lie above that of the proton. Your naming remained unsystematic until a comprehensive theory ( quark model, quantum chromodynamics ) was formulated that explains the relationships between the mesons. The following are the common names are used since 1988.

Mesons without flavor quantum number

Mesons without flavor quantum number consist either only of u-and d - quarks, or are states of a quark and its own antiquark, called quarkonium (ss, cc, bb, tt). This isospin triplets get uniform naming, also the charged mesons made ​​of light quarks (ud, du ) the expression " without flavor quantum number " within the meaning of that nomenclature. The wording is as follows:

( Isospin 1)

With ss ( isospin 0)

Notes: The TT mesons are hypothetical. 1 --- The ground state of cc is called for historical reasons, not ψ, but J / ψ.

• To distinguish between mesons with the same quantum numbers, the mass is given in MeV/c2, eg ψ ( 3770 ).
• For from heavy quarks (c, b, t) is formed quarkonia, if known, the spectroscopic designation given - for example, ψ (2S ) and J as another index - eg χc1 (1P). (See quarkonium. )
• At the lowest state may omit this information - ie φ = φ (1020 ) and? C =? C (1S).

Mesons with flavor quantum number

Mesons with flavor quantum numbers are quark-antiquark combinations in which ( anti) quark, a s, c, b or t and the other is not its antiparticle. For these mesons following nomenclature applies:

Mesons are green, Antimesonen in yellow * White deposited are the qq combinations whose nomenclature rules for mesons without flavor follows.

• The Symbol of the meson is determined by the heavier ( anti-) quark: Depending on this a s, c, b, or ( hypothetically ) a t, ie the meson K, D, B or T.
• If the lighter ( anti-) quark is not u or d, you are there in addition to a lower index. Example: The combination cs is a Ds meson.
• The electric charge Q is specified as a superscript.
• If the heavier (anti) Quark is positively charged (ie, an S, C, B or T ) is a meson; otherwise ( that is, when the heavier (anti) Quark S, C, B or T ) to a Antimeson. For example, the K0 has the composition sd; the K0 the composition sd Electrically neutral Antimesonen are marked with a slash; wherein the electrically charged, this is not necessary since, according to this convention, positively charged qq combinations always mesons and negatively charged qq combinations are always Antimesonen.
• Mesons with even-numbered total spin and positive parity (JP = 0 , 2 , ...) or odd total spin and negative parity (JP = 1 -, 3 -, ...) are additionally marked with a *. In these mesons, the spins of both quarks are parallel.
• To further distinguish the mass is given ( in MeV/c2 ) in parentheses. In the lightest mesons ( ground state ), this can be omitted.

List of some mesons

Currently ( Particle Data Group compilation of 2012) are known mesons 122; for a further 65 mesons, there are indications (We do not regard the other entries as established ). The following list is a selection of the most important mesons (long-lasting, ground states ):

Notes: KS and KL are quantum mechanical mixtures of K0 and K0 - see Kaon.

Anti-quarks and antiparticles are shown painted.