Neutrino
The neutrino is, in mainstream physics, an electrically neutral lepton of very small mass which interacts only through the weak interaction and gravity, and which therefore passes through ordinary matter almost without hindrance. It was postulated by Wolfgang Pauli in 1930 to rescue energy and momentum conservation in beta decay: the electron emitted by a decaying neutron comes out with a continuous range of energies rather than the single value a two-body decay requires, and Pauli's proposal was that an unseen third particle carries away the balance. Enrico Fermi incorporated it into his 1934 theory of beta decay and gave it its name. A first detection was reported by Clyde Cowan and Frederick Reines in 1956, using a reactor as the source and inverse beta decay in a tank of cadmium-loaded water as the signal.
Three flavours are recognised — electron, muon and tau — and the discovery that solar electron-neutrinos arrive at Earth at roughly a third of the predicted rate was resolved in the mainstream by neutrino oscillation: neutrinos change flavour in flight, which requires them to have non-zero mass. That result, confirmed by Super-Kamiokande and SNO around 1998–2001, was the first laboratory departure from the original Standard Model, which had assumed massless neutrinos. The absolute masses remain unmeasured; only differences of squared masses are known, together with upper bounds.
On this wiki
The neutrino is one of the most heavily contested objects in this archive, and the objection is unusually specific. It is not that neutrinos are improbable, but that they were introduced to save a conservation law rather than because anything was seen, that the detection experiments infer them from missing energy and rare secondary events rather than from a track, and that the flavour-oscillation solution to the solar deficit was fitted after the deficit was found.
The strongest position is taken by Ricardo L Carezani and his theory of Autodynamics. Carezani held that the continuous beta spectrum arises from a mistaken application of special relativity to the decay kinematics, and that once the redundant reference frame is removed the energy balance closes without any third particle: there is no neutrino to find. David de Hilster sets the argument out on this wiki in The Neutrino: Doomed from Inception (2011), whose thesis is that the neutrino's fate is tied to that of its progenitor theory — "if you believe special relativity to be wrong, then its bastard son the neutrino cannot exist." The Society for the Advancement of Autodynamics has promoted this position since 1994. Thomas N Lockyer attacks the experiments directly in Debunking Neutrino Detection Experiments (2010), arguing that what the detectors register does not establish what it is claimed to establish.
A second line of criticism comes from cosmology. William C Mitchell's No Neutrinos, No Big Bang (1997) argues that relic neutrinos are treated inconsistently with relic photons: quantum wave theory is applied to redshift the neutrino background into undetectability but is not applied to other particles supposedly surviving from the same era, and Mitchell takes this as an internal inconsistency in Big Bang cosmology rather than as a fact about neutrinos. The Electric Universe literature collected here likewise treats the solar neutrino deficit as evidence against the standard model of the Sun — against fusion confined to a stellar core — rather than as evidence for oscillation. Oliver K Manuel argues for a different solar composition and energy source on related grounds.
A third group does not deny the particle but rejects the standard description of it. Geoffrey Hunter models photons and neutrinos alike as electromagnetic solitons in Photons and Neutrinos as Electromagnetic Solitons (1989). Valeri V Dvoeglazov revisits the old proposal that light itself is composite in A Note on the Neutrino Theory of Light (1998). Daniel H Deutsch supplies a mechanical model of the neutrino alongside the other particles in Electromechanical Physical Models of the Electron, Proton, Neutron, and Neutrino (1991), and Don Briddell's Field Structure Theory assigns the neutrino a place among field knots. Jian-Miin Liu proposes in Relativistic Equilibrium Velocity Distribution, Nuclear Fusion Reaction Rate and the Solar Neutrino Problem (2003) that a corrected velocity distribution changes the predicted fusion rate, dissolving the solar neutrino deficit without oscillation.
Readers should note that the mainstream case for oscillation does not rest on the solar deficit alone — atmospheric and reactor experiments contribute independently — and the papers here vary a good deal in how much of that evidence they engage with.