Hadron
Encyclopedia
In particle physics
, a hadron is a composite particle made of quark
s held together
by the strong force (as atom
s and molecule
s are held together by the electromagnetic force). Hadrons are categorized into two families: baryon
s (made of three quarks) and meson
s (made of one quark and one antiquark
).
The best-known hadrons are proton
s and neutron
s (both baryons), which are components of atomic nuclei
. All hadrons except protons
are unstable and undergo particle decay
–however neutrons are stable inside atomic nuclei. The best-known mesons are the pion
and the kaon
, which were discovered during cosmic ray
experiments in the late 1940s and early 1950s. However, these are not the only hadrons; a great number of them have been discovered and continue to be discovered (see list of baryons and list of mesons).
Other types of hadron may exist, such as tetraquark
s (or, more generally, exotic meson
s) and pentaquark
s (exotic baryon
s), but no current evidence conclusively suggests their existence.
According to the quark model
, the properties of hadrons are primarily determined by their so-called valence quarks. For example, a proton
is composed of two up quark
s (each with electric charge
+, for a total of + together) and one down quark
(with electric charge −). Adding these together yields the proton charge of +1. Although quarks also carry color charge
, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". These are the simplest of the two ways: three quarks of different colors, or a quark of one color and an antiquark
carrying the corresponding anticolor. Hadrons with the first arrangement are called baryon
s, and those with the second arrangement are meson
s.
Like all subatomic particle
s, hadrons are assigned quantum number
s corresponding to the representations
of the Poincaré group
: JPC(m), where J is the spin
quantum number, P the intrinsic parity (or P-parity
), and C, the charge conjugation (or C-parity), and the particle's mass
, m. Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong interaction
. Hadrons may also carry flavor quantum numbers such as isospin
(or G parity), and strangeness
. All quarks carry an additive, conserved quantum number called a baryon number (B), which is + for quarks and − for antiquarks. This means that baryons (groups of three quarks) have B = 1 while mesons have B = 0.
Hadrons have excited state
s known as resonances
. Each ground state
hadron may have several excited states; several hundreds of resonances have been observed in particle physics experiments
. Resonances decay extremely quickly (within about 10−24 second
s) via the strong nuclear force.
In other phases
of QCD matter
the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics
(QCD) predicts that quarks and gluon
s will no longer be confined within hadrons because the strength
of the strong interaction diminishes with energy. This property, which is known as asymptotic freedom
, has been experimentally confirmed in the energy range between 1 GeV
(gigaelectronvolt) and 1 TeV
(teraelectronvolt).
All free
hadrons except the proton (and antiproton) are unstable.
s (i.e. they have odd half-integral spin because they have an odd number of quarks). As quarks possess baryon number B = , baryons have baryon number B = 1. The best-known baryons are the proton
and the neutron
.
One can hypothesise baryons with further quark–antiquark pairs in addition to their three quarks. Hypothetical baryons with one extra quark–antiquark pair (5 quarks in all) are called pentaquark
s. Several pentaquark candidates were found in the early 2000s, but upon further review these states have now been established as non-existent. (This does not rule against pentaquarks in general, only the candidates put forward). No evidence of baryon states with even more quark–antiquark pairs has been found either.
Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example: just as a proton is made of two up-quarks and one down-quark, its corresponding antiparticle, the antiproton, is made of two up-antiquarks and one down-antiquark.
s (integral spin - i.e. an even multiple of ½ - as they have an even number of quarks). They have baryon number B = 0. Examples of mesons commonly produced in particle physics experiments include pion
s and kaon
s. Pions also play a role in holding atomic nuclei
together via the residual strong force.
In principle, mesons with more than one quark–antiquark pair may exist; a hypothetical meson with two pairs is called a tetraquark
. Several tetraquark candidates were found in the 2000s, but their status is under debate. Several other hypothetical "exotic" mesons
lie outside the quark model of classification. These include glueball
s and hybrid mesons (mesons bound by excited gluon
s).
Particle physics
Particle physics is a branch of physics that studies the existence and interactions of particles that are the constituents of what is usually referred to as matter or radiation. In current understanding, particles are excitations of quantum fields and interact following their dynamics...
, a hadron is a composite particle made of quark
Quark
A quark is an 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. Due to a phenomenon known as color confinement, quarks are never directly...
s held together
Bound state
In physics, a bound state describes a system where a particle is subject to a potential such that the particle has a tendency to remain localised in one or more regions of space...
by the strong force (as atom
Atom
The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons...
s and molecule
Molecule
A molecule is an electrically neutral group of at least two atoms held together by covalent chemical bonds. Molecules are distinguished from ions by their electrical charge...
s are held together by the electromagnetic force). Hadrons are categorized into two families: baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
s (made of three quarks) and meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s (made of one quark and one antiquark
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
).
The best-known hadrons are proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
s and neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
s (both baryons), which are components of atomic nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
. All hadrons except protons
Proton decay
In particle physics, proton decay is a hypothetical form of radioactive decay in which the proton decays into lighter subatomic particles, such as a neutral pion and a positron...
are unstable and undergo particle decay
Particle decay
Particle decay is the spontaneous process of one elementary particle transforming into other elementary particles. During this process, an elementary particle becomes a different particle with less mass and an intermediate particle such as W boson in muon decay. The intermediate particle then...
–however neutrons are stable inside atomic nuclei. The best-known mesons are the pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
and the kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
, which were discovered during cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
experiments in the late 1940s and early 1950s. However, these are not the only hadrons; a great number of them have been discovered and continue to be discovered (see list of baryons and list of mesons).
Other types of hadron may exist, such as tetraquark
Tetraquark
In particle physics a tetraquark is a hypothetical meson composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date...
s (or, more generally, exotic meson
Exotic meson
Non-quark model mesons include#exotic mesons, which have quantum numbers not possible for mesons in the quark model;#glueballs or gluonium, which have no valence quarks at all;...
s) and pentaquark
Pentaquark
A pentaquark is a hypothetical subatomic particle consisting of four quarks and one antiquark bound together . As quarks have a baryon number of +, and antiquarks of −, it would have a total baryon number of 1, thus being classified as an exotic baryon...
s (exotic baryon
Exotic baryon
Exotic baryons are hypothetical composite particles which are bound states of 3 quarks and additional elementary particles. This is to be contrasted with ordinary baryons, which are bound states of just 3 quarks. The additional particles may include quarks, antiquarks or gluons.One such exotic...
s), but no current evidence conclusively suggests their existence.
Etymology
The term hadron was introduced by Lev B. Okun in a plenary talk at the 1962 International Conference on High Energy Physics. In this talk he said:Not withstanding the fact that this report deals with weak interactions, we shall frequently have to speak of strongly interacting particles. These particles pose not only numerous scientific problems, but also a terminological problem. The point is that "strongly interacting particles" is a very clumsy term which does not yield itself to the formation of an adjective. For this reason, to take but one instance, decays into strongly interacting particles are called non-leptonic. This definition is not exact because "non-leptonic" may also signify "photonic". In this report I shall call strongly interacting particles "hadrons", and the corresponding decays "hadronic" (the Greek ἁδρός signifies "large", "massive", in contrast to λεπτός which means "small", "light"). I hope that this terminology will prove to be convenient.
Properties
Quark model
In 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 properties of hadrons are primarily determined by their so-called valence quarks. For example, a proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
is composed of two up quark
Up quark
The up quark or u quark is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, forms the neutrons and protons of atomic nuclei...
s (each with electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
+, for a total of + together) and one down quark
Down quark
The down quark or d quark is the second-lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the up quark, forms the neutrons and protons of atomic nuclei...
(with electric charge −). Adding these together yields the proton charge of +1. Although quarks also carry color charge
Color charge
In particle physics, color charge is a property of quarks and gluons that is related to the particles' strong interactions in the theory of quantum chromodynamics . Color charge has analogies with the notion of electric charge of particles, but because of the mathematical complications of QCD,...
, hadrons must have zero total color charge because of a phenomenon called color confinement. That is, hadrons must be "colorless" or "white". These are the simplest of the two ways: three quarks of different colors, or a quark of one color and an antiquark
Antiparticle
Corresponding to most kinds of particles, there is an associated antiparticle with the same mass and opposite electric charge. For example, the antiparticle of the electron is the positively charged antielectron, or positron, which is produced naturally in certain types of radioactive decay.The...
carrying the corresponding anticolor. Hadrons with the first arrangement are called baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
s, and those with the second arrangement are meson
Meson
In particle physics, mesons are subatomic particles composed of one quark and one antiquark, bound together by the strong interaction. Because mesons are composed of sub-particles, they have a physical size, with a radius roughly one femtometer: 10−15 m, which is about the size of a proton...
s.
Like all subatomic particle
Subatomic particle
In physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
s, hadrons are assigned quantum number
Quantum number
Quantum numbers describe values of conserved quantities in the dynamics of the quantum system. Perhaps the most peculiar aspect of quantum mechanics is the quantization of observable quantities. This is distinguished from classical mechanics where the values can range continuously...
s corresponding to the representations
Representation theory
Representation theory is a branch of mathematics that studies abstract algebraic structures by representing their elements as linear transformations of vector spaces, and studiesmodules over these abstract algebraic structures...
of the Poincaré group
Poincaré group
In physics and mathematics, the Poincaré group, named after Henri Poincaré, is the group of isometries of Minkowski spacetime.-Simple explanation:...
: JPC(m), where J is the spin
Spin (physics)
In quantum mechanics and particle physics, spin is a fundamental characteristic property of elementary particles, composite particles , and atomic nuclei.It is worth noting that the intrinsic property of subatomic particles called spin and discussed in this article, is related in some small ways,...
quantum number, P the intrinsic parity (or P-parity
Parity (physics)
In physics, a parity transformation is the flip in the sign of one spatial coordinate. In three dimensions, it is also commonly described by the simultaneous flip in the sign of all three spatial coordinates:...
), and C, the charge conjugation (or C-parity), and the particle's mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
, m. Note that the mass of a hadron has very little to do with the mass of its valence quarks; rather, due to mass–energy equivalence, most of the mass comes from the large amount of energy associated with the strong interaction
Strong interaction
In particle physics, the strong interaction is one of the four fundamental interactions of nature, the others being electromagnetism, the weak interaction and gravitation. As with the other fundamental interactions, it is a non-contact force...
. Hadrons may also carry flavor quantum numbers such as isospin
Isospin
In physics, and specifically, particle physics, isospin is a quantum number related to the strong interaction. This term was derived from isotopic spin, but the term is confusing as two isotopes of a nucleus have different numbers of nucleons; in contrast, rotations of isospin maintain the number...
(or G parity), and strangeness
Strangeness
In particle physics, strangeness S is a property of particles, expressed as a quantum number, for describing decay of particles in strong and electromagnetic reactions, which occur in a short period of time...
. All quarks carry an additive, conserved quantum number called a baryon number (B), which is + for quarks and − for antiquarks. This means that baryons (groups of three quarks) have B = 1 while mesons have B = 0.
Hadrons have excited state
Excited state
Excitation is an elevation in energy level above an arbitrary baseline energy state. In physics there is a specific technical definition for energy level which is often associated with an atom being excited to an excited state....
s known as resonances
Resonance (particle physics)
In particle physics, a resonance is the peak located around a certain energy found in differential cross sections of scattering experiments. These peaks are associated with subatomic particles and their excitations...
. Each ground state
Ground state
The ground state of a quantum mechanical system is its lowest-energy state; 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...
hadron may have several excited states; several hundreds of resonances have been observed in particle physics experiments
Particle physics experiments
Particle physics experiments briefly discusses a number of past, present, and proposed experiments with particle accelerators, throughout the world. In addition, some important accelerator interactions are discussed...
. Resonances decay extremely quickly (within about 10−24 second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s) via the strong nuclear force.
In other phases
Phase (matter)
In the physical sciences, a phase is a region of space , throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, and chemical composition...
of QCD matter
QCD matter
Quark matter or QCD matter refers to any of a number of theorized phases of matter whose degrees of freedom include quarks and gluons. These theoretical phases would occur at extremely high temperatures and densities, billions of times higher than can be produced in equilibrium in laboratories...
the hadrons may disappear. For example, at very high temperature and high pressure, unless there are sufficiently many flavors of quarks, the theory of quantum chromodynamics
Quantum chromodynamics
In theoretical physics, quantum chromodynamics is a theory of the strong interaction , a fundamental force describing the interactions of the quarks and gluons making up hadrons . It is the study of the SU Yang–Mills theory of color-charged fermions...
(QCD) predicts that quarks and gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s will no longer be confined within hadrons because the strength
Coupling constant
In physics, a coupling constant, usually denoted g, is a number that determines the strength of an interaction. Usually the Lagrangian or the Hamiltonian of a system can be separated into a kinetic part and an interaction part...
of the strong interaction diminishes with energy. This property, which is known as asymptotic freedom
Asymptotic freedom
In physics, asymptotic freedom is a property of some gauge theories that causes interactions between particles to become arbitrarily weak at energy scales that become arbitrarily large, or, equivalently, at length scales that become arbitrarily small .Asymptotic freedom is a feature of quantum...
, has been experimentally confirmed in the energy range between 1 GeV
GEV
GEV or GeV may stand for:*GeV or gigaelectronvolt, a unit of energy equal to billion electron volts*GEV or Grid Enabled Vehicle that is fully or partially powered by the electric grid, see plug-in electric vehicle...
(gigaelectronvolt) and 1 TeV
TEV
TEV may refer to:* TeV, or teraelectronvolt, a measure of energy* Total Enterprise Value, a financial measure* Total Economic Value, an economic measure* Tobacco etch virus, a plant pathogenic virus of the family Potyviridae....
(teraelectronvolt).
All free
Free particle
In physics, a free particle is a particle that, in some sense, is not bound. In classical physics, this means the particle is present in a "field-free" space.-Classical Free Particle:The classical free particle is characterized simply by a fixed velocity...
hadrons except the proton (and antiproton) are unstable.
Baryons
All known baryons are made of three valence quarks, so they are fermionFermion
In particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s (i.e. they have odd half-integral spin because they have an odd number of quarks). As quarks possess baryon number B = , baryons have baryon number B = 1. The best-known baryons are the proton
Proton
The proton is a subatomic particle with the symbol or and a positive electric charge of 1 elementary charge. One or more protons are present in the nucleus of each atom, along with neutrons. The number of protons in each atom is its atomic number....
and the neutron
Neutron
The neutron is a subatomic hadron particle which has the symbol or , no net electric charge and a mass slightly larger than that of a proton. With the exception of hydrogen, nuclei of atoms consist of protons and neutrons, which are therefore collectively referred to as nucleons. The number of...
.
One can hypothesise baryons with further quark–antiquark pairs in addition to their three quarks. Hypothetical baryons with one extra quark–antiquark pair (5 quarks in all) are called pentaquark
Pentaquark
A pentaquark is a hypothetical subatomic particle consisting of four quarks and one antiquark bound together . As quarks have a baryon number of +, and antiquarks of −, it would have a total baryon number of 1, thus being classified as an exotic baryon...
s. Several pentaquark candidates were found in the early 2000s, but upon further review these states have now been established as non-existent. (This does not rule against pentaquarks in general, only the candidates put forward). No evidence of baryon states with even more quark–antiquark pairs has been found either.
Each type of baryon has a corresponding antiparticle (antibaryon) in which quarks are replaced by their corresponding antiquarks. For example: just as a proton is made of two up-quarks and one down-quark, its corresponding antiparticle, the antiproton, is made of two up-antiquarks and one down-antiquark.
Mesons
Mesons are hadrons composed of a quark–antiquark pair. They are bosonBoson
In particle physics, bosons are subatomic particles that obey Bose–Einstein statistics. Several bosons can occupy the same quantum state. The word boson derives from the name of Satyendra Nath Bose....
s (integral spin - i.e. an even multiple of ½ - as they have an even number of quarks). They have baryon number B = 0. Examples of mesons commonly produced in particle physics experiments include pion
Pion
In particle physics, a pion is any of three subatomic particles: , , and . Pions are the lightest mesons and they play an important role in explaining the low-energy properties of the strong nuclear force....
s and kaon
Kaon
In particle physics, a kaon is any one of a group of four mesons distinguished by the fact that they carry a quantum number called strangeness...
s. Pions also play a role in holding atomic nuclei
Atomic nucleus
The nucleus is the very dense region consisting of protons and neutrons at the center of an atom. It was discovered in 1911, as a result of Ernest Rutherford's interpretation of the famous 1909 Rutherford experiment performed by Hans Geiger and Ernest Marsden, under the direction of Rutherford. The...
together via the residual strong force.
In principle, mesons with more than one quark–antiquark pair may exist; a hypothetical meson with two pairs is called a tetraquark
Tetraquark
In particle physics a tetraquark is a hypothetical meson composed of four valence quarks. In principle, a tetraquark state may be allowed in quantum chromodynamics, the modern theory of strong interactions. However, there has been no confirmed report of a tetraquark state to date...
. Several tetraquark candidates were found in the 2000s, but their status is under debate. Several other hypothetical "exotic" mesons
Exotic meson
Non-quark model mesons include#exotic mesons, which have quantum numbers not possible for mesons in the quark model;#glueballs or gluonium, which have no valence quarks at all;...
lie outside the quark model of classification. These include glueball
Glueball
In particle physics, a glueball is a hypothetical composite particle. It consists solely of gluon particles, without valence quarks. Such a state is possible because gluons carry color charge and experience the strong interaction...
s and hybrid mesons (mesons bound by excited gluon
Gluon
Gluons are elementary particles which act as the exchange particles for the color force between quarks, analogous to the exchange of photons in the electromagnetic force between two charged particles....
s).
See also
- HadronizationHadronizationIn particle physics, hadronization is the process of the formation of hadrons out of quarks and gluons. This occurs after high-energy collisions in a particle collider in which free quarks or gluons are created. Due to postulated colour confinement, these cannot exist individually...
- Large Hadron ColliderLarge Hadron ColliderThe Large Hadron Collider is the world's largest and highest-energy particle accelerator. It is expected to address some of the most fundamental questions of physics, advancing the understanding of the deepest laws of nature....
(LHC) - LeptonLeptonA lepton is an elementary particle and a fundamental constituent of matter. The best known of all leptons is the electron which governs nearly all of chemistry as it is found in atoms and is directly tied to all chemical properties. Two main classes of leptons exist: charged leptons , and neutral...
s - List of particles
- Standard modelStandard ModelThe Standard Model of particle physics is a theory concerning the electromagnetic, weak, and strong nuclear interactions, which mediate the dynamics of the known subatomic particles. Developed throughout the mid to late 20th century, the current formulation was finalized in the mid 1970s upon...
- Subatomic particleSubatomic particleIn physics or chemistry, subatomic particles are the smaller particles composing nucleons and atoms. There are two types of subatomic particles: elementary particles, which are not made of other particles, and composite particles...
s