Neutrino Glossary: Key Terms in Neutrino Physics and Ambient-Energy Research

How to use this neutrino glossary

The six thematic sections below run from the particle outward: what a neutrino is, where neutrinos come from, how they are detected, the physics that surrounds them, and the energy and materials vocabulary that borders the field. A final section handles terms that are frequently misused in popular coverage.

Definitions here are deliberately conservative. Where something is unresolved - the neutrino mass ordering, whether sterile neutrinos exist, whether the neutrino is its own antiparticle - the entry says so rather than guessing. Where a research programme is in development rather than proven, the entry says that too, and every energy entry is written inside the limits set by the first and second laws of thermodynamics.

Terms are defined the way working physicists use them. Where popular usage has drifted from the technical meaning - 'free energy' is the clearest case - both meanings are given, together with the reason the drift matters.

The particle: core terms in any neutrino glossary

Neutrinos were proposed by Wolfgang Pauli in 1930 to rescue energy conservation in beta decay, and named by Enrico Fermi. They are the most abundant matter particles in the universe and by far the most reluctant to interact with anything else.

The entries below cover the particle, its three flavours, its mass and the oscillation phenomenon that proved that mass is not zero.

• Neutrino - an electrically neutral elementary particle with a tiny but non-zero mass, interacting only through gravity and the weak force, so it passes through ordinary matter almost untouched.
• Antineutrino - the antiparticle partner of the neutrino, emitted in ordinary beta-minus decay of a neutron alongside an electron.
• Flavour - the label linking a neutrino to a charged partner particle; a neutrino's flavour is revealed by the charged lepton it produces when it interacts.
• Electron, muon and tau neutrino - the three known flavours, named after the charged leptons they pair with. The tau neutrino was directly observed only in 2000, by the DONUT experiment at Fermilab.
• Sterile neutrino - a hypothetical additional type that would not feel the weak force at all and could be detected only indirectly, through mixing. No experiment has confirmed one.
• Lepton - the class of elementary fermions that do not feel the strong force: the electron, muon and tau, plus their three neutrinos.
• Fermion - a particle with half-integer spin that obeys the Pauli exclusion principle. All matter particles, neutrinos included, are fermions.
• Neutrino mass - non-zero but extraordinarily small. Oscillation experiments prove it exists; the KATRIN experiment's direct laboratory limit places the effective electron-neutrino mass below about half an electronvolt, roughly a million times lighter than the electron.
• Mass eigenstate - a state with a definite mass (labelled ν1, ν2, ν3). These do not coincide with the flavour states, and that mismatch is what makes oscillation possible.
• Majorana vs Dirac - the open question of whether a neutrino is its own antiparticle (Majorana) or has a distinct antiparticle (Dirac). Searches for neutrinoless double beta decay test this and have so far found nothing.
• Neutrino oscillation - the periodic change of flavour as a neutrino travels, possible only if neutrinos have mass. Takaaki Kajita and Arthur B. McDonald shared the 2015 Nobel Prize in Physics for its discovery.
• Mixing angle - a parameter (θ12, θ23, θ13) describing how strongly the flavour states overlap the mass states; it sets how large an oscillation effect can be.
• PMNS matrix - the Pontecorvo-Maki-Nakagawa-Sakata matrix, the bookkeeping that relates the three flavour states to the three mass states.
• MSW effect (matter effect) - the Mikheyev-Smirnov-Wolfenstein effect: oscillation is altered inside dense matter because electron neutrinos scatter off electrons differently from the other flavours. It explains the observed behaviour of solar neutrinos.
• Mass ordering - whether ν3 is heavier or lighter than ν1 and ν2 ('normal' or 'inverted'). Not yet settled, and a headline goal of current experiments.
• CP violation in the lepton sector - a possible difference between how neutrinos and antineutrinos oscillate, described by the phase δCP. It is measured but not yet pinned down, and is one motivation for the next generation of beam experiments.

Sources: where neutrinos come from

Neutrinos arrive from the Sun, the atmosphere, the Earth's interior, nuclear reactors, exploding stars and the distant universe. Their energies span more than fifteen orders of magnitude, and each source is studied with different instruments.

A fuller treatment lives on the neutrino sources page; the entries here are the short forms you will meet in captions and abstracts.

• Solar neutrinos - produced by fusion in the Sun's core. Roughly 6 × 10^10 pass through every square centimetre of Earth's surface each second.
• Atmospheric neutrinos - created when cosmic rays strike the upper atmosphere and produce pions that decay in flight. Their flavour ratios gave the first strong evidence for oscillation.
• Reactor neutrinos - electron antineutrinos from the beta decays of fission fragments in nuclear reactors; a controlled, well-understood source used by KamLAND, Daya Bay and JUNO.
• Geoneutrinos - antineutrinos released by the decay of uranium-238, thorium-232 and potassium-40 inside the Earth. They carry information about the planet's internal heat budget.
• Supernova neutrinos - an intense burst carrying the overwhelming majority of the energy released by a core-collapse supernova. About two dozen were recorded from SN 1987A, the first neutrinos ever detected from beyond the solar system.
• Cosmic neutrino background - relic neutrinos that decoupled roughly one second after the Big Bang and now fill space at about 1.95 K. Predicted with confidence, but never directly detected.
• Accelerator (beam) neutrinos - artificial beams made by firing protons at a target, aimed at detectors hundreds of kilometres away, as in T2K, NOvA and DUNE.
• Astrophysical neutrinos - very-high-energy neutrinos from sources outside the solar system, first established as a population by IceCube.
• Flux - the number of particles crossing a given area per unit time. Neutrino fluxes are enormous precisely because the interaction probability is so small.

Detection: how experiments catch a particle that barely interacts

Because interaction probabilities are minuscule, detection means huge target masses, extreme quiet and patience. A detector is essentially a very large volume of transparent matter, watched by light sensors, buried deep enough that almost nothing else gets in.

The methods and the leading facilities are covered in more depth under how neutrinos are detected and neutrino observatories.

• Cross section - a measure of interaction probability expressed as an effective area. Neutrino cross sections are so small that a typical solar neutrino could cross roughly a light-year of lead without interacting.
• Cherenkov radiation - the faint blue light emitted when a charged particle travels through a medium faster than light travels in that medium. Its ring patterns reveal the direction and type of interaction.
• Scintillator - a liquid or solid that emits light when a charged particle passes through it, favoured for low-energy detection.
• Photomultiplier tube (PMT) - a vacuum device that converts a single photon into a measurable electrical pulse. Thousands of them line the walls of large detectors.
• Inverse beta decay - the reaction in which an electron antineutrino strikes a proton and produces a neutron plus a positron; the classic reactor-antineutrino signature, used by Clyde Cowan and Frederick Reines in 1956 for the first detection of the neutrino.
• CEvNS - coherent elastic neutrino-nucleus scattering, in which a low-energy neutrino scatters off an entire nucleus at once. Predicted in 1974 and first measured by the COHERENT collaboration in 2017.
• Event - a single recorded interaction candidate. Many experiments collect only a handful per day.
• Background - anything that mimics a neutrino signal, such as natural radioactivity or cosmic muons. Suppressing it is why detectors sit deep underground behind heavy shielding.
• Neutrino floor / neutrino fog - the point at which neutrinos scattering coherently become an irreducible background for dark matter searches. 'Fog' is now the preferred term, because the boundary is gradual rather than sharp.
• IceCube - a cubic kilometre of instrumented Antarctic ice at the South Pole, built to catch high-energy astrophysical neutrinos.
• Super-Kamiokande - a Japanese detector holding 50,000 tonnes of ultrapure water watched by around 11,000 photomultiplier tubes.
• SNO - the Sudbury Neutrino Observatory in Canada, whose heavy-water target could count all three flavours and so resolved the long-standing solar neutrino problem.
• Borexino - an extremely low-background liquid-scintillator detector at Gran Sasso, Italy, which measured low-energy solar neutrinos and reported CNO-cycle neutrinos in 2020; it concluded data taking in 2021.
• DUNE - the Deep Underground Neutrino Experiment, sending a beam roughly 1,300 km from Fermilab to liquid-argon detectors in South Dakota; under construction.
• JUNO - the Jiangmen Underground Neutrino Observatory in southern China, a 20,000-tonne liquid-scintillator detector designed to settle the neutrino mass ordering using reactor antineutrinos.
• KATRIN - the Karlsruhe Tritium Neutrino experiment, which weighs the neutrino directly by measuring the shape of the tritium beta-decay spectrum near its endpoint.

Physics context: forces, particles and the wider cosmos

Neutrinos only make sense inside the broader framework of particle physics. These entries cover the surrounding concepts you will meet in almost any paper or popular article on the subject.

They are also the terms most often used loosely outside physics, which is another reason to have them written down plainly.

• Weak interaction - one of the four fundamental forces. It changes quark and lepton flavour and is the only non-gravitational way a neutrino interacts with anything.
• W and Z bosons - the heavy carriers of the weak force, discovered at CERN in 1983. The W mediates charged-current interactions, the Z neutral-current ones.
• Beta decay - a nucleus converting a neutron into a proton and emitting an electron plus an electron antineutrino, or the reverse process, emitting a positron plus an electron neutrino. The apparent energy imbalance in this process is what led Pauli to propose the neutrino in the first place.
• Half-life - the time needed for half the nuclei in a large sample of an unstable isotope to decay.
• Standard Model - the well-tested framework describing all known elementary particles and three of the four fundamental forces. It excludes gravity and, in its original form, assumed massless neutrinos.
• Quark - the constituents of protons and neutrons, six flavours in total, bound together by the strong force.
• Boson - an integer-spin particle. The force carriers (photon, gluon, W, Z) and the Higgs boson are bosons.
• Dark matter - unseen mass inferred from gravitational effects. Neutrinos contribute a small amount but cannot account for it, because they move too fast to clump the way galaxy formation requires.
• Antimatter - particles with the same mass and opposite charge to their ordinary counterparts. Whether the neutrino is its own antiparticle remains an open question.
• Cosmic rays - high-energy particles from space, mostly protons. Their collisions in the atmosphere produce atmospheric neutrinos and the muon backgrounds that detectors must shield against.
• Multimessenger astronomy - studying one astrophysical source through several channels at once: light, neutrinos, cosmic rays, gravitational waves. IceCube's 2017 high-energy neutrino coincident with the flaring blazar TXS 0506+056 is the standard example.

Energy and materials: harvesting, conversion and 2D materials

This is where neutrino physics borders applied research. The terms below describe how small amounts of energy already present in an environment can be captured, and the physical limits that bound any such attempt.

Every entry in this section is constrained by conservation of energy. Harvesting moves energy that is already there from one form into another; it never adds any.

• Energy harvesting - capturing small quantities of energy already present in the surroundings (light, heat gradients, vibration, radio-frequency fields) to power low-consumption electronics.
• Ambient energy - the energy present in ordinary environments. It is diffuse and finite, which is why small-scale harvesting realistically yields microwatts to milliwatts rather than kilowatts.
• Energy conversion - changing energy from one form into another. Every conversion is bounded by conservation of energy and incurs losses.
• First law of thermodynamics - energy can be converted but never created or destroyed. This is the reason no device can deliver more energy than it takes in.
• Second law of thermodynamics - every real conversion increases entropy and degrades some energy into unusable heat, so no process is perfectly efficient.
• Graphene - a single atomic layer of carbon in a hexagonal lattice, isolated in 2004 by Andre Geim and Konstantin Novoselov, who received the 2010 Nobel Prize in Physics for the work. It is exceptionally conductive, mechanically strong and highly sensitive to its surroundings.
• 2D materials - crystals one or a few atoms thick, such as graphene, hexagonal boron nitride and the transition-metal dichalcogenides, whose electronic behaviour differs sharply from that of the bulk material.
• Thermoelectric (Seebeck) effect - a temperature difference across a material generates a voltage. Described by Thomas Johann Seebeck in 1821 and used in thermoelectric generators.
• Piezoelectric effect - mechanical stress on certain crystals produces an electric charge. Discovered by Jacques and Pierre Curie in 1880 and used in vibration harvesters.
• Photovoltaic effect - light absorbed in a semiconductor junction frees charge carriers and drives a current. It is the best-established form of energy harvesting and a useful benchmark for any newer approach.
• Neutrinovoltaic - the Neutrino Energy Group's in-development research approach, investigating whether a patented graphene-silicon multilayer can convert a share of the ambient environmental flux around it - thermal motion, ambient radio-frequency waves and radiation of several kinds - into a small electrical current. (Static electromagnetic fields and ambient heat at equilibrium do not qualify as sources: a body in equilibrium emits and absorbs equally, and static fields carry no energy.) It is a research programme bounded by the first and second laws of thermodynamics: not proven, not independently validated as a power source, and not a purchasable product. A granted patent certifies novelty and disclosure, not that a device works as hoped.
• New energy technology - an umbrella term for emerging generation and storage approaches. It is useful only when attached to measured performance figures rather than expectations.

Terms to handle with care: free energy and neighbouring claims

Some words in this field are precise in physics and misleading in marketing. These entries exist so that the difference is unambiguous.

The short version: energy cannot be created. Any description of a device that appears to produce more energy than it consumes describes a measurement error, an unaccounted input, or a claim that has not survived independent testing.

• Free energy - in physics, a precise thermodynamic quantity (Gibbs free energy or Helmholtz free energy) describing the maximum work obtainable from a system under stated conditions. In popular usage the phrase is frequently misused to mean energy from nothing, which the first law of thermodynamics forbids. No such device exists, and Neutrino Energy Group neither offers nor claims one.
• Perpetual motion and over-unity - proposed machines that would run forever or produce more energy than they consume. Both are ruled out by the first and second laws of thermodynamics, and no over-unity claim has ever survived independent measurement. Patent offices in Europe and the United States routinely refuse applications for such machines on the grounds that they cannot work.
• Efficiency - the ratio of useful energy output to total energy input. Defined that way, it is always below 100 per cent in any real system.
• Power density - the power obtainable per unit area or volume. It is the number that decides whether an ambient-energy approach is practically useful, and the reason serious harvesting research talks about small loads rather than households.
• Independent replication - measurement of a claimed result by an unaffiliated group under controlled conditions. It is the threshold at which a research claim becomes an established fact, and it is the standard that neutrinovoltaic research still has to meet.
• Peer review - evaluation of a manuscript by independent specialists before publication. It filters out obvious errors but does not, on its own, establish that a result is correct; replication does that.