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.

Particle physics, particularly quantum chromodynamics, provides the fundamental equations that describe the properties of quarks and of the strong interaction.

However, when multiple nucleons are assembled into an atomic nucleus (nuclide), these fundamental equations become too difficult to solve directly (see lattice QCD).

These nucleons are acted upon equally by the strong interaction, which is invariant under rotation in isospin space.

[1]: 129–130 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.

The isospin and spin quantum numbers of the nucleon have two states each, resulting in four combinations in total.

In larger nuclei constituent nucleons, by Pauli exclusion, are compelled to have relative motion, which may also contribute to nuclear spin via the orbital quantum number.

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.

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.

^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.

When discussing nucleon resonances, sometimes the N is omitted and the order is reversed, in the form LIJ (m); for example, a proton can be denoted as "N(939) S11" or "S11 (939)".

The article on isospin 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 the equations of motion for quantum chromodynamics.

The topological stability of the skyrmion is interpreted as the conservation of baryon number, that is, the non-decay of the nucleon.

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.

The MIT bag model[8][9][10] confines quarks and gluons interacting through quantum 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.

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.

As of 2017[update], 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.

It is expected that a first-principles solution of the equations of QCD will demonstrate a similar duality of quark–meson descriptions.

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 .