Anatomic nucleus is shown here as a compact bundle of the two types of nucleons,protons (red) andneutrons (blue). In this picture, the protons and neutrons are shown as distinct, which is the conventional view inchemistry, for example. But in an actual nucleus, as understood by modernnuclear physics, the nucleons are partially delocalized and organize themselves according to the laws ofquantum chromodynamics.
Until the 1960s, nucleons were thought to beelementary particles, not made up of smaller parts. Now they are understood ascomposite particles, made of threequarks bound together by thestrong interaction. The interaction between two or more nucleons is calledinternucleon interaction ornuclear 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 whereparticle physics andnuclear physics overlap. Particle physics, particularlyquantum 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 otherhadrons). However, when multiple nucleons are assembled into an atomic nucleus (nuclide), these fundamental equations become too difficult to solve directly (seelattice QCD). Instead, nuclides are studied withinnuclear physics, which studies nucleons and their interactions by approximations and models, such as thenuclear shell model. These models can successfully describe nuclide properties, as for example, whether or not a particular nuclide undergoesradioactive decay.
The proton and neutron are in a scheme of categories being at oncefermions,hadrons andbaryons. The proton carries a positive netcharge, and the neutron carries a zero net charge; the proton'smass 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 anisospin doublet (I =1/2). In isospin space, neutrons can be transformed into protons and conversely bySU(2) symmetries. These nucleons are acted upon equally by the strong interaction, which is invariant under rotation in isospin space. According toNoether's theorem, isospin is conserved with respect to the strong interaction.[1]: 129–130
A proton (p) is composed of two up quarks (u) and one down quark (d): uud. A neutron (n) has one up quark (u) and two down quarks (d): udd. Anantiproton ( p ) has two upantiquarks ( u ) and one down antiquark ( d ): u u d . Anantineutron ( n ) has one up antiquark ( u ) and two down antiquarks ( d ): u d d . Thecolor charge (color assignment) of individual quarks is arbitrary, but all three colors (red, green, blue) must be present.
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 (seeneutron radiation andneutron scattering). Protons not bound to other nucleons are the nuclei of hydrogen atoms when bound with anelectron or – if not bound to anything – areions or cosmic rays.
Both the proton and the neutron arecomposite particles, meaning that each is composed of smaller parts, namely threequarks each; although once thought to be so, neither is anelementary particle. A proton is composed of twoup quarks and onedown quark, while the neutron has one up quark and two down quarks. Quarks are held together by thestrong force, or equivalently, bygluons, which mediate the strong force at the quark level.
An up quark haselectric charge++2/3e, and a down quark has charge−+1/3e, 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 is1.6726×10−27kg (938.27 MeV/c2), while for the neutron it is1.6749×10−27kg (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
Theisospin andspin quantum numbers of the nucleon have two states each, resulting in four combinations in total. Analpha particle is composed of four nucleons occupying all four combinations, namely, it has two protons (havingopposite spin) and two neutrons (also having opposite spin), and its netnuclear spin is zero. In larger nuclei constituent nucleons, by Pauli exclusion, are compelled to have relativemotion, which may also contribute to nuclear spin via theorbital quantum number. They spread out intonuclear shells analogous toelectron shells known from chemistry.
Both the proton and neutron havemagnetic moments, though thenucleon magnetic moments are anomalous and were unexpected when they were discovered in the 1930s. The proton's magnetic moment, symbolμp, is2.79 μN, whereas, if the proton were an elementaryDirac particle, it should have a magnetic moment of1.0 μN. Here the unit for the magnetic moments is thenuclear magneton, symbolμN, an atomic-scaleunit of measure. The neutron's magnetic moment isμn =−1.91 μN, whereas, since the neutron lacks an electric charge, it should have no magnetic moment. The value of the neutron's magnetic moment is negative because the direction of the moment is opposite to the neutron's spin. The nucleon magnetic moments arise from the quark substructure of the nucleons.[2][3] The proton magnetic moment is exploited forNMR / MRI scanning.
A neutron in free state is an unstable particle, with ahalf-life around ten minutes. It undergoes β− decay (a type ofradioactive decay) by turning into a proton while emitting an electron and anelectron antineutrino. This reaction can occur because the mass of the neutron is slightly greater than that of the proton. (See theNeutron 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 (seeProton decay).
Inside a nucleus, on the other hand, combined protons and neutrons (nucleons) can be stable or unstable depending on thenuclide, 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 orelectron capture. And inside still other nuclides, both protons and neutrons are stable and do not change form.
Both nucleons have correspondingantiparticles: theantiproton and theantineutron, 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 beexactly true, due toCPT 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 createdantideuterium[4][5] and antihelium-3[6] nuclei.
^a The masses of the proton and neutron are known with far greater precision indaltons (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.
^c Forfree neutrons; in most common nuclei, neutrons are stable.
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 than2×10−9[PDG 1] and the difference between the neutron and antineutron masses is on the order of(9±6)×10−5 MeV/c2.[PDG 2]
Nucleon resonances areexcited states of nucleon particles, often corresponding to one of the quarks having a flippedspin state, or with differentorbital angular momentum when the particle decays. Only resonances with a 3- or 4-star rating at theParticle 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, wherem is the particle's approximate mass,L is the orbital angular momentum (in thespectroscopic notation) of the nucleon–meson pair, produced when it decays, andI andJ are the particle'sisospin andtotal angular momentum respectively. Since nucleons are defined as having1/2 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, in the formLIJ (m); for example, a proton can be denoted 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 and their 2 antiparticles. Each resonance exists in a form with a positiveelectric 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 nostrange,charm,bottom, ortop quarks, these particles do not possessstrangeness, etc.
†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. inlithium-6.[7]
In thequark model withSU(2)flavour, the two nucleons are part of the ground-state doublet. The proton has quark content ofuud, and the neutron,udd. InSU(3) flavour, they are part of the ground-state octet (8) ofspin-1/2baryons, known as theEightfold way. The other members of this octet are thehyperonsstrangeisotriplet Σ+ , Σ0 , Σ− , the Λ and the strange isodoublet Ξ0 , Ξ− . One can extend this multiplet inSU(4) flavour (with the inclusion of thecharm quark) to the ground-state20-plet, or toSU(6) flavour (with the inclusion of thetop andbottom quarks) to the ground-state56-plet.
The article onisospin provides an explicit expression for the nucleon wave functions in terms of the quark flavour eigenstates.
Although it is known that the nucleon is made from three quarks, as of 2006[update], it is not known how to solve theequations of motion forquantum 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, usinglattice QCD. This requires complicated algorithms and very powerfulsupercomputers. However, several analytic models also exist:
Theskyrmion models the nucleon as atopological soliton in a nonlinearSU(2)pion field. The topological stability of the skyrmion is interpreted as the conservation ofbaryon number, that is, the non-decay of the nucleon. The localtopological winding number density is identified with the localbaryon number density of the nucleon. With the pion isospin vector field oriented in the shape of ahedgehog space, the model is readily solvable, and is thus sometimes called thehedgehog model. The hedgehog model is able to predict low-energy parameters, such as the nucleon mass, radius andaxial coupling constant, to approximately 30% of experimental values.
TheMIT bag model[8][9][10] confines quarks and gluons interacting throughquantum chromodynamics to a region of space determined by balancing the pressure exerted by the quarks and gluons against a hypothetical pressure exerted by the vacuum on all colored quantum fields. The simplest approximation to the model confines three non-interacting quarks to a spherical cavity, with theboundary condition that the quarkvector current vanish on the boundary. The non-interacting treatment of the quarks is justified by appealing to the idea ofasymptotic freedom, whereas the hard-boundary condition is justified byquark confinement.
Mathematically, the model vaguely resembles that of aradar cavity, with solutions to theDirac equation standing in for solutions to theMaxwell 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, thebag 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 bags-channel mechanism using theP-matrix.[11][12]
Thechiral bag model[13][14] merges theMIT bag model and theskyrmion 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 theaxial 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-zerovacuum expectation value (orspectral asymmetry) of the quark fields inside the bag. As of 2017[update], this remarkable trade-off betweentopology and thespectrum of an operator does not have any grounding or explanation in the mathematical theory ofHilbert spaces and their relationship togeometry.
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 theCheshire Cat principle,[15] after the fading ofLewis Carroll'sCheshire 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.
^The resultant coefficients are obtained by summation of the component charges:ΣQ =2/3 +2/3 +(−+1/3) =3/3 = +1 for proton, andΣQ =2/3 +(−+1/3) +(−+1/3) =0/3 = 0 for neutron.
