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particle physics Particle physics or high energy physics is the study of fundamental particles and forces that constitute matter and radiation. The fundamental particles in the universe are classified in the Standard Model as fermions (matter particles) an ...
, a meson ( or ) is a type of
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
ic
subatomic particle In physical sciences, a subatomic particle is a particle that composes an atom. According to the Standard Model of particle physics, a subatomic particle can be either a composite particle, which is composed of other particles (for example, a pr ...
composed of an equal number of
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly ...
s and
antiquark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly ...
s, usually one of each, bound together by the
strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called t ...
. Because mesons are composed of quark subparticles, they have a meaningful physical size, a diameter of roughly one
femtometre The magnitudes_.html" ;"title="Magnitude_(mathematics).html" ;"title="atom.html" ;"title="helium helium_atom_and_perspective_Magnitude_(mathematics)">magnitudes_">Magnitude_(mathematics).html"_;"title="atom.html"_;"title="helium_atom">helium_a ...
(10 m), which is about 0.6 times the size of a
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
or
neutron The neutron is a subatomic particle, symbol or , which has a neutral (not positive or negative) charge, and a mass slightly greater than that of a proton. Protons and neutrons constitute the nuclei of atoms. Since protons and neutrons beha ...
. All mesons are unstable, with the longest-lived lasting for only a few hundredths of a microsecond. Heavier mesons decay to lighter mesons and ultimately to stable
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
s,
neutrino A neutrino ( ; denoted by the Greek letter ) is a fermion (an elementary particle with spin of ) that interacts only via the weak interaction and gravity. The neutrino is so named because it is electrically neutral and because its rest mass ...
s and
photon A photon () is an elementary particle that is a quantum of the electromagnetic field, including electromagnetic radiation such as light and radio waves, and the force carrier for the electromagnetic force. Photons are Massless particle, massless ...
s. Outside the nucleus, mesons appear in nature only as short-lived products of very high-energy collisions between particles made of quarks, such as
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our o ...
s (high-energy protons and neutrons) and
baryonic matter In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classifi ...
. Mesons are routinely produced artificially in
cyclotron A cyclotron is a type of particle accelerator invented by Ernest O. Lawrence in 1929–1930 at the University of California, Berkeley, and patented in 1932. Lawrence, Ernest O. ''Method and apparatus for the acceleration of ions'', filed: Jan ...
s or other
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particl ...
s in the collisions of protons,
antiproton The antiproton, , (pronounced ''p-bar'') is the antiparticle of the proton. Antiprotons are stable, but they are typically short-lived, since any collision with a proton will cause both particles to be annihilated in a burst of energy. The exis ...
s, or other particles. Higher-energy (more massive) mesons were created momentarily in the
Big Bang The Big Bang event is a physical theory that describes how the universe expanded from an initial state of high density and temperature. Various cosmological models of the Big Bang explain the evolution of the observable universe from the ...
, but are not thought to play a role in nature today. However, such heavy mesons are regularly created in particle accelerator experiments that explore the nature of the heavier quarks that compose the heavier mesons. Mesons are part of the
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
particle family, which are defined simply as particles composed of two or more quarks. The other members of the hadron family are the
baryon In particle physics, a baryon is a type of composite subatomic particle which contains an odd number of valence quarks (at least 3). Baryons belong to the hadron family of particles; hadrons are composed of quarks. Baryons are also classifi ...
s: subatomic particles composed of odd numbers of valence quarks (at least 3), and some experiments show evidence of
exotic meson Exotic mesons are mesons that have quantum numbers not possible in the quark model; some proposals for non-standard quark model mesons could be: ; glueballs or gluonium: Glueballs have no valence quarks at all. ; tetraquarks: Tetraquarks have t ...
s, which do not have the conventional valence quark content of two quarks (one quark and one antiquark), but 4 or more. Because quarks have a spin , the difference in quark number between mesons and baryons results in conventional two-quark mesons being
boson In particle physics, a boson ( ) is a subatomic particle whose spin quantum number has an integer value (0,1,2 ...). Bosons form one of the two fundamental classes of subatomic particle, the other being fermions, which have odd half-integer s ...
s, whereas baryons are
fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks ...
s. Each type of meson has a corresponding
antiparticle In particle physics, every type of particle is associated with an antiparticle with the same mass but with opposite physical charges (such as electric charge). For example, the antiparticle of the electron is the positron (also known as an antie ...
(antimeson) in which quarks are replaced by their corresponding antiquarks and vice versa. For example, a positive
pion In particle physics, a pion (or a pi meson, denoted with the Greek letter pi: ) is any of three subatomic particles: , , and . Each pion consists of a quark and an antiquark and is therefore a meson. Pions are the lightest mesons and, more ge ...
() 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. Because mesons are composed of quarks, they participate in both the
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interactio ...
and
strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called t ...
. Mesons with net
electric charge Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons respecti ...
also participate in the
electromagnetic interaction In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
. Mesons are classified according to their quark content,
total angular momentum In quantum mechanics, the total angular momentum quantum number parametrises the total angular momentum of a given particle, by combining its orbital angular momentum and its intrinsic angular momentum (i.e., its spin). If s is the particle's s ...
, parity and various other properties, such as
C-parity In physics, the C parity or charge parity is a multiplicative quantum number of some particles that describes their behavior under the symmetry operation of charge conjugation. Charge conjugation changes the sign of all quantum charges (that is, ...
and G-parity. Although no meson is stable, those of lower
mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different eleme ...
are nonetheless more stable than the more massive, and hence are easier to observe and study in
particle accelerator A particle accelerator is a machine that uses electromagnetic fields to propel charged particles to very high speeds and energies, and to contain them in well-defined beams. Large accelerators are used for fundamental research in particl ...
s or in
cosmic ray Cosmic rays are high-energy particles or clusters of particles (primarily represented by protons or atomic nuclei) that move through space at nearly the speed of light. They originate from the Sun, from outside of the Solar System in our o ...
experiments. The lightest group of mesons is less massive than the lightest group of baryons, meaning that they are more easily produced in experiments, and thus exhibit certain higher-energy phenomena more readily than do baryons. But mesons can be quite massive: for example, the J/Psi meson () containing the
charm quark The charm quark, charmed quark or c quark (from its symbol, c) is the third-most massive of all quarks, a type of elementary particle. Charm quarks are found in hadrons, which are subatomic particles made of quarks. Examples of hadrons containin ...
, first seen 1974, is about three times as massive as a proton, and the
upsilon meson The Upsilon meson () is a quarkonium state (i.e. flavourless meson) formed from a bottom quark and its antiparticle. It was discovered by the E288 experiment team, headed by Leon Lederman, at Fermilab in 1977, and was the first particle containi ...
() containing the
bottom quark The bottom quark or b quark, also known as the beauty quark, is a third-generation heavy quark with a charge of −  ''e''. All quarks are described in a similar way by electroweak and quantum chromodynamics, but the bottom quark has exce ...
, first seen in 1977, is about ten times as massive.


History

From theoretical considerations, in 1934
Hideki Yukawa was a Japanese theoretical physicist and the first Japanese Nobel laureate for his prediction of the pi meson, or pion. Biography He was born as Hideki Ogawa in Tokyo and grew up in Kyoto with two older brothers, two older sisters, and two y ...
predicted the existence and the approximate mass of the "meson" as the carrier of the
nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nucle ...
that holds
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in ...
together. If there were no nuclear force, all nuclei with two or more
proton A proton is a stable subatomic particle, symbol , H+, or 1H+ with a positive electric charge of +1 ''e'' elementary charge. Its mass is slightly less than that of a neutron and 1,836 times the mass of an electron (the proton–electron mas ...
s would fly apart due to
electromagnetic In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
repulsion. Yukawa called his carrier particle the meson, from μέσος ''mesos'', the
Greek Greek may refer to: Greece Anything of, from, or related to Greece, a country in Southern Europe: *Greeks, an ethnic group. *Greek language, a branch of the Indo-European language family. **Proto-Greek language, the assumed last common ancestor ...
word for "intermediate", because its predicted mass was between that of the electron and that of the proton, which has about 1,836 times the mass of the electron. Yukawa or
Carl David Anderson Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist. He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936. B ...
, who discovered the
muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As ...
, had originally named the particle the "mesotron", but he was corrected by the physicist
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent serie ...
(whose father was a professor of Greek at the
University of Munich The Ludwig Maximilian University of Munich (simply University of Munich or LMU; german: Ludwig-Maximilians-Universität München) is a public research university in Munich, Germany. It is Germany's sixth-oldest university in continuous operati ...
). Heisenberg pointed out that there is no "tr" in the Greek word "mesos". The first candidate for Yukawa's meson, in modern terminology known as the
muon A muon ( ; from the Greek letter mu (μ) used to represent it) is an elementary particle similar to the electron, with an electric charge of −1 '' e'' and a spin of , but with a much greater mass. It is classified as a lepton. As ...
, was discovered in 1936 by
Carl David Anderson Carl David Anderson (September 3, 1905 – January 11, 1991) was an American physicist. He is best known for his discovery of the positron in 1932, an achievement for which he received the 1936 Nobel Prize in Physics, and of the muon in 1936. B ...
and others in the
decay product In nuclear physics, a decay product (also known as a daughter product, daughter isotope, radio-daughter, or daughter nuclide) is the remaining nuclide left over from radioactive decay. Radioactive decay often proceeds via a sequence of steps ( ...
s of cosmic ray interactions. The "mu meson" had about the right mass to be Yukawa's carrier of the strong nuclear force, but over the course of the next decade, it became evident that it was not the right particle. It was eventually found that the "mu meson" did not participate in the strong nuclear interaction at all, but rather behaved like a heavy version of the
electron The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family, and are generally thought to be elementary particles because they have no ...
, and was eventually classed as a
lepton In particle physics, a lepton is an elementary particle of half-integer spin ( spin ) that does not undergo strong interactions. Two main classes of leptons exist: charged leptons (also known as the electron-like leptons or muons), and n ...
like the electron, rather than a meson. Physicists in making this choice decided that properties other than particle mass should control their classification. There were years of delays in the subatomic particle research during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
(1939–1945), with most physicists working in applied projects for wartime necessities. When the war ended in August 1945, many physicists gradually returned to peacetime research. The first true meson to be discovered was what would later be called the "pi meson" (or pion). This discovery was made in 1947, by
Cecil Powell Cecil Frank Powell, FRS (5 December 1903 – 9 August 1969) was a British physicist, and Nobel Prize in Physics laureate for heading the team that developed the photographic method of studying nuclear processes and for the resulting discovery of ...
,
Hugh Muirhead Hugh Muirhead (1925 – 19 January 2007) was a British nuclear physicist and the last surviving author of the scientific paper announcing the discovery of the pion, a particle predicted by Hideki Yukawa. Muirhead did his PhD studies at the Univer ...
,
César Lattes Cesare Mansueto Giulio Lattes (11 July 1924 – 8 March 2005), also known as César Lattes, was a Brazilian experimental physicist, one of the discoverers of the pion, a composite subatomic particle made of a quark and an antiquark. Life Latte ...
, and
Giuseppe Occhialini Giuseppe Paolo Stanislao "Beppo" Occhialini ForMemRS (; 5 December 1907 – 30 December 1993) was an Italian physicist who contributed to the discovery of the pion or pi-meson decay in 1947 with César Lattes and Cecil Frank Powell, the latt ...
, who were investigating cosmic ray products at the
University of Bristol , mottoeng = earningpromotes one's innate power (from Horace, ''Ode 4.4'') , established = 1595 – Merchant Venturers School1876 – University College, Bristol1909 – received royal charter , type ...
in
England England is a Countries of the United Kingdom, country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separa ...
, based on photographic films placed in the Andes mountains. Some of those mesons had about the same mass as the already-known mu "meson", yet seemed to decay into it, leading physicist Robert Marshak to hypothesize in 1947 that it was actually a new and different meson. Over the next few years, more experiments showed that the pion was indeed involved in strong interactions. The pion (as a
virtual particle A virtual particle is a theoretical transient particle that exhibits some of the characteristics of an ordinary particle, while having its existence limited by the uncertainty principle. The concept of virtual particles arises in the perturba ...
) is also believed to be the primary force carrier for the
nuclear force The nuclear force (or nucleon–nucleon interaction, residual strong force, or, historically, strong nuclear force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nucle ...
in
atomic nuclei The atomic nucleus is the small, dense region consisting of protons and neutrons at the center of an atom, discovered in 1911 by Ernest Rutherford based on the 1909 Geiger–Marsden gold foil experiment. After the discovery of the neutron in ...
. Other mesons, such as the virtual
rho meson Rho (uppercase Ρ, lowercase ρ or ; el, ρο or el, ρω, label=none) is the 17th letter of the Greek alphabet. In the system of Greek numerals it has a value of 100. It is derived from Phoenician letter res . Its uppercase form uses the ...
s are involved in mediating this force as well, but to a lesser extent. Following the discovery of the pion, Yukawa was awarded the 1949
Nobel Prize in Physics ) , image = Nobel Prize.png , alt = A golden medallion with an embossed image of a bearded man facing left in profile. To the left of the man is the text "ALFR•" then "NOBEL", and on the right, the text (smaller) "NAT•" then " ...
for his predictions. For a while in the past, the word ''meson'' was sometimes used to mean ''any'' force carrier, such as "the Z meson", which is involved in mediating the
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interactio ...
. However, this use has fallen out of favor, and mesons are now defined as particles composed of pairs of quarks and antiquarks.


Overview


Spin, orbital angular momentum, and total angular momentum

Spin (quantum number ) is a
vector Vector most often refers to: *Euclidean vector, a quantity with a magnitude and a direction *Vector (epidemiology), an agent that carries and transmits an infectious pathogen into another living organism Vector may also refer to: Mathematic ...
quantity that represents the "intrinsic"
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed sys ...
of a particle. It comes in increments of   .
Quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly ...
s are
fermion In particle physics, a fermion is a particle that follows Fermi–Dirac statistics. Generally, it has a half-odd-integer spin: spin , spin , etc. In addition, these particles obey the Pauli exclusion principle. Fermions include all quarks ...
s—specifically in this case, particles having spin Because spin projections vary in increments of 1 (that is 1 ), a single quark has a spin vector of length , and has two spin projections, either or Two quarks can have their spins aligned, in which case the two spin vectors add to make a vector of length with three possible spin projections and and their combination is called a ''
vector meson In high energy physics, a vector meson is a meson with total spin 1 and odd parity (usually noted as ). Vector mesons have been seen in experiments since the 1960s, and are well known for their spectroscopic pattern of masses. The vector meso ...
'' or spin-1 triplet. If two quarks have oppositely aligned spins, the spin vectors add up to make a vector of length and only one spin projection called a ''
scalar meson In high energy physics, a scalar meson is a meson with total spin 0 and even parity (usually noted as ''JP''=0+). Compare to ''pseudoscalar meson''. The first known scalar mesons have been observed since the late 1950s, with observations of numer ...
'' or spin-0 singlet. Because mesons are made of one quark and one antiquark, they are found in triplet and singlet spin states. The latter are called
scalar meson In high energy physics, a scalar meson is a meson with total spin 0 and even parity (usually noted as ''JP''=0+). Compare to ''pseudoscalar meson''. The first known scalar mesons have been observed since the late 1950s, with observations of numer ...
s or
pseudoscalar meson In high-energy physics, a pseudoscalar meson is a meson with total spin 0 and odd parity (usually notated as Pseudoscalar mesons are commonly seen in proton-proton scattering and proton-antiproton annihilation, and include the pion (), ...
s, depending on their parity (see below). There is another quantity of quantized
angular momentum In physics, angular momentum (rarely, moment of momentum or rotational momentum) is the rotational analog of linear momentum. It is an important physical quantity because it is a conserved quantity—the total angular momentum of a closed sys ...
, called the orbital angular momentum (quantum number ), that is the angular momentum due to quarks orbiting each other, and also comes in increments of 1 . The total angular momentum (quantum number ) of a particle is the combination of the two intrinsic angular momentums (spin) and the orbital angular momentum. It can take any value from up to in increments of 1. Particle physicists are most interested in mesons with no orbital angular momentum ( = 0), therefore the two groups of mesons most studied are the  = 1;  = 0 and  = 0;  = 0, which corresponds to  = 1 and  = 0, although they are not the only ones. It is also possible to obtain  = 1 particles from  = 0 and  = 1. How to distinguish between the  = 1,  = 0 and  = 0,  = 1 mesons is an active area of research in meson spectroscopy.


-parity

-parity is left-right parity, or spatial parity, and was the first of several "parities" discovered, and so is often called just “parity”. If the universe were reflected in a mirror, most 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 ().
Gravity In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the str ...
, the
electromagnetic force In physics, electromagnetism is an interaction that occurs between particles with electric charge. It is the second-strongest of the four fundamental interactions, after the strong force, and it is the dominant force in the interactions of ...
, and the
strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called t ...
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 (-symmetry). However, the
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interactio ...
does'' ''distinguish "left" from "right", a phenomenon called
parity violation In physics, a parity transformation (also called parity inversion) is the flip in the sign of ''one'' spatial coordinate. In three dimensions, it can also refer to the simultaneous flip in the sign of all three spatial coordinates (a point refl ...
(-violation). Based on this, one might think that, if the
wavefunction A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements ma ...
for each particle (more precisely, the
quantum field In theoretical physics, quantum field theory (QFT) is a theoretical framework that combines classical field theory, special relativity, and quantum mechanics. QFT is used in particle physics to construct physical models of subatomic particles and ...
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 ( = −1, or alternatively  = −), whereas the other particles are said to have ''positive'' or ''even'' parity ( = +1, or alternatively  = +). For mesons, parity is related to the orbital angular momentum by the relation: :P = \left( -1 \right)^ where the is a result of the parity of the corresponding
spherical harmonic In mathematics and physical science, spherical harmonics are special functions defined on the surface of a sphere. They are often employed in solving partial differential equations in many scientific fields. Since the spherical harmonics for ...
of the
wavefunction A wave function in quantum physics is a mathematical description of the quantum state of an isolated quantum system. The wave function is a complex-valued probability amplitude, and the probabilities for the possible results of measurements ma ...
. The "+1" comes from the fact that, according to the
Dirac equation In particle physics, the Dirac equation is a relativistic wave equation derived by British physicist Paul Dirac in 1928. In its free form, or including electromagnetic interactions, it describes all spin- massive particles, called "Dirac pa ...
, a quark and an antiquark have opposite intrinsic parities. Therefore, the intrinsic parity of a meson is the product of the intrinsic parities of the quark (+1) and antiquark (−1). As these are different, their product is −1, and so it contributes the "+1" that appears in the exponent. As a consequence, all mesons with no orbital angular momentum ( = 0) have odd parity ( = −1).


C-parity

-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 :, q\bar\rangle = , \barq\rangle then, the meson is " even" ( = +1). On the other hand, if :, q\bar\rangle = -, \barq\rangle then the meson is " odd" ( = −1). -parity rarely is studied on its own, but more commonly in combination with P-parity into CP-parity. -parity was originally thought to be conserved, but was later found to be violated on rare occasions in
weak interaction In nuclear physics and particle physics, the weak interaction, which is also often called the weak force or weak nuclear force, is one of the four known fundamental interactions, with the others being electromagnetism, the strong interactio ...
s.


-parity

-parity is a generalization of the -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. If :, q_1\bar_2\rangle = , \bar_1 q_2\rangle then, the meson is " even" ( = +1). On the other hand, if :, q_1\bar_2\rangle = -, \bar_1 q_2\rangle then the meson is " odd" ( = −1).


Isospin and charge


Original isospin model

The concept of isospin was first proposed by
Werner Heisenberg Werner Karl Heisenberg () (5 December 1901 – 1 February 1976) was a German theoretical physicist and one of the main pioneers of the theory of quantum mechanics. He published his work in 1925 in a breakthrough paper. In the subsequent serie ...
in 1932 to explain the similarities between protons and neutrons under the
strong interaction The strong interaction or strong force is a fundamental interaction that confines quarks into proton, neutron, and other hadron particles. The strong interaction also binds neutrons and protons to create atomic nuclei, where it is called t ...
. Although they had different electric charges, their masses were so similar that physicists believed that 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 Eugene Paul "E. P." Wigner ( hu, Wigner Jenő Pál, ; November 17, 1902 – January 1, 1995) was a Hungarian-American theoretical physicist who also contributed to mathematical physics. He received the Nobel Prize in Physics in 1963 "for his con ...
in 1937. When the first mesons were discovered, they too were seen through the eyes of isospin and so the three pions were believed to be the same particle, but in different isospin states. The mathematics of isospin was modeled after the mathematics of spin. Isospin projections varied in increments of 1 just like those of spin, and to each projection was associated a " charged state". Because the "pion particle" had three "charged states", it was said to be of isospin Its "charged states" , , and , corresponded to the isospin projections and respectively. Another example is the " rho particle", also with three charged states. Its "charged states" , , and , corresponded to the isospin projections and respectively.


Replacement by the quark model

This belief lasted until
Murray Gell-Mann Murray Gell-Mann (; September 15, 1929 – May 24, 2019) was an American physicist who received the 1969 Nobel Prize in Physics for his work on the theory of elementary particles. He was the Robert Andrews Millikan Professor of Theoretica ...
proposed the
quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the ...
in 1964 (containing originally only the , , and quarks). The success of the isospin model is now understood to be an artifact of the similar masses of the and quarks. Because the and quarks have similar masses, particles made of the same number of them also have similar masses. The exact and quark composition determines the charge, because quarks carry charge whereas quarks carry charge . For example, the three pions all have different charges * * = a
quantum superposition Quantum superposition is a fundamental principle of quantum mechanics. It states that, much like waves in classical physics, any two (or more) quantum states can be added together ("superposed") and the result will be another valid quantum s ...
of ) and states * but they all have similar masses ( ) as they are each composed of a same total number of up and down quarks and antiquarks. Under the isospin model, they were considered a single particle in different charged states. After the
quark model In particle physics, the quark model is a classification scheme for hadrons in terms of their valence quarks—the quarks and antiquarks which give rise to the quantum numbers of the hadrons. The quark model underlies "flavor SU(3)", or the ...
was adopted, physicists noted that the isospin projections were related to the up and down quark content of particles by the relation :I_3 = \frac\left left(n_\text - n_\bar\right) - \left(n_\text - n_\bar\right)\right where the -symbols are the count 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. Because mesons are hadrons, the isospin classification is also used for them all, with the quantum number calculated by adding for each positively charged up-or-down quark-or-antiquark (up quarks and down antiquarks), and for each negatively charged up-or-down quark-or-antiquark (up antiquarks and down quarks).


Flavour quantum numbers

The
strangeness In particle physics, strangeness ("''S''") is a property of particles, expressed as a quantum number In quantum physics and chemistry, quantum numbers describe values of conserved quantities in the dynamics of a quantum system. Quantum num ...
quantum number In quantum physics and chemistry, quantum numbers describe values of conserved quantities in the dynamics of a quantum system. Quantum numbers correspond to eigenvalues of operators that commute with the Hamiltonian—quantities that can be k ...
''S'' (not to be confused with spin) was noticed to go up and down along with particle mass. The higher the mass, the lower (more negative) the strangeness (the more s quarks). Particles could be described with isospin projections (related to charge) and strangeness (mass) (see the uds nonet figures). As other quarks were discovered, new quantum numbers were made to have similar description of udc and udb nonets. Because 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 other nonets (for example ucb nonet). If the quarks all had the same mass, their behaviour would be called ''symmetric'', because they would all behave in exactly the same way with respect to the strong interaction. However, as 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 (''I''3), the
baryon number In particle physics, the baryon number is a strictly conserved additive quantum number of a system. It is defined as ::B = \frac\left(n_\text - n_\bar\right), where ''n''q is the number of quarks, and ''n'' is the number of antiquarks. Bary ...
(''B'') and flavour quantum numbers (''S'', ''C'', ''B''′, ''T'') by the Gell-Mann–Nishijima formula: :Q = I_3 + \frac(B + S + C + B^\prime + T), where ''S'', ''C'', ''B''′, and ''T'' represent the
strangeness In particle physics, strangeness ("''S''") is a property of particles, expressed as a quantum number In quantum physics and chemistry, quantum numbers describe values of conserved quantities in the dynamics of a quantum system. Quantum num ...
, charm,
bottomness In physics, bottomness (symbol ''B''′ using a prime as plain ''B'' is used already for baryon number) or beauty is a flavour quantum number reflecting the difference between the number of bottom antiquarks (''n'') and the number of bottom ...
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: :\begin S &= -(n_\text - n_\bar) \\ C &= +(n_\text - n_\bar) \\ B^\prime &= -(n_\text - n_\bar) \\ T &= +(n_\text - n_\bar), \end meaning that the Gell-Mann–Nishijima formula is equivalent to the expression of charge in terms of quark content: :Q=\frac n_\text-n_\bar)+(n_\text-n_\bar)+(n_\text-n_\bar)\frac n_\text-n_\bar)+(n_\text-n_\bar)+(n_\text-n_\bar)


Classification

Mesons are classified into groups according to their
isospin In nuclear physics and particle physics, isospin (''I'') is a quantum number related to the up- and down quark content of the particle. More specifically, isospin symmetry is a subset of the flavour symmetry seen more broadly in the interactions ...
(''I''),
total angular momentum In quantum mechanics, the total angular momentum quantum number parametrises the total angular momentum of a given particle, by combining its orbital angular momentum and its intrinsic angular momentum (i.e., its spin). If s is the particle's s ...
(''J''), parity (''P''), G-parity (''G'') or
C-parity In physics, the C parity or charge parity is a multiplicative quantum number of some particles that describes their behavior under the symmetry operation of charge conjugation. Charge conjugation changes the sign of all quantum charges (that is, ...
(''C'') when applicable, and
quark A quark () is a type of elementary particle and a fundamental constituent of matter. Quarks combine to form composite particles called hadrons, the most stable of which are protons and neutrons, the components of atomic nuclei. All commonly ...
(q) content. The rules for classification are defined by the
Particle Data Group The Particle Data Group (or PDG) is an international collaboration of particle physicists that compiles and reanalyzes published results related to the properties of particles and fundamental interactions. It also publishes reviews of theoretical ...
, 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: = 0, = 0, = 0, = 0). The rules for flavourless mesons are: ;In addition: * When the spectroscopic state of the meson is known, it is added in parentheses. * When the spectroscopic state is unknown, mass (in MeV/''c''2) is added in parentheses. * When the meson is in its
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
, nothing is added in parentheses.


Flavoured mesons

Flavoured 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 heavier quark, the superscript depends on the charge, and the subscript (if any) depends on the lighter quark. In table form, they are: ;In addition: * If P is in the "normal series" (i.e., P = 0+, 1, 2+, 3, ...), a superscript ∗ is added. * If the meson is not pseudoscalar ( P = 0) or vector ( P = 1), is added as a subscript. * When the spectroscopic state of the meson is known, it is added in parentheses. * When the spectroscopic state is unknown, mass (in MeV/''c''2) is added in parentheses. * When the meson is in its
ground state The ground state of a quantum-mechanical system is its stationary state of lowest energy; the energy of the ground state is known as the zero-point energy of the system. An excited state is any state with energy greater than the ground state. ...
, nothing is added in parentheses.


Exotic mesons

There is experimental evidence for particles that are
hadron In particle physics, a hadron (; grc, ἁδρός, hadrós; "stout, thick") is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the ...
s (i.e., are composed of quarks) and are color-neutral with zero baryon number, and thus by conventional definition are mesons. Yet, these particles do not consist of a single quark/antiquark pair, as all the other conventional mesons discussed above do. A tentative category for these particles is
exotic meson Exotic mesons are mesons that have quantum numbers not possible in the quark model; some proposals for non-standard quark model mesons could be: ; glueballs or gluonium: Glueballs have no valence quarks at all. ; tetraquarks: Tetraquarks have t ...
s. There are at least five exotic meson resonances that have been experimentally confirmed to exist by two or more independent experiments. The most statistically significant of these is the
Z(4430) Z(4430) is a mesonic resonance discovered by the Belle experiment. It has a mass of . The resonant nature of the peak has been confirmed by the LHCb experiment with a significance of at least 13.9 σ. The particle is charged and is thought ...
, discovered by the
Belle experiment The Belle experiment was a particle physics experiment conducted by the Belle Collaboration, an international collaboration of more than 400 physicists and engineers, at the High Energy Accelerator Research Organisation (KEK) in Tsukuba, Ibarak ...
in 2007 and confirmed by
LHCb The LHCb (Large Hadron Collider beauty) experiment is one of eight particle physics detector experiments collecting data at the Large Hadron Collider at CERN. LHCb is a specialized b-physics experiment, designed primarily to measure the parame ...
in 2014. It is a candidate for being a
tetraquark A tetraquark, in particle physics, is an exotic meson composed of four valence quarks. A tetraquark state has long been suspected to be allowed by quantum chromodynamics, the modern theory of strong interactions. A tetraquark state is an exampl ...
: a particle composed of two quarks and two antiquarks.LHCb collaborators (2014)
Observation of the resonant character of the Z(4430)− state
/ref> See the main article above for other particle resonances that are candidates for being exotic mesons.


List


See also

* Mesonic molecule *
Standard Model The Standard Model of particle physics is the theory describing three of the four known fundamental forces ( electromagnetic, weak and strong interactions - excluding gravity) in the universe and classifying all known elementary particles. It ...


Footnotes


References


External links

* * — Compiles authoritative information on particle properties * * * — An interactive visualisation allowing physical properties to be compared * * {{Authority control Bosons Hadrons Force carriers