Neutron
The neutron is the electrically neutral constituent of the atomic nucleus, discovered by James Chadwick in 1932 in the radiation emitted when beryllium is bombarded with alpha particles. Its mass, 939.6 MeV/c2, is slightly greater than the proton's; its spin is ½; and although it carries no net charge it has a magnetic moment, which is taken as direct evidence that it has internal charge structure. Free neutrons are unstable, decaying to a proton, an electron and an antineutrino with a mean lifetime of about fifteen minutes; bound in most nuclei they are stable indefinitely.
In the Standard Model the neutron is a baryon composed of one up quark and two down quarks, and its decay is a weak-interaction process in which a down quark converts to an up quark. Chadwick's discovery immediately explained isotopes — nuclei of the same element differing only in neutron number — and made controlled fission possible, since neutrons carry no charge and so are not repelled by the nucleus they approach.
There is a genuine unresolved measurement in mainstream neutron physics: the neutron lifetime discrepancy. Experiments that trap ultracold neutrons in a bottle and count the survivors give a lifetime several seconds shorter than experiments that count the protons produced by a neutron beam. The two methods have been refined for decades and the gap has not closed. No consensus explanation exists.
On this wiki
Two questions dominate this wiki's treatment of the neutron. The first is whether it is an elementary particle at all or a bound proton–electron pair; the second is whether its decay really requires a neutrino.
The proton–electron composite view is old, predates the quark model, and is defended here on the grounds that the neutron's decay products are exactly a proton and an electron, that its magnetic moment implies internal charge, and that a neutral elementary particle with a magnetic moment is a contradiction. Charles William Lucas builds nuclei from finite-size electrodynamic particles on this basis in A Classical Electrodynamic Theory of the Nucleus (2006) and A Physical Model for Atoms and Nuclei, Part 2: Structure of the Nucleus (2003), replacing the strong nuclear force with ordinary electrodynamics between extended charges. David L Bergman computes the neutron's fine-structure quantities from ring geometry in Fine-Structure Properties of the Electron, Proton and Neutron (2006), and Thomas G Barnes set out classical alternatives in New Proton and Neutron Models (1980).
Toroidal and vortex models are the other main family. Jaroslav G Klyushin extends his torus model of the electron and proton to the neutral particle in On a Toroidal Model of the Neutron (2006) and Neutron Construction (2006), deriving charge from equatorial rotation and spin from meridional rotation, so that a neutron is a configuration in which the two cancel. Don Briddell treats the neutron as a knot in a field rather than an object in space in The Neutron: Modeled as a Fieldstructure (2011), part of his wider Field Structure Theory. Stoyan Sarg assembles nucleons from aether sub-structures in Basic Structures of Matter - Supergravitation Unified Theory - a new approach in Physics, and William R Hohenberger gives an aether-fractal account in The Structure of Aether and the Mechanics of the Electromagnetic Wave Spectrum. Daniel H Deutsch models the neutron mechanically in Electromechanical Physical Models of the Electron, Proton, Neutron, and Neutrino (1991), and Karl Teppo surveys the particle's anomalies in The Secret Life of the Neutron (2009).
The decay question connects the neutron to the most contested particle on this wiki. Neutron beta decay is where the neutrino was first postulated, to account for the continuous energy spectrum of the emitted electron, and Ricardo L Carezani's Autodynamics holds that the missing energy is an artefact of applying special relativity to the decay and that no third particle is needed — the case is made on this wiki by David de Hilster in The Neutrino: Doomed from Inception (2011). Wladimir Guglinski reanalyses the Wu parity experiment in his paper Beta Decay and argues that its results support an axial distribution of nucleons within the nucleus.
Neutrons also appear here in cosmological arguments. Glen W Deen proposes a gravitational asymmetry between neutrons and ordinary matter in Neutron Gravity and the Anti-universe (2009), and the degenerate-matter account of neutron stars is disputed in several papers collected under Category:Astronomy. Nucleon fusion geometry is treated by Carl R Littmann in Fusion Mass Losses and Tunnels Formed between Touching Nucleons, and neutron-mediated reactions recur throughout the cold fusion literature collected here.