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Meson
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In particle physics, mesons are subatomic particles composed of one quark and one antiquark. They are part of the hadron particle family – particles made of quarks. The other members of the hadron family are the baryons – subatomic particles composed of three quarks. The main difference between mesons and baryons is that mesons are bosons while baryons are fermions – that is mesons have integer spin while baryons have half-integer spin – meaning that the Pauli exclusion principle does not apply to them.
Since mesons are composed of quarks, they participate in both the weak and strong interactions.
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Encyclopedia
In particle physics, mesons are subatomic particles composed of one quark and one antiquark. They are part of the hadron particle family – particles made of quarks. The other members of the hadron family are the baryons – subatomic particles composed of three quarks. The main difference between mesons and baryons is that mesons are bosons while baryons are fermions – that is mesons have integer spin while baryons have half-integer spin – meaning that the Pauli exclusion principle does not apply to them.
Since mesons are composed of quarks, they participate in both the weak and strong interactions. Mesons with net electric charge also participate in the electromagnetic interaction. They are classified according to their quark content, total angular momentum, parity, and various other properties such as C-parity and G-parity. While no meson is stable, those of lower mass are nonetheless more stable than the most massive mesons, and are easier to observe and study in particle accelerators or in cosmic ray experiments. They are also typically less massive than baryons, meaning that they are more easily produced in experiments, and will exhibit higher energy phenomena sooner than baryons would. For example, the charm quark was first seen in the J/Psi meson in 1974, and the bottom quark in the upsilon meson in 1977.
Each meson has a corresponding antiparticle (antimeson) where quarks are replaced by their corresponding antiquarks and vice-versa. For example, a positive pion is made of one up quark and one down antiquark; and its corresponding antiparticle, the negative pion , is made of one up antiquark and one down quark. Some experiments show the evidence of tetraquarks – "exotic" mesons made of two quarks and two antiquark, but the particle physics community as a whole does not view their existence as likely, although still possible.
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'.
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 in today's nomenclature. The pion was the first true meson to be discovered. The word meson has at times also been used to mean any force carrier, as in, for instance "Z0 meson". This usage has also fallen out of favor.
Overview
Spin, orbital angular momentum, and total angular momentum
Spin (quantum number S) is a vector quantity that represents the "intrinsic" angular momentum of a particle. It comes in increments of h (pronounced "h-bar"). The h is often dropped because it is the "fundamental" unit of spin, and it is implied that "spin 1" means "spin 1 h". In some systems of natural units, h is chosen to be 1, therefore does not appear anywhere.
Quarks are fermionic particles of spin (S = ). Because spin projections varies in increments of 1 (that is 1 h), a single quark has a spin vector of length , and has two spin projections (Sz = + and Sz = -). Two quarks can have their spins aligned, in which case the two spin vectors add to make a vector of length S = 1 and three spin projections (Sz = +1, Sz = 0, and Sz = -1) called the spin-1 triplet. If two quarks have unaligned spins, the spin vectors add up to make a vector of length S = 0 and has only one spin projection (Sz = 0), called the spin-0 singlet etc. Since mesons are made of one quark and one antiquark, they can be found in triplets and singlets spin states.
There is another quantity of angular momentum, called the orbital angular momentum (quantum number L), that comes in increments of 1 h, which represent the angular moment due to quarks orbiting around each other. The total angular momentum (quantum number J) of a particle is therefore the combination of intrinsic angular momentum (spin) and orbital angular momentum. It can take any value from to , in increments of 1.
Meson angular momentum quantum numbers for L = 0, 1, 2, 3 | Spin (S) | Orbital angular momentum (L) | Total angular momentum (J) | Parity (P)
(See below) | Condensed notation (JP) |
|---|
| 0 | 0 | 0 | - | 0- | | 1 | 1 | + | 1+ | | 2 | 2 | - | 2-, 3- | | 3 | 3 | + | 3+, 3+ | | 1 | 0 | 1 | - | 1- | | 1 | 1, 0 | + | 1+, 0+ | | 2 | 3, 2, 1 | - | 3-, 2-, 1- | | 3 | 4, 3, 2 | + | 4+, 3+, 2+ |
Particles physicists are most interested in mesons with no orbital angular momentum (L = 0), therefore the two groups of mesons most studied are the S = 1; L = 0 and S = 0; L = 0, which corresponds to J = 1 and J = 0, although they are not the only ones. It is also possible to obtain J = 1 particles from S = 0 and L = 1. How to distinguish between the S = 1, L = 0 and S = 0, L = 1 mesons is an active area of research in meson spectroscopy.
Parity
If the universe were reflected in a mirror, most of the laws of physics would be identical – things would behave the same way regardless of what we call "left" and what we call "right". This concept of mirror reflection is called parity (P). Gravity, the electromagnetic force, and the strong interaction all behave in the same way regardless of whether or not the universe is reflected in a mirror, and thus are said to conserve parity (P-symmetry). However, the weak interaction does distinguish "left" from "right", a phenomenon called parity violation (P-violation).
Based on this, one might think that if the wavefunction for each particle (more precisely, the quantum field for each particle type) were simultaneously mirror-reversed, then the new set of wavefunctions would perfectly satisfy the laws of physics (apart from the weak interaction). It turns out that this is not quite true: In order for the equations to be satisfied, the wavefunctions of certain types of particles have to be multiplied by -1, in addition to being mirror-reversed. Such particle types are said to have negative or odd parity (P = -1, or alternatively P = –), while the other particles are said to have positive or even parity (P = +1, or alternatively P = +).
For mesons, the parity is related to the orbital angular momentum by the relation:
As a consequence, mesons with no orbital angular momentum (L = 0) all have odd parity (P = -).
C-parity
C-parity is only defined for mesons that are their own antiparticle (i.e neutral mesons). It represents whether or not the wavefunction of the meson remains the same under the interchange of their quark with their antiquark. If
then, the meson is "C even" (C = +1). On the other hand, if
then the meson is "C odd" (C = -1).
C-parity rarely is studied on its own, but the combination of C- and P-parity into CP-parity. CP-parity was thought to be conserved, but was later found to violated in weak interactions.
G-parity
G parity is a generalizations of the C-parity. Instead of simply comparing the wavefunction after exchanging quarks and antiquarks, it compares the wavefunction after exchanging the meson for the corresponding antimeson, regardless of quark content. In the case of neutral meson, G-parity is equivalent to C-parity because neutral mesons are their own antiparticles.
If
then, the meson is "G even" (G = +1). On the other hand, if
then the meson is "G odd" (G = -1).
Isospin and charge
The concept of isospin was first proposed by Werner Heisenberg in 1932 to explain the similarities between protons and neutrons under the strong interaction. Although they had different electric charges, their masses were so similar that physicists believed they were actually the same particle. The different electric charges were explained as being the result of some unknown excitation similar to spin. This unknown excitation was later dubbed isospin by Eugene Wigner in 1937. When the first mesons were discovered, they too were seen through the eyes of isospin. The three pions were believed to be the same particle, but in different isospin states.
This belief lasted until Murray Gell-Mann proposed the quark model in 1964 (containing originally only the u, d, and s quarks). The success of the isospin model is now understood to be the result of the similar masses of the u and d quarks. Since the u and d quarks have similar masses, particles made of the same number then also have similar masses. The exact specific u and d quark composition determines the charge, as u quarks carry charge + while d quarks carry charge -. For example the three pions all have different charges ( , (a quantum superposition of and states), , but have similar masses (~140 MeV/c2) as they are each made of a total of pairs of up and down quarks and antiquarks. Under the isospin model, they were considered to be a single particle in different charged states.
The mathematics of isospin was modeled after that of spin. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a "charged state". Since the "pion particle" had three "charged states", it was said to be of isospin I = 1. Its "charged states" , , and , corresponded to the isospin projections Iz = +1, Iz = 0, and Iz = -1 respectively. Another example is the "rho particle", also with three charged states. Its "charged states" , , and , corresponded to the isospin projections Iz = +1, Iz = 0, and Iz = -1 respectively. It was later noted that the isospin projections were related to the up and down quark content of particles by the relation:
where the n's are the number of up and down quarks and antiquarks.
In the "isospin picture", the three pions and three rhos were thought to be the different states of two particles. However in the quark model, the rhos are excited states of pions. Isospin, although conveying an inaccurate picture of things, is still used to classify hadrons, leading to unnatural and often confusing nomenclature. Since mesons are hadrons, the isospin classification is also used, with Iz = + for up quarks and down antiquarks, and Iz = - for up antiquarks and down quarks.
Flavour quantum numbers
The strangeness flavour quantum number S (not to be confused with spin) was noticed to go up and down along with particle mass. The higher the mass, the lower the strangeness (the more s quarks). Particles could be described with isospin projections (related to charge) and strangeness (mass) (see the uds nonet figures on the right). As other quarks where discovered, new quantum numbers were made to have similar description of udc and udb nonets. Since only the u and d mass are similar, this description of particle mass and charge in terms of isospin and flavour quantum numbers only works well for the nonets made of one u, one d and one other quark and breaks down for the nonets (for example ucb nonet). If the quarks all had the same mass, their behaviour would be called symmetric, as they would all behave in exactly the same way with respect to the strong interaction. Since quarks do not have the same mass, they do not interact in the same way (exactly like an electron placed in an electric field will accelerate more than a proton placed in the same field because of its lighter mass), and the symmetry is said to be broken.
It was noted that charge (Q) was related to the isospin projection (Iz), the baryon number (B) and flavour quantum numbers (S, C, B′, T) by the Gell-Mann–Nishijima formula:
where S, C, B′, and T represent the strangeness, charmness, bottomness and topness flavour quantum numbers respectively. They are related to the number of strange, charm, bottom, and top quarks and antiquark according to the relations:
meaning that the Gell-Man–Nishijima formula is equivalent to the expression of charge in terms of quark content:
Classification
Mesons are classified into groups according to their isospin (I), total angular momentum (J), parity (P), G-parity (G) or C-parity (C) when applicable, and quark (q) content. The rules for classification are defined by the Particle Data Group, and are rather convoluted. The rules are presented below, in table form for simplicity.
Types of Meson
Mesons are classified into types according to their spin configurations. Some specific configurations are given special names based on the mathematical properties of their spin configuration.
Nomenclature
Flavourless mesons
Flavourless mesons are mesons made of pair of quark and antiquarks of the same flavour (all their flavour quantum numbers are zero (Iz = 0, S = 0, C = 0, B′ = 0, T = 0)). The rules for flavourless mesons are:
Nomenclature of flavourless mesons | J PC?
I ? | 0-+, 2-+, 4-+, ... | 1+-, 3+-, 5+-, ... | 1--, 2--, 3--, ... | 0++, 1++, 2++, ... |
|---|
-
| 1 |
| b+
b0
b- |
| a+
a0
a- |
| 0 | | h
h′ | | f
f′ | | 0 | | hc | | ?c | | 0 | | hb | | ?b | | 0 | | ht | | ?t | |
† The C parity is only relevant to neutral mesons.
†† For J PC=1 --, the ? is often called the
In addition:
- When the spectroscopic state of the meson is known, it is added in parenthesis.
- When the spectroscopic state is unknown, mass (in MeV/c2) is added in parenthesis.
- When the meson is in its ground state, nothing is added in parenthesis.
Flavourful mesons
Flavourful mesons are mesons made of pair of quark and antiquarks of different flavours. The rules are simpler in this case; The main symbol depends on the quark, and the subscript (if any) depends on the antiquark. In table form, they are:
Nomenclature of flavourful mesons antiquark ?
quark ? | up | down | strange | charm | bottom | top |
|---|
| up | — | — | | | | | | down | — | — | | | | | | strange | | | — | | | | | charm | | | | — | | | | bottom | | | | | — | | | top | | | | | | — | |
In addition:
- If JP is in the "normal series" (i.e., JP = 0+, 1-, 2+, 3-, ...), a superscript * is added.
- If the meson is not pseudoscalar (JP = 0+) or vector (JP = 1-), J is added as a subscript.
- When the spectroscopic state of the meson is known, it is added in parenthesis.
- When the spectroscopic state is unknown, mass (in MeV/c2) is added in parenthesis.
- When the meson is in its ground state, nothing is added in parenthesis.
Lists of mesons
These lists detail all known and predicted pseudoscalar (JP = 0-) and vector (JP = 1-) mesons.
The symbols encountered in these lists are: I (isospin), J (total angular momentum), P (parity), C (C-parity), G (G-parity), u (up quark), d (down quark), s (strange quark), c (charm quark), b (bottom quark), Q (charge), B (baryon number), S (strangeness), C (charmness), and B' (bottomness), as well as a wide array of subatomic particles (hover for name).
The properties and quark content of the particles are given in the tables; for antiparticles, simply change quarks in antiquarks (and vice-versa), and Q, B, S, C, and B' would be of opposite signs. Particles with † next to their names have been predicted by the standard model but not yet observed. Values in red have not been firmly established by experiments, but are predicted by the quark model and are consistent with the measurements.
See also
External links
-
- Authoritative information on particle properties is compiled by the
Particle Data Group http://pdg.lbl.gov (a pdf file)
Recent Findings
Bibliography
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