# Nucleon

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 fermions, hadrons and baryons. 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 = ​12). 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.[1]:129–130

An atomic nucleus is shown here as a compact bundle of the two types of nucleons, protons (red) and neutrons (blue). In this picture, the protons and neutrons are shown as distinct, which is the conventional view in chemistry, for example. But in an actual nucleus, as understood by modern nuclear physics, the nucleons are partially delocalized and organize themselves according to the laws of quantum chromodynamics.

## Overview

### Properties

Proton (
p
):
u

u

d
Neutron (
n
):
u

d

d
Antiproton (
p
):
u

u

d
Antineutron (
n
):
u

d

d

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 +23 e, and a down quark has charge 13 e, so the summed electric charges of proton and neutron are +e and 0, respectively.[note 1] 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.[1]:135–136

The spin of both protons and neutrons is ​12, 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 magnetonsN), 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[2][3] and antihelium-3[4] nuclei.

## Tables of detailed properties

### Nucleons

Nucleons (I = ​12; S = C = B = 0)
Particle
name
Symbol Quark
content
Mass (MeV/c2) Mass (u)[a] I3 JP Q (e) Magnetic moment Mean lifetime (s) Commonly decays to
proton[PDG 1]
p
/
p+
/
N+

u

u

d
938.272013±0.000023 1.00727646677±0.00000000010 +​12 12+ +1 2.792847356±0.000000023 Stable[b] Unobserved
neutron[PDG 2]
n
/
n0
/
N0

u

d

d
939.565346±0.000023 1.00866491597±0.00000000043 -​12 12+ 0 −1.91304273±0.00000045 (8.857±0.008)×10+2[c]
p
+
e
+
ν
e
antiproton
p
/
p
/
N

u

u

d
938.272013±0.000023 1.00727646677±0.00000000010 -​12 12+ −1 −2.793±0.006 Stable[b] Unobserved
antineutron
n
/
n0
/
N0

u

d

d
939.485±0.051 1.00866491597±0.00000000043 +​12 12+ 0 ? (8.857±0.008)×10+2[c]
p
+
e+
+
ν
e

^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[PDG 1] and the difference between the neutron and antineutron masses is on the order of (9±6)×10−5 MeV/c2.[PDG 2]

Proton-antiproton CPT invariance tests
Test Formula PDG Result[PDG 1]
Mass ${\displaystyle {\frac {|m_{p}-m_{\bar {p}}|}{m_{p}}}}$ < 2×10−9
Charge-to-mass ratio ${\displaystyle {\frac {|{\frac {q_{\bar {p}}}{m_{\bar {p}}}}|}{({\frac {q_{p}}{m_{p}}})}}}$ 0.99999999991±0.00000000009
Charge-to-mass-to-mass ratio ${\displaystyle {\frac {|{\frac {q_{\bar {p}}}{m_{\bar {p}}}}|-{\frac {q_{p}}{m_{p}}}}{\frac {q_{p}}{m_{p}}}}}$ (−9±9)×10−11
Charge ${\displaystyle {\frac {|q_{p}+q_{\bar {p}}|}{e}}}$ < 2×10−9
Electron charge ${\displaystyle {\frac {|q_{p}+q_{e}|}{e}}}$ <1×10−21
Magnetic moment ${\displaystyle {\frac {|\mu _{p}+\mu _{\bar {p}}|}{\mu _{p}}}}$ (−0.1±2.1)×10−3

^b At least 1035 years. See proton decay.

### 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 ​12 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
u

u

d
like the proton, and a neutral form, with a quark composition of
u

d

d
like the neutron, as well as the corresponding antiparticles with antiquark compositions of
u

u

d
and
u

d

d
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 ​12. For resonances with ​32 isospin, see the Delta baryon article.

Nucleon resonances (I = ​12)
Symbol JP PDG mass average
(MeV/c2)
Full Width
(MeV/c2)
Pole Position
(Real Part)
Pole Position
(−2 × Imaginary Part)
Common decays
i /Γ > 50%)
N(939) P11
[PDG 3]
12+ 939
N(1440) P11
[PDG 4]
aka the Roper resonance
12+ 1440
(1420–1470)
300
(200–450)
1365
(1350–1380)
190
(160–220)
N +
π
N(1520) D13
[PDG 5]
32 1520
(1515–1525)
115
(100–125)
1510
(1505–1515)
110
(105–120)
N +
π
N(1535) S11
[PDG 6]
12 1535
(1525–1545)
150
(125–175)
1510
1490 — 1530)
170
(90–250)
N +
π
or

N +
η

N(1650) S11
[PDG 7]
12 1650
(1645–1670)
165
(145–185)
1665
(1640–1670)
165
(150–180)
N +
π
N(1675) D15
[PDG 8]
52 1675
(1670–1680)
150
(135–165)
1660
(1655–1665)
135
(125–150)
N +
π
+
π
or

Δ
+
π

N(1680) F15
[PDG 9]
52+ 1685
(1680–1690)
130
(120–140)
1675
(1665–1680)
120
(110–135)
N +
π
N(1700) D13
[PDG 10]
32 1700
(1650–1750)
100
(50–150)
1680
(1630–1730)
100
(50–150)
N +
π
+
π
N(1710) P11
[PDG 11]
12+ 1710
(1680–1740)
100
(50–250)
1720
(1670–1770)
230
(80–380)
N +
π
+
π
N(1720) P13
[PDG 12]
32+ 1720
(1700–1750)
200
(150–300)
1675
(1660–1690)
115–275 N +
π
+
π
or

N +
ρ

N(2190) G17
[PDG 13]
72 2190
(2100–2200)
500
(300–700)
2075
(2050–2100)
450
(400–520)
N +
π
(10—20%)
N(2220) H19
[PDG 14]
92+ 2250
(2200–2300)
400
(350–500)
2170
(2130–2200)
480
(400–560)
N +
π
(10—20%)
N(2250) G19
[PDG 15]
92 2250
(2200–2350)
500
(230–800)
2200
(2150–2250)
450
(350–550)
N +
π
(5—15%)

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 spin12 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.[5] [6]

### Chiral bag model

The chiral bag model[7][8] 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,[9] 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.

• 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

1. ^ The resultant coefficients are obtained by summation of the component charges: Σ= +23 + +23 + 13 = +33 = +1, and Σ= +23 + 13 + 13 = ​03 = 0.

## References

1. ^ a b Griffiths, David J. (2008), Introduction to Elementary Particles (2nd revised ed.), WILEY-VCH, ISBN 978-3-527-40601-2
2. ^ Massam, T; Muller, Th.; Righini, B.; Schneegans, M.; Zichichi, A. (1965). "Experimental observation of antideuteron production". Il Nuovo Cimento. 39 (1): 10–14. Bibcode:1965NCimS..39...10M. doi:10.1007/BF02814251.
3. ^ Dorfan, D. E; Eades, J.; Lederman, L. M.; Lee, W.; Ting, C. C. (June 1965). "Observation of Antideuterons". Phys. Rev. Lett. 14 (24): 1003–1006. Bibcode:1965PhRvL..14.1003D. doi:10.1103/PhysRevLett.14.1003.
4. ^ R. Arsenescu; et al. (2003). "Antihelium-3 production in lead-lead collisions at 158 A GeV/c". New Journal of Physics. 5 (1): 1. Bibcode:2003NJPh....5....1A. doi:10.1088/1367-2630/5/1/301.
5. ^ Jaffe, R. L.; Low, F. E. (1979). "Connection between quark-model eigenstates and low-energy scattering". Phys. Rev. D. 19 (7): 2105. Bibcode:1979PhRvD..19.2105J. doi:10.1103/PhysRevD.19.2105.
6. ^ Yu; Simonov, A. (1981). "The quark compound bag model and the Jaffe-Low P matrix". Phys. Lett. B. 107 (1–2): 1. Bibcode:1981PhLB..107....1S. doi:10.1016/0370-2693(81)91133-3.
7. ^ Gerald E. Brown and Mannque Rho (March 1979). "The little bag". Phys. Lett. B. 82 (2): 177–180. Bibcode:1979PhLB...82..177B. doi:10.1016/0370-2693(79)90729-9.
8. ^ Vepstas, L.; Jackson, A.D.; Goldhaber, A.S. (1984). "Two-phase models of baryons and the chiral Casimir effect". Physics Letters B. 140 (5–6): 280–284. Bibcode:1984PhLB..140..280V. doi:10.1016/0370-2693(84)90753-6.
9. ^ Vepstas, L.; Jackson, A. D. (1990). "Justifying the chiral bag". Physics Reports. 187 (3): 109–143. Bibcode:1990PhR...187..109V. doi:10.1016/0370-1573(90)90056-8.

### Particle listings

Action Masters

Action Masters are a sub-line of the Transformers toy franchise, first released in 1990, with a wave of new releases released in Europe in 1991. It featured Transformers action figures who were unable to transform, but came with transforming partners, weapons or exo-suits. Some of the larger sets came with transforming vehicles or bases. This was the last sub-line release as part of the original Transformers toyline before the launch of Generation 2.

Atomic number

The atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of an atom. It is identical to the charge number of the nucleus. The atomic number uniquely identifies a chemical element. In an uncharged atom, the atomic number is also equal to the number of electrons.

The sum of the atomic number Z and the number of neutrons, N, gives the mass number A of an atom. Since protons and neutrons have approximately the same mass (and the mass of the electrons is negligible for many purposes) and the mass defect of nucleon binding is always small compared to the nucleon mass, the atomic mass of any atom, when expressed in unified atomic mass units (making a quantity called the "relative isotopic mass"), is within 1% of the whole number A.

Atoms with the same atomic number Z but different neutron numbers N, and hence different atomic masses, are known as isotopes. A little more than three-quarters of naturally occurring elements exist as a mixture of isotopes (see monoisotopic elements), and the average isotopic mass of an isotopic mixture for an element (called the relative atomic mass) in a defined environment on Earth, determines the element's standard atomic weight. Historically, it was these atomic weights of elements (in comparison to hydrogen) that were the quantities measurable by chemists in the 19th century.

The conventional symbol Z comes from the German word Zahl meaning number, which, before the modern synthesis of ideas from chemistry and physics, merely denoted an element's numerical place in the periodic table, whose order is approximately, but not completely, consistent with the order of the elements by atomic weights. Only after 1915, with the suggestion and evidence that this Z number was also the nuclear charge and a physical characteristic of atoms, did the word Atomzahl (and its English equivalent atomic number) come into common use in this context.

Diquark

In particle physics, a diquark, or diquark correlation/clustering, is a hypothetical state of two quarks grouped inside a baryon (that consists of three quarks) (Lichtenberg 1982). Corresponding models of baryons are referred to as quark–diquark models. The diquark is often treated as a single subatomic particle with which the third quark interacts via the strong interaction. The existence of diquarks inside the nucleons is a disputed issue, but it helps to explain some nucleon properties and to reproduce experimental data sensitive to the nucleon structure. Diquark–antidiquark pairs have also been advanced for anomalous particles such as the X(3872).

Exotic hadrons are subatomic particles composed of quarks and gluons, but which - unlike "well-known" hadrons such as protons , neutrons and mesons - consist of more than three valence quarks. By contrast, "ordinary" hadrons contain just two or three quarks. Hadrons with explicit valence gluon content would also be considered exotic. In theory, there is no limit on the number of quarks in a hadron, as long as the hadron's color charge is white, or color-neutral.Consistent with ordinary hadrons, exotic hadrons are classified as being either fermions, like ordinary baryons, or bosons, like ordinary mesons. According to this classification scheme, pentaquarks, containing five valence quarks, are exotic baryons, while tetraquarks (four valence quarks) and hexaquarks (six quarks, consisting of either a dibaryon or three quark-antiquark pairs) would be considered exotic mesons. Tetraquark and pentaquark particles are believed to have been observed and are being investigated; Hexaquarks have not yet been confirmed as observed.

Exotic hadrons can be searched for by looking for S-matrix poles with quantum numbers forbidden to ordinary hadrons. Experimental signatures for such exotic hadrons have been seen by at least 2003 but remain a topic of controversy in particle physics.

Jaffe and Low suggested that the exotic hadrons manifest themselves as poles of the P matrix, and not of the S matrix. Experimental P-matrix poles are determined reliably in both the meson-meson channels and nucleon-nucleon channels.

The Faddeev equations, named after their inventor Ludvig Faddeev, are equations that describe, at once, all the possible exchanges/interactions in a system of three particles in a fully quantum mechanical formulation. They can be solved iteratively.

In general, Faddeev equations need as input a potential that describes the interaction between two individual particles. It is also possible to introduce a term in the equation in order to take also three-body forces into account.

The Faddeev equations are the most often used non-perturbative formulations of the quantum-mechanical three-body problem.

Unlike the three body problem in classical mechanics, the quantum three body problem is uniformly soluble.

In nuclear physics, the off the energy shell nucleon-nucleon interaction has been studied by analyzing (n,2n) and (p,2p) reactions on deuterium targets, using the Faddeev Equations. The nucleon-nucleon interaction is expanded (approximated) as a series of separable potentials. The Coulomb interaction between two protons is a special problem, in that its expansion in separable potentials does not converge, but this is handled by matching the Faddeev solutions to long range Coulomb solutions, instead of to plane waves.

Separable potentials are interactions that do not preserve a particle's location. Ordinary local potentials can be expressed as sums of separable potentials. The physical nucleon-nucleon interaction, which involves exchange of mesons, is not expected to be either local or separable.

Ford Nucleon

The Ford Nucleon is a concept car developed by Ford in 1957 designed as a future nuclear-powered car, one of a handful of such designs during the 1950s and '60s. The concept was only demonstrated as a scale model. The design did not include an internal-combustion engine; rather, the vehicle was to be powered by a small nuclear reactor in the rear of the vehicle, based on the assumption that this would one day be possible by reducing sizes. The car was to use a steam engine powered by uranium fission similar to those found in nuclear submarines.The mock-up of the car can be viewed at the Henry Ford Museum in Dearborn, Michigan.

High energy nuclear physics

High-energy nuclear physics studies the behavior of nuclear matter in energy regimes typical of high energy physics. The primary focus of this field is the study of heavy-ion collisions, as compared to lower atomic mass atoms in other particle accelerators. At sufficient collision energies, these types of collisions are theorized to produce the quark–gluon plasma. In peripheral nuclear collisions at high energies one expects to obtain information on the electromagnetic production of leptons and mesons which are not accessible in electron-positron colliders due to their much smaller luminosities.

Previous high-energy nuclear accelerator experiments have studied heavy-ion collisions using projectile energies of 1 GeV/nucleon up to 158 GeV/nucleon. Experiments of this type, called "fixed target" experiments, primarily accelerate a "bunch" of ions (typically around ${\displaystyle 10^{6}}$ to ${\displaystyle 10^{8}}$ ions per bunch) to speeds approaching the speed of light (0.999c) and smash them into a target of similar heavy ions. While all collision systems are interesting, great focus was applied in the late 1990s to symmetric collision systems of gold beams on gold targets at Brookhaven National Laboratory's Alternating Gradient Synchrotron (AGS) and uranium beams on uranium targets at CERN's Super Proton Synchrotron.

Currently, high-energy nuclear physics experiments are being conducted at Brookhaven National Laboratory's Relativistic Heavy Ion Collider (RHIC) and in CERN's new Large Hadron Collider. The four primary experiments at RHIC (PHENIX, STAR, PHOBOS, and BRAHMS) study collisions of highly relativistic nuclei. Unlike fixed target experiments, collider experiments steer two accelerated beams of ions toward each other at (in the case of RHIC) six interaction regions. At RHIC, ions can be accelerated (depending on the ion size) from 100 GeV/nucleon to 250GeV/nucleon. Since each colliding ion possesses this energy moving in opposite directions, the maximum energy of the collisions can achieve a center of mass collision energy of 200GeV/nucleon for gold and 500GeV/nucleon for protons.

The ALICE (A Large Ion Collider Experiment) detector at the LHC at CERN is specialized in studying Pb-Pb nuclei collisions at a centre of mass energy of 2.76 TeV per nucleon pair. Other LHC detectors like CMS, ATLAS, and LHCb also have heavy ion programs.

Iron-56

Iron-56 (56Fe) is the most common isotope of iron. About 91.754% of all iron is iron-56.

Of all nuclides, iron-56 has the lowest mass per nucleon. With 8.8 MeV binding energy per nucleon, iron-56 is one of the most tightly bound nuclei.Nickel-62, a relatively rare isotope of nickel, has a higher nuclear binding energy per nucleon; this is consistent with having a higher mass per nucleon because nickel-62 has a greater proportion of neutrons, which are slightly more massive than protons. See the nickel-62 article for more information regarding the ordering of binding energy per nucleon, and mass-per-nucleon, for various nuclides.

Thus, light elements undergoing nuclear fusion and heavy elements undergoing nuclear fission release energy as their nucleons bind more tightly, and the resulting nuclei approach the maximum total energy per nucleon, which occurs at 62Ni. However, during nucleosynthesis in stars the competition between photodisintegration and alpha capturing causes more 56Ni to be produced than 62Ni (56Fe is produced later in the star's ejection shell as 56Ni decays). This means that as the Universe ages, more matter is converted into extremely tightly bound nuclei, such as 56Fe, ultimately leading to the formation of iron stars in around 101500 years.Production of these elements has decreased considerably from what it was at the beginning of the stelliferous era.

Kamioka Observatory

The Kamioka Observatory, Institute for Cosmic Ray Research (神岡宇宙素粒子研究施設, Kamioka Uchū Soryūshi Kenkyū Shisetsu, Japanese pronunciation: [ka.mi.o.ka ɯtɕɯː soɾʲɯːɕi̥ keŋkʲɯː ɕi̥setsɯ]) is a neutrino and gravitational waves laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan. A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades. All of the experiments have been very large and have contributed substantially to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation.

Mass number

The mass number (symbol A, from the German word Atomgewicht (atomic weight), also called atomic mass number or nucleon number, is the total number of protons and neutrons (together known as nucleons) in an atomic nucleus. It determines the atomic mass of atoms. Because protons and neutrons both are baryons, the mass number A is identical with the baryon number B as of the nucleus as of the whole atom or ion. The mass number is different for each different isotope of a chemical element. This is not the same as the atomic number (Z) which denotes the number of protons in a nucleus, and thus uniquely identifies an element. Hence, the difference between the mass number and the atomic number gives the number of neutrons (N) in a given nucleus: ${\displaystyle N=A-Z}$.

The mass number is written either after the element name or as a superscript to the left of an element's symbol. For example, the most common isotope of carbon is carbon-12, or 12
C
, which has 6 protons and 6 neutrons. The full isotope symbol would also have the atomic number (Z) as a subscript to the left of the element symbol directly below the mass number: 12
6
C
. This is technically redundant, as each element is defined by its atomic number, so it is often omitted.

Nickel-62

Nickel-62 is an isotope of nickel having 28 protons and 34 neutrons.

It is a stable isotope, with the highest binding energy per nucleon of any known nuclide (8.7945 MeV). It is often stated that 56Fe is the "most stable nucleus", but only because 56Fe has the lowest mass per nucleon (not binding energy per nucleon) of all nuclides. The lower mass per nucleon of Fe-56 is possible because 56Fe has 26/56 = 46.43% protons, while 62Ni has only 28/62 = 45.16% protons; and the larger fraction of lighter protons in 56Fe lowers its mean mass-per-nucleon ratio, despite having a slightly higher binding energy in a way that has no effect on its binding energy.

Nuclear binding energy

Nuclear binding energy is the minimum energy that would be required to disassemble the nucleus of an atom into its component parts. These component parts are neutrons and protons, which are collectively called nucleons. The binding is always a positive number, as we need to spend energy in moving these nucleons, attracted to each other by the strong nuclear force, away from each other. The mass of an atomic nucleus is less than the sum of the individual masses of the free constituent protons and neutrons, according to Einstein's equation E=mc2. This 'missing mass' is known as the mass defect, and represents the energy that was released when the nucleus was formed.

The term "nuclear binding energy" may also refer to the energy balance in processes in which the nucleus splits into fragments composed of more than one nucleon. If new binding energy is available when light nuclei fuse (nuclear fusion), or when heavy nuclei split (nuclear fission), either process can result in release of this binding energy. This energy may be made available as nuclear energy and can be used to produce electricity, as in nuclear power, or in a nuclear weapon. When a large nucleus splits into pieces, excess energy is emitted as photon (gamma rays) and as the kinetic energy of a number of different ejected particles (nuclear fission products).

These nuclear binding energies and forces are on the order of a million times greater than the electron binding energies of light atoms like hydrogen.The mass defect of a nucleus represents the amount of mass equivalent to the

binding energy of the nucleus (E=mc2), which is the difference between the mass of a nucleus and the sum of the individual masses of the nucleons of which it is composed.

Nuclear drip line

The nuclear drip line is the boundary delimiting the zone beyond which atomic nuclei decay by the emission of a proton or neutron.

An arbitrary combination of protons and neutrons does not necessarily yield a stable nucleus. One can think of moving up and/or to the right across the table of nuclides by adding one type of nucleon to a given nucleus. However, adding nucleons one at a time to a given nucleus will eventually lead to a newly formed nucleus that immediately decays by emitting a proton (or neutron). Colloquially speaking, the nucleon has 'leaked' or 'dripped' out of the nucleus, hence giving rise to the term "drip line".

Drip lines are defined for protons and neutrons at the extreme of the proton-to-neutron ratio; at p:n ratios at or beyond the drip lines, no bound nuclei can exist. While the location of the proton drip line is well known for many elements, the location of the neutron drip line is only known for elements up to neon.

Nuclear force

The nuclear force (or nucleon–nucleon interaction or residual strong force) is a force that acts between the protons and neutrons of atoms. Neutrons and protons, both nucleons, are affected by the nuclear force almost identically. Since protons have charge +1 e, they experience an electric force that tends to push them apart, but at short range the attractive nuclear force is strong enough to overcome the electromagnetic force. The nuclear force binds nucleons into atomic nuclei.

The nuclear force is powerfully attractive between nucleons at distances of about 1 femtometre (fm, or 1.0 × 10−15 metres), but it rapidly decreases to insignificance at distances beyond about 2.5 fm. At distances less than 0.7 fm, the nuclear force becomes repulsive. This repulsive component is responsible for the physical size of nuclei, since the nucleons can come no closer than the force allows. By comparison, the size of an atom, measured in angstroms (Å, or 1.0 × 10−10 m), is five orders of magnitude larger. The nuclear force is not simple, however, since it depends on the nucleon spins, has a tensor component, and may depend on the relative momentum of the nucleons. The strong nuclear force is one of the fundamental forces of nature.

The nuclear force plays an essential role in storing energy that is used in nuclear power and nuclear weapons. Work (energy) is required to bring charged protons together against their electric repulsion. This energy is stored when the protons and neutrons are bound together by the nuclear force to form a nucleus. The mass of a nucleus is less than the sum total of the individual masses of the protons and neutrons. The difference in masses is known as the mass defect, which can be expressed as an energy equivalent. Energy is released when a heavy nucleus breaks apart into two or more lighter nuclei. This energy is the electromagnetic potential energy that is released when the nuclear force no longer holds the charged nuclear fragments together.A quantitative description of the nuclear force relies on equations that are partly empirical. These equations model the internucleon potential energies, or potentials. (Generally, forces within a system of particles can be more simply modeled by describing the system's potential energy; the negative gradient of a potential is equal to the vector force.) The constants for the equations are phenomenological, that is, determined by fitting the equations to experimental data. The internucleon potentials attempt to describe the properties of nucleon–nucleon interaction. Once determined, any given potential can be used in, e.g., the Schrödinger equation to determine the quantum mechanical properties of the nucleon system.

The discovery of the neutron in 1932 revealed that atomic nuclei were made of protons and neutrons, held together by an attractive force. By 1935 the nuclear force was conceived to be transmitted by particles called mesons. This theoretical development included a description of the Yukawa potential, an early example of a nuclear potential. Mesons, predicted by theory, were discovered experimentally in 1947. By the 1970s, the quark model had been developed, by which the mesons and nucleons were viewed as composed of quarks and gluons. By this new model, the nuclear force, resulting from the exchange of mesons between neighboring nucleons, is a residual effect of the strong force.

Nuclear structure

Understanding the structure of the atomic nucleus is one of the central challenges in nuclear physics.

Nucleon pair breaking in fission

Nucleon pair breaking in fission has been an important topic in nuclear physics for decades. "Nucleon pair" refers to nucleon pairing effects which strongly influence the nuclear properties of a nuclide.

The most measured quantities in research on nuclear fission are the charge and mass fragments yields for uranium-235 and other fissile nuclides. In this sense, experimental results on charge distribution for low-energy fission of actinides present a preference to an even Z fragment, which is called odd-even effect on charge yield.The importance of these distributions is because they are the result of rearrangement of nucleons on the fission process due to the interplay between collective variables and individual particle levels; therefore they permit to understand several aspects of dynamics of fission process. The process from saddle (when nucleus begins its irreversible evolution to fragmentation) to scission point (when fragments are formed and nuclear interaction between fragments dispels), fissioning system shape changes but also promote nucleons to excited particle levels.

Because, for even Z (proton number) and even N (neutron number) nuclei, there is a gap from ground state to first excited particle state—which is reached by nucleon pair breaking—fragments with even Z is expected to have a higher probability to be produced than those with odd Z.

The preference even Z even N divisions is interpreted as the preservation of superfluidity during the descent from saddle to scission. The absence of odd-even effect means that process is rather viscous.Contrary to observed for charge distributions no odd-even effect on fragments mass number (A) is observed. This result is interpreted by the hypothesis that in fission process always there will be nucleon pair breaking, which may be proton pair or neutron pair breaking in low energy fission of uranium-234, uranium-236, and plutonium-240 studied by Modesto Montoya.

Nucleon spin structure

Nucleon spin structure describes the partonic structure of nucleon (proton and neutron) intrinsic angular momentum (spin). The key question is how the nucleon's spin, whose magnitude is 1/2ħ, is carried by its constituent partons (quarks and gluons). It was originally expected before the 1980s that quarks carry all of the nucleon spin, but later experiments contradict this expectation. In the late 1980s, the European Muon Collaboration (EMC) conducted experiments that suggested the spin carried by quarks is not sufficient to account for the total spin of the nucleons. This finding astonished particle physicists at that time, and the problem of where the missing spin lies is sometimes referred to as the proton spin crisis.

Experimental research on these topics has been continued by the Spin Muon Collaboration (SMC) and the COMPASS experiment at CERN, experiments E142, E143, E154 and E155 at SLAC, HERMES at DESY, experiments at JLab and RHIC, and others. Global analysis of data from all major experiments confirmed the original EMC discovery and showed that the quark spin did contribute about 30% to the total spin of the nucleon. A major topic of modern particle physics is to find the missing angular momentum, which is believed to be carried either by gluon spin, or by gluon and quark orbital angular momentum. This fact is expressed by the sum rule,

${\displaystyle {\frac {1}{2}}={\frac {1}{2}}\Sigma _{q}+\Sigma _{g}+L_{q}+L_{g}.}$

The gluon spin components ${\displaystyle \Sigma _{g}}$ are being measured by many experiments. Quark and gluon angular momenta will be studied by measuring so-called generalized parton distributions (GPD) through deeply virtual compton scattering (DVCS) experiments, conducted at CERN (COMPASS) and at Jefferson Lab, among other laboratories.

Silicon-burning process

In astrophysics, silicon burning is a very brief sequence of nuclear fusion reactions that occur in massive stars with a minimum of about 8-11 solar masses. Silicon burning is the final stage of fusion for massive stars that have run out of the fuels that power them for their long lives in the main sequence on the Hertzsprung-Russell diagram. It follows the previous stages of hydrogen, helium, carbon, neon and oxygen burning processes.

Silicon burning begins when gravitational contraction raises the star's core temperature to 2.7–3.5 billion Kelvin (GK). The exact temperature depends on mass. When a star has completed the silicon-burning phase, no further fusion is possible. The star catastrophically collapses and may explode in what is known as a Type II supernova.

Skyrmion

In particle theory, the skyrmion () is a topologically stable field configuration of a certain class of non-linear sigma models. It was originally proposed as a model of the nucleon by Tony Skyrme in 1962. As a topological soliton in the pion field, it has the remarkable property of being able to model, with reasonable accuracy, multiple low-energy properties of the nucleon, simply by fixing the nucleon radius. It has since found application in solid state physics, as well as having ties to certain areas of string theory.

Skyrmions as topological objects are important in solid state physics, especially in the emerging technology of spintronics. A two-dimensional magnetic skyrmion, as a topological object, is formed, e.g., from a 3D effective-spin "hedgehog" (in the field of micromagnetics: out of a so-called "Bloch point" singularity of homotopy degree +1) by a stereographic projection, whereby the positive north-pole spin is mapped onto a far-off edge circle of a 2D-disk, while the negative south-pole spin is mapped onto the center of the disk. In a spinor field such as for example photonic or polariton fluids the skyrmion topology corresponds to a full Poincaré beam (which is, a quantum vortex of spin comprising all the states of polarization).Skyrmions have been reported, but not conclusively proven, to be in Bose-Einstein condensates, superconductors, thin magnetic films and in chiral nematic liquid crystals.As a model of the nucleon, the topological stability of the Skyrmion can be interpreted as a statement that the baryon number is conserved; i.e. that the proton does not decay. The Skyrme Lagrangian is essentially a one-parameter model of the nucleon. Fixing the parameter fixes the proton radius, and also fixes all other low-energy properties, which appear to be correct to about 30%. It is this predictive power of the model that makes it so appealing as a model of the nucleon.

Hollowed-out skyrmions form the basis for the chiral bag model (Chesire cat model) of the nucleon. Exact results for the duality between the fermion spectrum and the topological winding number of the non-linear sigma model have been obtained by Dan Freed. This can be interpreted as a foundation for the duality between a QCD description of the nucleon (but consisting only of quarks, and without gluons) and the Skyrme model for the nucleon.

The skyrmion can be quantized to form a quantum superposition of baryons and resonance states. It could be predicted from some nuclear matter properties.

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