In
physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, a
nucleon is a collective name for two particles: the
neutronThe 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...
and the
protonThe 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....
. These are the two constituents of the
atomic nucleusThe 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...
. Until the 1960s, the nucleons were thought to be
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s. Now they are known to be composite particles, each made of three
quarkA 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 bound together by the socalled
strong interactionIn 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 noncontact force...
.
The interaction between two or more nucleons is called internucleon interactions or
nuclear forceThe nuclear force is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them...
, which is also ultimately caused by the strong interaction. (Before the discovery of quarks, the term "strong interaction" referred to just internucleon interactions.)
Nucleons sit at the boundary where
particle physicsParticle 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...
and
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
overlap. Particle physics, particularly
quantum chromodynamicsIn 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 colorcharged fermions...
, provides the fundamental equations that explain the properties of quarks and of the strong interaction. These equations explain quantitatively how quarks can bind together into protons and neutrons (and all the other
hadronIn particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s). However, when multiple nucleons are assembled into an atomic nucleus (
nuclideA nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
), these fundamental equations become too difficult to solve directly (see
lattice QCDLattice QCD is a wellestablished nonperturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
). Instead, nuclides are studied within
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
, which studies nucleons and their interactions by approximations and models, such as the nuclear shell model. These models can successfully explain nuclide properties, for example, whether or not a certain nuclide undergoes
radioactive decayRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
The proton and neutron are both
baryonA baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quarkbased particles...
s and both
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s. In the terminology of
particle physicsParticle 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...
, these two particles make up an
isospin doubletIn 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...
. This explains why their masses are so similar, with the neutron just 0.1% heavier than the proton.
Properties
Protons and neutrons are most important and best known for constituting atomic nuclei, but they can also be found on their own, not part of a larger nucleus. A proton on its own is the nucleus of the hydrogen1 atom (
^{1}H). A neutron on its own is unstable (see below), but they can be found in nuclear reactions (see
neutron radiationNeutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
) and are used in scientific analysis (see
neutron scatteringNeutron scattering,the scattering of free neutrons by matter,is a physical processand an experimental technique using this processfor the investigation of materials.Neutron scattering as a physical process is of primordial importance...
).
Both the proton and neutron are made of three quarks. The proton is made of two
up quarkThe 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 and one
down quarkThe down quark or d quark is the secondlightest 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...
, while the neutron is one up quark and two down quarks. The quarks are held together by the
strong forceIn 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 noncontact force...
. It is also said that the quarks are held together by
gluonGluons 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, but this is just a different way to say the same thing (gluons mediate the strong force).
An up quark has
electric chargeElectric 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...
+
eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
, and a down quark has charge − e, so the total electric charge of the proton and neutron are
+eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
and 0, respectively. The word "neutron" comes from the fact that it is electrically "neutral".
The mass of the proton and neutron is quite similar: The proton is or , while the neutron is or . The neutron is roughly 0.1% heavier. The similarity in mass is explained by the
isospinIn 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...
approximatesymmetry in
particle physicsParticle 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...
(also see below).
The
spinIn 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,...
of both protons and neutrons is . This means they are
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s not
bosonIn 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, and therefore, like electrons, they are subject to the
Pauli exclusion principleThe Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is antisymmetric with respect to exchange of the particles...
. This is a very important fact in
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
: Protons and neutrons in an atomic nucleus cannot all be in the same
quantum state, but instead they spread out into nuclear shells analogous to
electron shellAn electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
s in chemistry. Another reason that the spin of the proton and neutron is important is because it is the source of nuclear spin in larger nuclei. Nuclear spin is best known for its crucial role in the
NMRNMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solidstate nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon13 NMR...
/MRI technique for chemistry and biochemistry analysis.
The
magnetic momentThe magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
of a proton, denoted μ
_{p}, is , while the magnetic moment of a neutron is μ
_{n} = . These parameters are also important in NMR/MRI.
Stability
A neutron by itself is an unstable particle: It undergoes
{{Aboutthe Ford concept carFord Nucleonthe fictional power source in the Transformers universeNucleon (Transformers)}}
In physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, a
nucleon is a collective name for two particles: the
neutronThe 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...
and the
protonThe 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....
. These are the two constituents of the
atomic nucleusThe 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...
. Until the 1960s, the nucleons were thought to be
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s. Now they are known to be composite particles, each made of three
quarkA 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 bound together by the socalled
strong interactionIn 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 noncontact force...
.
The interaction between two or more nucleons is called internucleon interactions or
nuclear forceThe nuclear force is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them...
, which is also ultimately caused by the strong interaction. (Before the discovery of quarks, the term "strong interaction" referred to just internucleon interactions.)
Nucleons sit at the boundary where
particle physicsParticle 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...
and
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
overlap. Particle physics, particularly
quantum chromodynamicsIn 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 colorcharged fermions...
, provides the fundamental equations that explain the properties of quarks and of the strong interaction. These equations explain quantitatively how quarks can bind together into protons and neutrons (and all the other
hadronIn particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s). However, when multiple nucleons are assembled into an atomic nucleus (
nuclideA nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
), these fundamental equations become too difficult to solve directly (see
lattice QCDLattice QCD is a wellestablished nonperturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
). Instead, nuclides are studied within
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
, which studies nucleons and their interactions by approximations and models, such as the nuclear shell model. These models can successfully explain nuclide properties, for example, whether or not a certain nuclide undergoes
radioactive decayRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
The proton and neutron are both
baryonA baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quarkbased particles...
s and both
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s. In the terminology of
particle physicsParticle 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...
, these two particles make up an
isospin doubletIn 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...
({{nowrap
I {{=}} {{frac12}}}}). This explains why their masses are so similar, with the neutron just 0.1% heavier than the proton.
Properties
{{multiple image
 align = right
 direction = vertical
 header = Nucleon
quarkA 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...
composition
 width1 = 128
 image1 = Quark structure proton.svg
 alt1 = Proton
 caption1 = Proton ({{SubatomicParticleproton}}): {{SubatomicParticleUp quark}}{{SubatomicParticleUp quark}}{{SubatomicParticleDown quark}}
 width2 = 128
 image2 = Quark structure neutron.svg
 alt2 = Neutron
 caption2 = Neutron ({{SubatomicParticleneutron}}): {{SubatomicParticleUp quark}}{{SubatomicParticleDown quark}}{{SubatomicParticleDown quark}}
 width3 = 128
 image3 = Quark structure antiproton.svg
 alt3 = Antiproton
 caption3 = Antiproton ({{SubatomicParticleantiproton}}): {{SubatomicParticleUp antiquark}}{{SubatomicParticleUp antiquark}}{{SubatomicParticleDown antiquark}}
 width4 = 128
 image4 = Quark structure antineutron.svg
 alt4 = Antineutron
 caption4 = Antineutron ({{SubatomicParticleantineutron}}): {{SubatomicParticleUp antiquark}}{{SubatomicParticleDown antiquark}}{{SubatomicParticleDown antiquark}}
}}
Protons and neutrons are most important and best known for constituting atomic nuclei, but they can also be found on their own, not part of a larger nucleus. A proton on its own is the nucleus of the hydrogen1 atom (
^{1}H). A neutron on its own is unstable (see below), but they can be found in nuclear reactions (see
neutron radiationNeutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
) and are used in scientific analysis (see
neutron scatteringNeutron scattering,the scattering of free neutrons by matter,is a physical processand an experimental technique using this processfor the investigation of materials.Neutron scattering as a physical process is of primordial importance...
).
Both the proton and neutron are made of three quarks. The proton is made of two
up quarkThe 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 and one
down quarkThe down quark or d quark is the secondlightest 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...
, while the neutron is one up quark and two down quarks. The quarks are held together by the
strong forceIn 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 noncontact force...
. It is also said that the quarks are held together by
gluonGluons 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, but this is just a different way to say the same thing (gluons mediate the strong force).
An up quark has
electric chargeElectric 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...
+{{frac23}}
eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
, and a down quark has charge −{{frac13}} e, so the total electric charge of the proton and neutron are
+eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
and 0, respectively. The word "neutron" comes from the fact that it is electrically "neutral".
The mass of the proton and neutron is quite similar: The proton is {{val1.6726e=27ul=kg}} or {{val938.27ul=MeV/c2}}, while the neutron is {{val1.6749e=27ul=kg}} or {{val939.57ul=MeV/c2}}. The neutron is roughly 0.1% heavier. The similarity in mass is explained by the
isospinIn 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...
approximatesymmetry in
particle physicsParticle 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...
(also see below).
The
spinIn 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,...
of both protons and neutrons is {{frac12}}. This means they are
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s not
bosonIn 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, and therefore, like electrons, they are subject to the
Pauli exclusion principleThe Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is antisymmetric with respect to exchange of the particles...
. This is a very important fact in
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
: Protons and neutrons in an atomic nucleus cannot all be in the same
quantum state, but instead they spread out into nuclear shells analogous to
electron shellAn electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
s in chemistry. Another reason that the spin of the proton and neutron is important is because it is the source of nuclear spin in larger nuclei. Nuclear spin is best known for its crucial role in the
NMRNMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solidstate nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon13 NMR...
/MRI technique for chemistry and biochemistry analysis.
The
magnetic momentThe magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
of a proton, denoted μ
_{p}, is {{val2.79u=
nuclear magnetons (μ
_{N})}}, while the magnetic moment of a neutron is μ
_{n} = {{val1.91u=μ
_{N}}}. These parameters are also important in NMR/MRI.
Stability
A neutron by itself is an unstable particle: It undergoes
{{Aboutthe Ford concept carFord Nucleonthe fictional power source in the Transformers universeNucleon (Transformers)}}
In physicsPhysics is a natural science that involves the study of matter and its motion through spacetime, along with related concepts such as energy and force. More broadly, it is the general analysis of nature, conducted in order to understand how the universe behaves.Physics is one of the oldest academic...
, a
nucleon is a collective name for two particles: the
neutronThe 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...
and the
protonThe 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....
. These are the two constituents of the
atomic nucleusThe 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...
. Until the 1960s, the nucleons were thought to be
elementary particleIn particle physics, an elementary particle or fundamental particle is a particle not known to have substructure; that is, it is not known to be made up of smaller particles. If an elementary particle truly has no substructure, then it is one of the basic building blocks of the universe from which...
s. Now they are known to be composite particles, each made of three
quarkA 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 bound together by the socalled
strong interactionIn 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 noncontact force...
.
The interaction between two or more nucleons is called internucleon interactions or
nuclear forceThe nuclear force is the force between two or more nucleons. It is responsible for binding of protons and neutrons into atomic nuclei. The energy released causes the masses of nuclei to be less than the total mass of the protons and neutrons which form them...
, which is also ultimately caused by the strong interaction. (Before the discovery of quarks, the term "strong interaction" referred to just internucleon interactions.)
Nucleons sit at the boundary where
particle physicsParticle 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...
and
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
overlap. Particle physics, particularly
quantum chromodynamicsIn 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 colorcharged fermions...
, provides the fundamental equations that explain the properties of quarks and of the strong interaction. These equations explain quantitatively how quarks can bind together into protons and neutrons (and all the other
hadronIn particle physics, a hadron is a composite particle made of quarks held together by the strong force...
s). However, when multiple nucleons are assembled into an atomic nucleus (
nuclideA nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
), these fundamental equations become too difficult to solve directly (see
lattice QCDLattice QCD is a wellestablished nonperturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
). Instead, nuclides are studied within
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
, which studies nucleons and their interactions by approximations and models, such as the nuclear shell model. These models can successfully explain nuclide properties, for example, whether or not a certain nuclide undergoes
radioactive decayRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
.
The proton and neutron are both
baryonA baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quarkbased particles...
s and both
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s. In the terminology of
particle physicsParticle 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...
, these two particles make up an
isospin doubletIn 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...
({{nowrap
I {{=}} {{frac12}}}}). This explains why their masses are so similar, with the neutron just 0.1% heavier than the proton.
Properties
{{multiple image
 align = right
 direction = vertical
 header = Nucleon
quarkA 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...
composition
 width1 = 128
 image1 = Quark structure proton.svg
 alt1 = Proton
 caption1 = Proton ({{SubatomicParticleproton}}): {{SubatomicParticleUp quark}}{{SubatomicParticleUp quark}}{{SubatomicParticleDown quark}}
 width2 = 128
 image2 = Quark structure neutron.svg
 alt2 = Neutron
 caption2 = Neutron ({{SubatomicParticleneutron}}): {{SubatomicParticleUp quark}}{{SubatomicParticleDown quark}}{{SubatomicParticleDown quark}}
 width3 = 128
 image3 = Quark structure antiproton.svg
 alt3 = Antiproton
 caption3 = Antiproton ({{SubatomicParticleantiproton}}): {{SubatomicParticleUp antiquark}}{{SubatomicParticleUp antiquark}}{{SubatomicParticleDown antiquark}}
 width4 = 128
 image4 = Quark structure antineutron.svg
 alt4 = Antineutron
 caption4 = Antineutron ({{SubatomicParticleantineutron}}): {{SubatomicParticleUp antiquark}}{{SubatomicParticleDown antiquark}}{{SubatomicParticleDown antiquark}}
}}
Protons and neutrons are most important and best known for constituting atomic nuclei, but they can also be found on their own, not part of a larger nucleus. A proton on its own is the nucleus of the hydrogen1 atom (
^{1}H). A neutron on its own is unstable (see below), but they can be found in nuclear reactions (see
neutron radiationNeutron radiation is a kind of ionizing radiation which consists of free neutrons. A result of nuclear fission or nuclear fusion, it consists of the release of free neutrons from atoms, and these free neutrons react with nuclei of other atoms to form new isotopes, which, in turn, may produce...
) and are used in scientific analysis (see
neutron scatteringNeutron scattering,the scattering of free neutrons by matter,is a physical processand an experimental technique using this processfor the investigation of materials.Neutron scattering as a physical process is of primordial importance...
).
Both the proton and neutron are made of three quarks. The proton is made of two
up quarkThe 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 and one
down quarkThe down quark or d quark is the secondlightest 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...
, while the neutron is one up quark and two down quarks. The quarks are held together by the
strong forceIn 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 noncontact force...
. It is also said that the quarks are held together by
gluonGluons 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, but this is just a different way to say the same thing (gluons mediate the strong force).
An up quark has
electric chargeElectric 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...
+{{frac23}}
eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
, and a down quark has charge −{{frac13}} e, so the total electric charge of the proton and neutron are
+eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
and 0, respectively. The word "neutron" comes from the fact that it is electrically "neutral".
The mass of the proton and neutron is quite similar: The proton is {{val1.6726e=27ul=kg}} or {{val938.27ul=MeV/c2}}, while the neutron is {{val1.6749e=27ul=kg}} or {{val939.57ul=MeV/c2}}. The neutron is roughly 0.1% heavier. The similarity in mass is explained by the
isospinIn 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...
approximatesymmetry in
particle physicsParticle 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...
(also see below).
The
spinIn 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,...
of both protons and neutrons is {{frac12}}. This means they are
fermionIn particle physics, a fermion is any particle which obeys the Fermi–Dirac statistics . Fermions contrast with bosons which obey Bose–Einstein statistics....
s not
bosonIn 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, and therefore, like electrons, they are subject to the
Pauli exclusion principleThe Pauli exclusion principle is the quantum mechanical principle that no two identical fermions may occupy the same quantum state simultaneously. A more rigorous statement is that the total wave function for two identical fermions is antisymmetric with respect to exchange of the particles...
. This is a very important fact in
nuclear physicsNuclear physics is the field of physics that studies the building blocks and interactions of atomic nuclei. The most commonly known applications of nuclear physics are nuclear power generation and nuclear weapons technology, but the research has provided application in many fields, including those...
: Protons and neutrons in an atomic nucleus cannot all be in the same
quantum state, but instead they spread out into nuclear shells analogous to
electron shellAn electron shell may be thought of as an orbit followed by electrons around an atom's nucleus. The closest shell to the nucleus is called the "1 shell" , followed by the "2 shell" , then the "3 shell" , and so on further and further from the nucleus. The shell letters K,L,M,.....
s in chemistry. Another reason that the spin of the proton and neutron is important is because it is the source of nuclear spin in larger nuclei. Nuclear spin is best known for its crucial role in the
NMRNMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solidstate nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon13 NMR...
/MRI technique for chemistry and biochemistry analysis.
The
magnetic momentThe magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
of a proton, denoted μ
_{p}, is {{val2.79u=
nuclear magnetons (μ
_{N})}}, while the magnetic moment of a neutron is μ
_{n} = {{val1.91u=μ
_{N}}}. These parameters are also important in NMR/MRI.
Stability
A neutron by itself is an unstable particle: It undergoes
{{SubatomicParticleIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
(a type of
radioactive decayRadioactive decay is the process by which an atomic nucleus of an unstable atom loses energy by emitting ionizing particles . The emission is spontaneous, in that the atom decays without any physical interaction with another particle from outside the atom...
) by turning into a proton, electron, and electron antineutrino, with a halflife around ten minutes. (See the
NeutronThe 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...
article for further discussion of neutron decay.) A proton by itself is thought to be stable, or at least its lifetime is too long to measure. (This is an important issue in particle physics, see
Proton decayIn 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...
.)
Inside a nucleus, on the other hand, both protons and neutrons can be stable or unstable, depending on the
nuclideA nuclide is an atomic species characterized by the specific constitution of its nucleus, i.e., by its number of protons Z, its number of neutrons N, and its nuclear energy state....
. Inside some nuclides, a neutron can turn into a proton (plus other particles) as described above; inside other nuclides the reverse can happen, where a proton turns into a neutron (plus other particles) through
{{SubatomicParticleIn nuclear physics, beta decay is a type of radioactive decay in which a beta particle is emitted from an atom. There are two types of beta decay: beta minus and beta plus. In the case of beta decay that produces an electron emission, it is referred to as beta minus , while in the case of a...
or
electron captureElectron capture is a process in which a protonrich nuclide absorbs an inner atomic electron and simultaneously emits a neutrino...
; and inside still other nuclides, both protons and neutrons are stable and do not change form.
Antinucleons
{{mainAntineutronAntiprotonAntimatter}}
Both of the nucleons have corresponding
antiparticleCorresponding 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...
s: The
antiprotonThe antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically shortlived since any collision with a proton will cause both particles to be annihilated in a burst of energy....
and the
antineutronThe antineutron is the antiparticle of the neutron with symbol . It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, but has opposite baryon number...
. These antimatter particles have the same mass and opposite charge as the proton and neutron respectively, and they interact in the same way. (This is generally believed to be
exactly true, due to
CPT symmetryCPT symmetry is a fundamental symmetry of physical laws under transformations that involve the inversions of charge, parity, and time simultaneously.History:...
. If there is a difference, it is too small to measure in all experiments to date.) In particular, antinucleons can bind into an "antinucleus". So far, scientists have created antideuterium and antihelium3 nuclei.
Nucleons
Nucleons (SIn 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...
= C = BIn physics, bottomness also called beauty, is a flavour quantum number reflecting the difference between the number of bottom antiquarks and the number of bottom quarks that are present in a particle: B^\prime = Bottom quarks have a bottomness of −1 while bottom antiquarks have a...
= 0)
Particle name 
Symbol 
Quark content 
Rest mass (MeV/cThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time... ^{2}) 
u The unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon12 in its nuclear and electronic ground state, and has a value of... )^{{{ref>nucleonmass[a]}}} 
IIn 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...

I_{3} 
J^{PParity (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:...} 
QElectric 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... (eThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called... ) 
Mean lifetime (s 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.... ) 
Commonly decays to 
protonThe 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....

{{SubatomicParticleProton}} / {{SubatomicParticleProton+}} / {{SubatomicParticleNucleon+}} 
{{SubatomicParticlelink=yesUp quark}}{{SubatomicParticleUp quark}}{{SubatomicParticlelink=yesDown quark}} 
{{sort0938.272013{{val938.2720130.000023}}}} 
{{sort1.00727646688{{val1.00727646688(13)}}}} 
{{sort0.5{{frac12}}}} 
{{sort0.5+{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
{{sort1+1}} 
{{sort+10Stable}}^{{{refprotonlifetime[b]}}} 
Unobserved 
neutronThe 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...

{{SubatomicParticleNeutron}} / {{SubatomicParticleNeutron0}} / {{SubatomicParticleNucleon0}} 
{{SubatomicParticleUp quark}}{{SubatomicParticleDown quark}}{{SubatomicParticleDown quark}} 
{{sort0939.565346{{val939.5653460.000023}}}} 
{{sort1.00866491560{{val1.00866491560(55)}}}} 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
0 
{{sort+2{{val8.8570.008e=+2}}}}^{{{refneutronlifetime[c]}}} 
{{nowrap{{SubatomicParticlelink=yesProton}} + {{SubatomicParticlelink=yesElectron}} + {{SubatomicParticlelink=yesElectron antineutrino}}}} 
antiprotonThe antiproton is the antiparticle of the proton. Antiprotons are stable, but they are typically shortlived since any collision with a proton will cause both particles to be annihilated in a burst of energy....

{{SubatomicParticleAntiproton}} / {{PhysicsParticlepTR=−}} / {{PhysicsParticleNTR=−}} 
{{SubatomicParticleUp antiquark}}{{SubatomicParticleUp antiquark}}{{SubatomicParticleDown antiquark}} 
{{sort0938.272013{{val938.2720130.000023}}}} 
{{sort1.00727646688{{val1.00727646688(13)}}}} 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
{{sort1−1}} 
{{sort+10Stable}}^{{{refprotonlifetime[b]}}} 
Unobserved 
antineutronThe antineutron is the antiparticle of the neutron with symbol . It differs from the neutron only in that some of its properties have equal magnitude but opposite sign. It has the same mass as the neutron, and no net electric charge, but has opposite baryon number...

{{SubatomicParticleAntineutron}} / {{SubatomicParticleAntineutron0}} / {{SubatomicParticleAntinucleon0}} 
{{SubatomicParticleUp antiquark}}{{SubatomicParticleDown antiquark}}{{SubatomicParticleDown antiquark}} 
{{sort0939.565346{{val939.4850.051}}}} 
{{sort1.00866491560{{val1.00866491560(55)}}}} 
{{sort0.5{{frac12}}}} 
{{sort0.5+{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
0 
{{sort+2{{val8.8570.008e=+2}}}}^{{{refneutronlifetime[c]}}} 
{{nowrap{{SubatomicParticlelink=yesAntiproton}} + {{SubatomicParticlelink=yesPositron}} + {{SubatomicParticlelink=yesElectron neutrino}}}} 
{{notenucleonmassa}} The masses of the proton and neutron are known with much better precision in
atomic mass unitThe unified atomic mass unit or dalton is a unit that is used for indicating mass on an atomic or molecular scale. It is defined as one twelfth of the rest mass of an unbound neutral atom of carbon12 in its nuclear and electronic ground state, and has a value of...
s (u) than in MeV/c
^{2}, due to the relatively poorly known value of the
elementary chargeThe elementary charge, usually denoted as e, is the electric charge carried by a single proton, or equivalently, the absolute value of the electric charge carried by a single electron. This elementary charge is a fundamental physical constant. To avoid confusion over its sign, e is sometimes called...
. The masses of their antiparticles are assumed to be identical, and no experiments have refuted this to date. Current experiments show the difference between the masses of the proton and antiproton, if it exists, is less than {{val2e=9}} MeV/c
^{2}, and the difference between the neutron and antineutron masses is on the order of {{val96e=5}} MeV/c
^{2}.
{{noteprotonlifetimeb}} At least 10
^{35} years. See
proton decayIn 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...
.
{{noteneutronlifetimec}} For free neutrons; in most common nuclei, neutrons are stable.
Nucleon resonances
Nucleon resonances are excited states of nucleon particles, often corresponding to one of the quarks having a flipped
spinIn 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,...
state, or with different
orbital angular momentumThe azimuthal quantum number is a quantum number for an atomic orbital that determines its orbital angular momentum and describes the shape of the orbital...
when the particle decays. Only resonances with a 3 or 4 star rating at the
Particle Data GroupThe Particle Data Group 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 results that are phenomenologically relevant, including...
(PDG) are included in this table. Due to their extraordinarily short lifetimes, many properties of these particles are still under investigation. The symbol format is given as N(M) L
_{2I2J}, where M is the particle's approximate mass, L is the orbital angular momentum of the Nucleonmeson pair produced when it decays, and I and J are the particle's
isospinIn 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...
and total angular momentum respectively.
The table below lists only the base resonance; each individual entry represents 4
baryonA baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quarkbased particles...
s: 2 nucleon resonances particles, as well as their 2 antiparticles. Each resonance exists in a form with a positive
electric chargeElectric 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...
(Q), with a quark composition of uud like the proton, and a neutral form, with a quark composition of udd like the neutron, as well as the corresponding antiparticles with antiquark compositions of ūūd̄ and ūd̄d̄ respectively. Since they contain no
strangeThe strange quark or s quark is the thirdlightest of all quarks, a type of elementary particle. Strange quarks are found in hadrons, which are subatomic particles. Example of hadrons containing strange quarks include kaons , strange D mesons , Sigma baryons , and other strange particles...
,
charmThe charm quark or c quark 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...
,
bottomThe bottom quark, also known as the beauty quark, is a thirdgeneration quark with a charge of − e. Although all quarks are described in a similar way by the quantum chromodynamics, the bottom quark's large bare mass , combined with low values of the CKM matrix elements Vub and Vcb, gives it a...
, or
topThe top quark, also known as the t quark or truth quark, is an elementary particle and a fundamental constituent of matter. Like all quarks, the top quark is an elementary fermion with spin, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and...
quarks, these particles do not possess
strangenessIn 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...
, etc. The table only lists the resonances with an isospin of {{frac12}}. For resonances with {{frac32}} isospin, see the
Delta baryonThe Delta baryons are a family of subatomic hadron particles which have the symbols , , , and and electric charges +2, +1, 0 and 1 elementary charge respectively...
article.
Nucleon resonances
Particle name 
Symbol 
PDG mass average (MeV/cThe speed of light in vacuum, usually denoted by c, is a physical constant important in many areas of physics. Its value is 299,792,458 metres per second, a figure that is exact since the length of the metre is defined from this constant and the international standard for time... ^{2}) 
IIn 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...

J^{PParity (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:...} 
Full Width (MeV/c^{2}) 
Common decays (Γ_{i} /Γ > 50%) 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles... /proton/neutron† 
N(939) P_{11} 
939 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
† 
† 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...
aka the Roper resonanceThe Roper resonance, also known as P11, is an unstable baryon state/particle with a mass of about 1,440 MeV/c2 and with a relatively wide full BreitWigner width Γ≈300 MeV/c2. It contains three quarks with total spin J=1/2 and total isospin I=1/2...

N(1440) P_{11} 
1440 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
200 — 450 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1520) D_{13} 
1520 
{{sort0.5{{frac12}}}} 
{{sort1.5{{frac32}}}}^{} 
100 — 125 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1535) S_{11} 
1535 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{} 
125 — 175 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} or}}
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesEta}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1650) S_{11} 
1650 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{} 
145 — 185 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1675) D_{15} 
1675 
{{sort0.5{{frac12}}}} 
{{sort2.5{{frac52}}}}^{} 
135 — 165 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} + {{SubatomicParticlelink=yesPion}} or}}
{{nowrap{{SubatomicParticlelink=yesDelta}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1680) F_{15} 
1680 
{{sort0.5{{frac12}}}} 
{{sort2.5{{frac52}}}}^{+} 
120 — 140 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1700) D_{13} 
1700 
{{sort0.5{{frac12}}}} 
{{sort1.5{{frac32}}}}^{} 
50 — 150 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1710) P_{11} 
1710 
{{sort0.5{{frac12}}}} 
{{sort0.5{{frac12}}}}^{+} 
50 — 250 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} + {{SubatomicParticlelink=yesPion}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(1720) P_{13} 
1720 
{{sort0.5{{frac12}}}} 
{{sort1.5{{frac32}}}}^{+} 
150 — 300 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} + {{SubatomicParticlelink=yesPion}} or}}
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesRho}}}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(2190) G_{17} 
2190 
{{sort0.5{{frac12}}}} 
{{sort3.5{{frac72}}}}^{} 
300 — 700 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} (10—20%)}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(2220) H_{19} 
2220 
{{sort0.5{{frac12}}}} 
{{sort4.5{{frac92}}}}^{+} 
350 — 500 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} (10—20%)}} 
nucleon In physics, a nucleon is a collective name for two particles: the neutron and the proton. These are the two constituents of the atomic nucleus. Until the 1960s, the nucleons were thought to be elementary particles...

N(2250) G_{19} 
2250 
{{sort0.5{{frac12}}}} 
{{sort4.5{{frac92}}}}^{} 
230 — 800 
{{nowrap{{SubatomicParticlelink=yesNucleon}} + {{SubatomicParticlelink=yesPion}} (5—15%)}} 
†
The P_{11}(939) nucleon represents the excited state of a normal proton or neutron, for example, within the nucleus of an atom. Such particles are usually stable within the nucleus, i.e. Lithium6.
Quark model classification
In the
quark modelIn 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....
with SU(2)
flavourIn particle physics, flavour or flavor is a quantum number of elementary particles. In quantum chromodynamics, flavour is a global symmetry...
, the two nucleons are part of the ground state doublet. The proton has quark content of
uud, and the neutron,
udd. In SU(3) flavour, they are part of the ground state octet (
8) of
spinIn 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,...
{{frac12}}
baryonA baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quarkbased particles...
s, known as the
Eightfold wayIn physics, the Eightfold Way is a term coined by American physicist Murray GellMann for a theory organizing subatomic baryons and mesons into octets...
. The other members of this octet are the
hyperonIn particle physics, a hyperon is any baryon containing one or more strange quarks, but no charm quarks or bottom quarks.Properties and behavior of hyperons:...
s
strangeIn 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...
isotriplet
{{SubatomicParticleThe Sigma baryons are a family of subatomic hadron particles which have a +2, +1 or 1 elementary charge or are neutral. They are baryons containing three quarks: two up and/or down quarks, and one third quark, which can be either a strange , a charm , a bottom or a top quark...
, the {{SubatomicParticle and the strange isodoublet {{SubatomicParticle. One can extend this multiplet in SU(4) flavour (with the inclusion of the charm quark) to the ground state
20plet, or to SU(6) flavour (with the inclusion of the top and bottom quarks) to the ground state
56plet.
The article on
isospinIn 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...
provides an explicit expression for the nucleon wave functions in terms of the quark flavour eigenstates.
Models
{{Confusingdate=August 2007}}
Although it is known that the nucleon is made from three quarks, {{As of2006lc=on}}, it is not known how to solve the equations of motion for
quantum chromodynamicsIn 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 colorcharged fermions...
. Thus, the study of the lowenergy properties of the nucleon are performed by means of models. The only firstprinciples approach available is to attempt to solve the equations of QCD numerically, using
lattice QCDLattice QCD is a wellestablished nonperturbative approach to solving the quantum chromodynamics theory of quarks and gluons. It is a lattice gauge theory formulated on a grid or lattice of points in space and time....
. This requires complicated algorithms and very powerful
supercomputerA supercomputer is a computer at the frontline of current processing capacity, particularly speed of calculation.Supercomputers are used for highly calculationintensive tasks such as problems including quantum physics, weather forecasting, climate research, molecular modeling A supercomputer is a...
s. However, several analytic models also exist:
The
SkyrmionIn theoretical physics, a skyrmion is a mathematical model used to model baryons . It was conceived by Tony Skyrme.Overview:...
models the nucleon as a topological soliton in a nonlinear SU(2)
pionIn 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 lowenergy properties of the strong nuclear force....
field. The topological stability of the Skyrmion is interpreted as the conservation of
baryon number, that is, the nondecay of the nucleon. The local topological winding number density is identified with the local
baryon number density of the nucleon. With the pion isospin vector field oriented in the shape of a
hedgehog spaceIn mathematics, a hedgehog space is a topological space, consisting of a set of spines joined at a point.For any cardinal number K, the Khedgehog space is formed by taking the disjoint union of K real unit intervals identified at the origin...
, the model is readily solvable, and is thus sometimes called the
hedgehog model. The hedgehog model is able to predict lowenergy parameters, such as the nucleon mass, radius and axial coupling constant, to approximately 30% of experimental values.
The MIT bag model confines three noninteracting quarks to a spherical cavity, with the boundary condition that the quark vector current vanish on the boundary. The noninteracting treatment of the quarks is justified by appealing to the idea of
asymptotic freedomIn 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...
, whereas the hard boundary condition is justified by quark confinement. Mathematically, the model vaguely resembles that of a radar cavity, with solutions to the
Dirac equationThe Dirac equation is a relativistic quantum mechanical wave equation formulated by British physicist Paul Dirac in 1928. It provided a description of elementary spin½ particles, such as electrons, consistent with both the principles of quantum mechanics and the theory of special relativity, and...
standing in for solutions to the Maxwell equations and the vanishing vector current boundary condition standing for the conducting metal walls of the radar cavity. If the radius of the bag is set to the radius of the nucleon, the bag model predicts a nucleon mass that is within 30% of the actual mass. An important failure of the basic bag model is its failure to provide a pionmediated interaction.
The
chiral bag model merges the MIT bag model and the Skyrmion model. In this model, a hole is punched out of the middle of the Skyrmion, and replaced with a bag model. The boundary condition is provided by the requirement of continuity of the axial vector current across the bag boundary. Very curiously, the missing part of the topological winding number (the baryon number) of the hole punched into the Skyrmion is exactly made up by the nonzero
vacuum expectation valueIn quantum field theory the vacuum expectation value of an operator is its average, expected value in the vacuum. The vacuum expectation value of an operator O is usually denoted by \langle O\rangle...
(or
spectral asymmetryIn mathematics and physics, the spectral asymmetry is the asymmetry in the distribution of the spectrum of eigenvalues of an operator. In mathematics, the spectral asymmetry arises in the study of elliptic operators on compact manifolds, and is given a deep meaning by the AtiyahSinger index theorem...
) of the quark fields inside the bag. {{As of2006}}, this remarkable tradeoff between
topologyTopology is a major area of mathematics concerned with properties that are preserved under continuous deformations of objects, such as deformations that involve stretching, but no tearing or gluing...
and the spectrum of an operator does not have any grounding or explanation in the mathematical theory of
Hilbert spaceThe mathematical concept of a Hilbert space, named after David Hilbert, generalizes the notion of Euclidean space. It extends the methods of vector algebra and calculus from the twodimensional Euclidean plane and threedimensional space to spaces with any finite or infinite number of dimensions...
s and their relationship to
geometryGeometry arose as the field of knowledge dealing with spatial relationships. Geometry was one of the two fields of premodern mathematics, the other being the study of numbers ....
. Several other properties of the chiral bag are notable: it provides a better fit to the low energy nucleon properties, to within 5–10%, and these are almost completely independent of the chiral bag radius (as long as the radius is less than the nucleon radius). This independence of radius is referred to as the
Cheshire Cat principle, after the fading to a smile of
Lewis CarrollCharles Lutwidge Dodgson , better known by the pseudonym Lewis Carroll , was an English author, mathematician, logician, Anglican deacon and photographer. His most famous writings are Alice's Adventures in Wonderland and its sequel Through the LookingGlass, as well as the poems "The Hunting of the...
's
Cheshire CatThe Cheshire Cat is a fictional cat popularised by Lewis Carroll's depiction of it in Alice's Adventures in Wonderland. Known for his distinctive mischievous grin, the Cheshire Cat has had a notable impact on popular culture.Origins:...
. It is expected that a firstprinciples solution of the equations of QCD will demonstrate a similar duality of quarkpion descriptions.
Further reading