In chemistry and physics, a nucleon is either
a proton or a neutron, considered in its role
as a component of an atomic nucleus. The number
of nucleons in a nucleus defines an isotope's
mass number (nucleon number).
Until the 1960s, nucleons were thought to
be elementary particles, not made up of smaller
parts. Now they are known to be composite
particles, made of three quarks bound together
by the so-called strong interaction. The interaction
between two or more nucleons is called internucleon
interaction or nuclear force, 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
physics and nuclear physics overlap. Particle
physics, particularly quantum chromodynamics,
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 hadrons).
However, when multiple nucleons are assembled
into an atomic nucleus (nuclide), these fundamental
equations become too difficult to solve directly
(see lattice QCD). Instead, nuclides are studied
within nuclear physics, which studies nucleons
and their interactions by approximations and
models, such as the nuclear shell model. These
models can successfully explain nuclide properties,
as for example, whether or not a particular
nuclide undergoes radioactive decay.
The proton and neutron are both baryons and
both fermions. The proton carries a positive
net charge and the neutron carries a zero
net charge; the proton's mass is only about
0.13% less than the neutron's. Thus, they
can be viewed as two states of the same nucleon,
and together form an isospin doublet (I = ​1⁄2).
In isospin space, neutrons can be transformed
into protons via SU(2) symmetries, and vice
versa. These nucleons are acted upon equally
by the strong interaction, which is invariant
under rotation in isospin space. According
to the Noether theorem, isospin is conserved
with respect to the strong interaction.
== Overview ==
=== Properties ===
Protons and neutrons are best known in their
role as nucleons, i.e., as the components
of atomic nuclei, but they also exist as free
particles. Free neutrons are unstable, with
a half-life of around 13 minutes, but they
are common in nature and have important applications
(see neutron radiation and neutron scattering).
Singular protons, not bound to other nucleons,
are usually regarded as the nuclei of hydrogen
atoms or ions, but in some extreme cases (cosmic
rays, proton beams), they may be regarded
as free protons.
Neither the proton nor neutron is an elementary
particle, meaning each is composed of smaller
parts, namely three quarks each. A proton
is composed of two up quarks and one down
quark, while the neutron has one up quark
and two down quarks. Quarks are held together
by the strong force, or equivalently, by gluons,
which mediate the strong force.
An up quark has electric charge +​2⁄3
e, and a down quark has charge −​1⁄3
e, so the summed electric charges of proton
and neutron are +e and 0, respectively. Thus,
the neutron has a charge of 0 (zero), and
therefore is electrically neutral; indeed,
the term "neutron" comes from the fact that
a neutron is electrically neutral.
The mass of the proton and neutron is quite
similar: The proton is 1.6726×10−27 kg
or 938.27 MeV/c2, while the neutron is 1.6749×10−27
kg or 939.57 MeV/c2. The neutron is roughly
0.13% heavier. The similarity in mass can
be explained roughly by the slight difference
in masses of up quarks and down quarks composing
the nucleons. However, a detailed explanation
remains an unsolved problem in particle physics.The
spin of both protons and neutrons is ​1⁄2,
which means they are fermions and, like electrons
(and unlike bosons), are subject to the Pauli
exclusion principle, a very important phenomenon
in nuclear physics: protons and neutrons in
an atomic nucleus cannot all be in the same
quantum state; instead they spread out into
nuclear shells analogous to electron shells
in chemistry. Also important, this spin (of
proton and neutron) is the source of nuclear
spin in larger nuclei. Nuclear spin is best
known for its crucial role in the NMR/MRI
technique for chemical and biochemical analyses.
The magnetic moment of a proton, denoted μp,
is 2.79 nuclear magnetons (μN), while the
magnetic moment of a neutron is μn = −1.91
μN. These parameters are also important in
NMR/MRI.
=== Stability ===
A neutron in free state is an unstable particle,
with a half-life around ten minutes. It undergoes
β− decay (a type of radioactive decay)
by turning into a proton while emitting an
electron and an electron antineutrino. (See
the Neutron article for more 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
discussion in particle physics, (see Proton
decay).
Inside a nucleus, on the other hand, combined
protons and neutrons (nucleons) can be stable
or unstable depending on the nuclide, or nuclear
species. Inside some nuclides, a neutron can
turn into a proton (producing other particles)
as described above; the reverse can happen
inside other nuclides, where a proton turns
into a neutron (producing other particles)
through β+ decay, or electron capture. And
inside still other nuclides, both protons
and neutrons are stable and do not change
form.
=== Antinucleons ===
Both nucleons have corresponding antiparticles:
the antiproton and the antineutron, which
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 symmetry.
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 antihelium-3 nuclei.
== Tables of detailed properties ==
=== 
Nucleons ===
^a The masses of the proton and neutron are
known with far greater precision in atomic
mass units (u) than in MeV/c2, due to the
relatively poorly known value of the elementary
charge. The conversion factor used is 1 u
= 931.494028±0.000023 MeV/c2.
The masses of their antiparticles are assumed
to be identical, and no experiments have refuted
this to date. Current experiments show any
percent difference between the masses of the
proton and antiproton must be less than 2×10−9
and the difference between the neutron and
antineutron masses is on the order of (9±6)×10−5
MeV/c2.
^b At least 1035 years. See proton decay.
^c 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 spin state, or with
different orbital angular momentum when the
particle decays. Only resonances with a 3
or 4 star rating at the Particle Data Group
(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) L2I2J,
where M is the particle's approximate mass,
L is the orbital angular momentum of the Nucleon-meson
pair produced when it decays, and I and J
are the particle's isospin and total angular
momentum respectively. Since nucleons are
defined as having ​1⁄2 isospin, the first
number will always be 1, and the second number
will always be odd. When discussing nucleon
resonances, sometimes the N is omitted and
the order is reversed, giving L2I2J (M). For
example, a proton can be symbolized as "N(939)
S11" or "S11 (939)".
The table below lists only the base resonance;
each individual entry represents 4 baryons:
2 nucleon resonances particles, as well as
their 2 antiparticles. Each resonance exists
in a form with a positive electric charge
(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 uud and udd respectively.
Since they contain no strange, charm, bottom,
or top quarks, these particles do not possess
strangeness, etc. The table only lists the
resonances with an isospin of ​1⁄2. For
resonances with ​3⁄2 isospin, see the
Delta baryon article.
† The P11(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.
Lithium-6.
== 
Quark model classification ==
In the quark model with SU(2) flavour, 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
spin ​1⁄2 baryons, known as the Eightfold
way. The other members of this octet are the
hyperons strange isotriplet Σ+, Σ0, Σ−,
the Λ and the strange isodoublet Ξ0, Ξ−.
One can extend this multiplet in SU(4) flavour
(with the inclusion of the charm quark) to
the ground state 20-plet, or to SU(6) flavour
(with the inclusion of the top and bottom
quarks) to the ground state 56-plet.
The article on isospin provides an explicit
expression for the nucleon wave functions
in terms of the quark flavour eigenstates.
== Models ==
Although it is known that the nucleon is made
from three quarks, as of 2006, it is not known
how to solve the equations of motion for quantum
chromodynamics. Thus, the study of the low-energy
properties of the nucleon are performed by
means of models. The only first-principles
approach available is to attempt to solve
the equations of QCD numerically, using lattice
QCD. This requires complicated algorithms
and very powerful supercomputers. However,
several analytic models also exist:
=== Skyrmion models ===
The Skyrmion models the nucleon as a topological
soliton in a non-linear SU(2) pion field.
The topological stability of the Skyrmion
is interpreted as the conservation of baryon
number, that is, the non-decay 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
space, the model is readily solvable, and
is thus sometimes called the hedgehog model.
The hedgehog model is able to predict low-energy
parameters, such as the nucleon mass, radius
and axial coupling constant, to approximately
30% of experimental values.
=== MIT bag model ===
The MIT bag model confines three non-interacting
quarks to a spherical cavity, with the boundary
condition that the quark vector current vanish
on the boundary. The non-interacting treatment
of the quarks is justified by appealing to
the idea of asymptotic freedom, 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 equation 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.
Although the basic bag model does not provide
a pion-mediated interaction, it describes
excellently the nucleon-nucleon forces through
the 6 quark bag s-channel mechanism using
the P matrix.
=== Chiral bag model ===
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 non-zero vacuum expectation
value (or spectral asymmetry) of the quark
fields inside the bag. As of 2017, this remarkable
trade-off between topology and the spectrum
of an operator does not have any grounding
or explanation in the mathematical theory
of Hilbert spaces and their relationship to
geometry. 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 Carroll's Cheshire Cat.
It is expected that a first-principles solution
of the equations of QCD will demonstrate a
similar duality of quark-pion descriptions.
== See also ==
Hadrons
Electroweak interaction
== 
Further reading ==
A.W. Thomas and W.Weise, The Structure of
the Nucleon, (2001) Wiley-WCH, Berlin, ISBN
3-527-40297-7
Brown, G. E.; Jackson, A. D. (1976). The Nucleon–Nucleon
Interaction. North-Holland Publishing. ISBN
978-0-7204-0335-0.
Nakamura, N.; Particle Data Group; et al.
(2011). "Review of Particle Physics". Journal
of Physics 
G. 37 (7): 075021. Bibcode:2010JPhG...37g5021N.
doi:10.1088/0954-3899/37/7A/075021.
== Notes
