Nucleon
In physics and chemistry, 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 the atom's mass number (nucleon number).
Until the 1960s, nucleons were thought to be
Nucleons sit at the boundary where particle physics and nuclear physics overlap. Particle physics, particularly quantum chromodynamics, provides the fundamental equations that describe the properties of quarks and of the strong interaction. These equations describe 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 describe nuclide properties, as for example, whether or not a particular nuclide undergoes radioactive decay.
The proton and neutron are in a scheme of categories being at once
Overview
Properties
u
) and one down antiquark (
d
):
u
u
d
. An antineutron (
n
) has one up antiquark (
u
) and two down antiquarks (
d
):
u
d
d
. The color charge (color assignment
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 have important applications (see neutron radiation and neutron scattering). Protons not bound to other nucleons are the nuclei of hydrogen atoms when bound with an electron or – if not bound to anything – are ions or cosmic rays.
Both the proton and the neutron are
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.[a] 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 masses of the proton and neutron are similar: for the proton it is 1.6726×10−27 kg (938.27 MeV/c2), while for the neutron it is 1.6749×10−27 kg (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 description remains an unsolved problem in particle physics.[1]: 135–136
The spin of the nucleon is 1/2, which means that they are fermions and, like electrons, are subject to the Pauli exclusion principle: no more than one nucleon, e.g. in an atomic nucleus, may occupy the same quantum state.
The
Both the proton and neutron have
Stability
A neutron in free state is an unstable particle, with a
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
nuclei.Tables of detailed properties
Nucleons
Particle name |
Symbol | Quark content |
Mass[a] | I3 | JP
|
Q | Magnetic moment (μN) | Mean lifetime
|
Commonly decays to |
---|---|---|---|---|---|---|---|---|---|
proton[PDG 1] | p / p+ / N+ |
u u d |
938.272013(23) MeV/c2 1.00727646677(10) Da | +1/2 | 1/2+ | +1 e | 2.792847356(23) | stable[b] | unobserved |
neutron[PDG 2] | n / n0 / N0 |
u d d |
939.565346(23) MeV/c2 1.00866491597(43) Da | −+1/2 | 1/2+ | 0 e | −1.91304273(45) | 885.7(8) s[c] | ν e |
antiproton | p / p− / N− |
u u d |
938.272013(23) MeV/c2 1.00727646677(10) Da | −+1/2 | 1/2+ | −1 e | −2.793(6) | stable[b] | unobserved |
antineutron | n / n0 / N0 |
u d d |
939.485(51) MeV/c2 1.00866491597(43) Da | ++1/2 | 1/2+ | 0 e | ? | 885.7(8) s[c] | p + e+ + ν e |
^a The masses of the proton and neutron are known with far greater precision in daltons (Da) than in MeV/c2 due to the way in which these are defined. The conversion factor used is 1 Da = 931.494028(23) MeV/c2.
^b At least 1035 years. See proton decay.
The masses of their antiparticles are assumed to be identical, and no experiments have refuted this to date. Current experiments show any relative 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]
Test | Formula | PDG result[PDG 1] |
---|---|---|
Mass | <2×10−9 | |
Charge-to-mass ratio
|
0.99999999991(9) | |
Charge-to-mass-to-mass ratio | (−9±9)×10−11 | |
Charge | <2×10−9 | |
Electron charge | <1×10−21 | |
Magnetic moment | (−0.1±2.1)×10−3 |
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) LIJ, where m is the particle's approximate mass, L is the orbital angular momentum (in the
The table below lists only the base resonance; each individual entry represents 4 baryons: 2 nucleon resonances particles and 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 = 1/2. For resonances with isospin = 3/2, see the article on Delta baryons.
Symbol | JP
|
PDG MeV/c 2)
|
Full width (MeV/c2) |
Pole position (real part) |
Pole position (−2 × imaginary part) |
Common decays (Γi/Γ > 50%) |
---|---|---|---|---|---|---|
N(939) P11 [PDG 3]† |
1/2+ | 939 | † | † | † | † |
N(1440) P11 [PDG 4] (the Roper resonance) |
1/2+ | 1440 (1420–1470) |
300 (200–450) |
1365 (1350–1380) |
190 (160–220) |
N + π |
N(1520) D13 [PDG 5] |
3/2− | 1520 (1515–1525) |
115 (100–125) |
1510 (1505–1515) |
110 (105–120) |
N + π |
N(1535) S11 [PDG 6] |
1/2− | 1535 (1525–1545) |
150 (125–175) |
1510 (1490–1530) |
170 (90–250) |
N + η |
N(1650) S11 [PDG 7] |
1/2− | 1650 (1645–1670) |
165 (145–185) |
1665 (1640–1670) |
165 (150–180) |
N + π |
N(1675) D15 [PDG 8] |
5/2− | 1675 (1670–1680) |
150 (135–165) |
1660 (1655–1665) |
135 (125–150) |
N + π + π or Δ + π |
N(1680) F15 [PDG 9] |
5/2+ | 1685 (1680–1690) |
130 (120–140) |
1675 (1665–1680) |
120 (110–135) |
N + π |
N(1700) D13 [PDG 10] |
3/2− | 1700 (1650–1750) |
100 (50–150) |
1680 (1630–1730) |
100 (50–150) |
N + π + π |
N(1710) P11 [PDG 11] |
1/2+ | 1710 (1680–1740) |
100 (50–250) |
1720 (1670–1770) |
230 (80–380) |
N + π + π |
N(1720) P13 [PDG 12] |
3/2+ | 1720 (1700–1750) |
200 (150–300) |
1675 (1660–1690) |
115–275 | N + π + π or N + ρ |
N(2190) G17 [PDG 13] |
7/2− | 2190 (2100–2200) |
500 (300–700) |
2075 (2050–2100) |
450 (400–520) |
N + π (10—20%) |
N(2220) H19 [PDG 14] |
9/2+ | 2250 (2200–2300) |
400 (350–500) |
2170 (2130–2200) |
480 (400–560) |
N + π (10—20%) |
N(2250) G19 [PDG 15] |
9/2− | 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. Such a particle may be stable when in an atomic nucleus, e.g. in
Quark model classification
In the
The article on isospin provides an explicit expression for the nucleon wave functions in terms of the quark flavour eigenstates.
Models
This section may be confusing or unclear to readers. (August 2007) |
Although it is known that the nucleon is made from three quarks, as of 2006[update], 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
MIT bag model
The MIT bag model
Mathematically, the model vaguely resembles that of a
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.[11][12]
Chiral bag model
The chiral bag model[13][14] 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
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,[15] after the fading of Lewis Carroll's Cheshire Cat to just its smile. It is expected that a first-principles solution of the equations of QCD will demonstrate a similar duality of quark–meson descriptions.
See also
Footnotes
- ^ The resultant coefficients are obtained by summation of the component charges: ΣQ = 2/3 + 2/3 + (−+1/3) = 3/3 = +1 for proton, and ΣQ = 2/3 + (−+1/3) + (−+1/3) = 0/3 = 0 for neutron.
References
- ^ ISBN 978-3-527-40601-2.
- ISBN 978-0-201-05757-7.
- ^ Kincade, Kathy (2 February 2015). "Pinpointing the magnetic moments of nuclear matter". Phys.org. Archived from the original on 2 May 2015. Retrieved May 8, 2015.
- S2CID 122952224.
- .
- ^ R. Arsenescu; et al. (2003). "Antihelium-3 production in lead-lead collisions at 158 A GeV/c". .
- ^ "Lithium-6. Compound summary". PubChem. National Library of Medicine. Archived from the original on 2021-11-19. Retrieved 2021-04-08.
- ^ Chodos et al. "New extended model of hadrons" Archived 2023-12-30 at the Wayback Machine, Phys. Rev. D 9, 3471 (1974).
- ^ Chodos et al. "Baryon structure in the bag theory" Archived 2023-12-30 at the Wayback Machine, Phys. Rev. D 10, 2599 (1974).
- ^ DeGrand et al. "Masses and other parameters of the light hadrons" Archived 2023-12-30 at the Wayback Machine, Phys. Rev. D 12, 2060 (1975).
- .
- ^ Yu; Simonov, A. (1981). "The quark compound bag model and the Jaffe-Low P-matrix". .
- .
- .
- .
Particle listings
- ^ a b c Particle listings –
p
Archived 2017-01-27 at the Wayback Machine. - ^ a b Particle listings –
n
Archived 2018-10-03 at the Wayback Machine. - ^ Particle listings — Note on N and Delta Resonances Archived 2021-03-27 at the Wayback Machine.
- ^ Particle listings — N(1440) Archived 2021-03-30 at the Wayback Machine.
- ^ Particle listings — N(1520) Archived 2021-03-29 at the Wayback Machine.
- ^ Particle listings — N(1535) Archived 2021-03-29 at the Wayback Machine.
- ^ Particle listings — N(1650) Archived 2021-03-30 at the Wayback Machine.
- ^ Particle listings — N(1675) Archived 2021-03-28 at the Wayback Machine.
- ^ Particle listings — N(1680) Archived 2021-03-29 at the Wayback Machine.
- ^ Particle listings — N(1700) Archived 2021-03-28 at the Wayback Machine.
- ^ Particle listings — N(1710) Archived 2021-03-28 at the Wayback Machine.
- ^ Particle listings — N(1720) Archived 2021-03-30 at the Wayback Machine.
- ^ Particle listings — N(2190) Archived 2021-03-29 at the Wayback Machine.
- ^ Particle listings — N(2220) Archived 2021-03-29 at the Wayback Machine.
- ^ Particle listings — N(2250) Archived 2021-03-29 at the Wayback Machine.
Further reading
- Thomas, A. W.; Weise, W. (2001). The Structure of the Nucleon. Berlin, DE: Wiley-WCH. ISBN 3-527-40297-7.
- Brown, G .E.; Jackson, A. D. (1976). The Nucleon–Nucleon Interaction. ISBN 978-0-7204-0335-0.
- Nakamura, N.; hdl:10481/34593.