Neutrino glossary

A neutrino produced by colliding a high-energy proton beam with a fixed target to make pions and kaons, which then decay in flight to muon neutrinos. The energy and direction of the resulting beam can be tuned. Used by T2K, NOvA, MINOS, and the future DUNE.

A galaxy whose central supermassive black hole is actively pulling in matter and producing intense radiation, often with relativistic jets. AGN are among the most promising extragalactic cosmic-ray and neutrino accelerators.

• → NGC 1068 as a steady neutrino source
• → TXS 0506+056

In MSW oscillation, the slow change of the mass eigenstates as a neutrino propagates through matter of varying density. If the change is slow enough (adiabatic), the neutrino stays in its instantaneous mass eigenstate. Solar neutrinos undergo adiabatic conversion in the Sun.

The antimatter partner of the neutrino. Antineutrinos are produced abundantly in nuclear reactors and in beta-minus decay. Whether neutrinos and antineutrinos are actually distinct particles or the same particle is still an open question (see Majorana vs Dirac).

The probability that a neutrino produced as one flavor will be detected as a different flavor. Most modern long-baseline experiments measure appearance probabilities (e.g., muon-neutrino to electron-neutrino) to probe CP violation.

Neutrinos produced when cosmic rays collide with nuclei in Earth's upper atmosphere. The most useful natural source for studying oscillations across a wide range of energies and baselines.

• → The atmospheric neutrino flux

A hypothetical very-light particle proposed to solve the strong-CP problem in quantum chromodynamics. Axions can also be a dark-matter candidate. Searched for by resonant-cavity experiments like ADMX and HAYSTAC.

The distance a neutrino travels from production to detection. The oscillation pattern is set by the dimensionless ratio L/E (baseline over energy). Short baselines: a few hundred metres. Long baselines: hundreds to thousands of kilometres.

A type of radioactive decay in which a nucleus emits an electron (beta-minus) and an antineutrino, or a positron (beta-plus) and a neutrino. The shape of the electron energy spectrum at the endpoint is sensitive to the neutrino mass — the basis of the KATRIN experiment.

A nuclear decay where a proton converts to a neutron, emitting a positron and an electron neutrino. Occurs in proton-rich isotopes. The Sun's pp chain is mostly beta-plus reactions, which is why solar neutrinos are all electron flavor.

The formation of the lightest atomic nuclei (hydrogen, helium-4, deuterium, lithium-7) in the first few minutes after the Big Bang. The observed light-element abundances constrain the number of relativistic neutrino species during that era.

• → BBN and N_eff

The production of the lightest elements (hydrogen, helium-4, helium-3, deuterium, lithium-7) in the first few minutes after the Big Bang. Its predicted abundances depend sensitively on the number of relativistic neutrino species ($N_{eff}$), which BBN measures at $N_{eff} = 2.99 \pm 0.17$. Provides an independent cross-check on the Standard Model's three neutrino species.

An active galactic nucleus whose relativistic jet points almost directly toward Earth. Doppler-boosted emission makes blazars extraordinarily bright across the electromagnetic spectrum. TXS 0506+056 was the first identified cosmic neutrino source.

Coherent elastic neutrino-nucleus scattering — a low-energy neutrino interaction in which the entire nucleus recoils as one unit. Predicted in 1974, first detected by COHERENT in 2017. Now central to reactor monitoring and the dark-matter 'neutrino floor.'

• → CEvNS: the 43-year detection

A neutrino interaction mediated by a charged W boson. The neutrino converts into its charged-lepton partner (electron, muon, or tau), and the target nucleon changes accordingly. This is the channel that produces a visible charged lepton in the detector.

The faint blue light emitted when a charged particle moves through a medium faster than light can in that medium. Used by water and ice detectors (Super-K, IceCube) to reconstruct neutrino interactions.

A property of fundamental fermions describing how they transform under spatial reflection. Left-handed and right-handed components behave differently under the weak interaction. Neutrinos in the Standard Model are produced only in left-handed states.

• → Why neutrinos are left-handed

Oscillations in the cosmic microwave background power spectrum caused by acoustic waves in the primordial photon-baryon fluid. Their positions and amplitudes encode the matter content of the Universe, including the number and mass of neutrino species.

The CERN Neutrinos to Gran Sasso beam, operated 2006–2012. Sent muon neutrinos 730 km from CERN to the OPERA and ICARUS detectors at Gran Sasso. Demonstrated muon-to-tau oscillation by direct tau-lepton appearance in emulsion targets.

A secondary fusion sequence in the Sun, using carbon, nitrogen, and oxygen as catalysts. Contributes about 1 % of solar energy. First directly observed by Borexino in 2020 via its CNO neutrino flux.

A neutrino beam (or natural source) in which the wave-function phases of the three mass states are still aligned. Coherence is required for oscillation to be observable. Cosmic-distance propagation can in principle decohere a beam, but no decoherence has been observed in practice.

The relic radiation left over from when the universe became transparent to light, about 380,000 years after the Big Bang. Precision CMB measurements constrain both the neutrino mass sum and the effective neutrino species count N_eff.

The predicted relic flux of neutrinos left over from the Big Bang, at a density of about 336 per cubic centimeter everywhere in space. Inferred indirectly with high confidence; direct laboratory detection is the goal of PTOLEMY and similar experiments.

• → The cosmic neutrino background

Very high-energy neutrinos arriving from outside our galaxy. IceCube's discovery of an astrophysical flux opened multi-messenger astronomy with neutrinos.

• → Cosmic messengers from blazars

High-energy charged particles arriving at Earth from outside the solar system — mostly protons, plus heavier nuclei. Discovered by Victor Hess in 1912. They produce atmospheric neutrinos when they hit the upper atmosphere.

• → Hess 1912 balloon flights

A flattening of the cosmic-ray spectrum at about 5 × 10¹⁸ eV. Interpreted as the energy at which extragalactic cosmic rays begin to dominate over the declining galactic component.

• → The cosmic-ray ankle

A steepening of the cosmic-ray energy spectrum at about 3 × 10¹⁵ eV. Interpreted as the maximum energy reachable by galactic accelerators (most likely supernova remnants). Sets the upper energy of the galactic neutrino flux.

• → The cosmic-ray knee

Predicted ultra-high-energy neutrinos produced when cosmic-ray protons collide with cosmic microwave background photons over intergalactic distances. The flux is small but guaranteed; detection is the target of next-generation radio experiments.

• → Cosmogenic neutrinos

A small difference in how nature treats matter and antimatter. Confirmed in quark systems but still being measured in neutrinos. CP violation in the neutrino sector is the leading candidate to explain why the universe is made of matter.

• → CP violation in the neutrino sector

A foundational result in quantum field theory: any local, Lorentz-invariant theory must be invariant under the combined operation of charge conjugation (C), parity (P), and time reversal (T). Predicts that particles and antiparticles have identical masses.

• → CPT and Lorentz invariance tests

A measure of how likely a neutrino is to interact with a target particle, expressed in cm² or barns. Neutrino cross sections are tiny: about 10⁻⁴⁴ cm² at MeV energies, the smallest of any Standard-Model particle interaction.

A hypothetical gauge boson of a hidden-sector U(1) symmetry, kinetically mixed with the ordinary photon. A candidate mediator between visible matter and a hidden dark sector. Searched for by FASER and other beam-dump experiments.

The CP-violating phase of the PMNS mixing matrix. Currently being measured by T2K and NOvA; will be pinned down by DUNE and Hyper-Kamiokande. Its value determines how much CP violation neutrinos exhibit.

The Dark Energy Spectroscopic Instrument, a galaxy redshift survey using a wide-field spectrograph at the Mayall telescope, Kitt Peak. Its 2024 Year-1 cosmology release dramatically tightened the bound on the sum of neutrino masses to $\sum m_\nu < 0.07$ eV.

The accumulated neutrino flux from every core-collapse supernova throughout cosmic history. Not yet directly detected; Super-Kamiokande with gadolinium loading is approaching the sensitivity needed.

• → The DSNB

A mass term in the Standard Model Lagrangian that conserves lepton number. All charged leptons and quarks have Dirac mass via Higgs coupling. If neutrinos have only Dirac mass, they require additional right-handed neutrinos and the Higgs coupling must be extraordinarily small.

A neutrino whose antiparticle is a distinct particle, like the electron and positron. The alternative is Majorana — neutrinos that are their own antiparticles. The neutrinoless double-beta-decay search aims to tell them apart.

A nuclear process where a proton captures an inner-shell atomic electron, converting to a neutron and emitting an electron neutrino. The resulting neutrino is monoenergetic, set by the mass difference between parent and daughter. Used by HOLMES for direct mass measurement on holmium-163.

The number of neutrino interactions per unit time in a detector. For a 1 GW reactor at 1 km distance with a 1-ton scintillator detector, the rate is about 1 event per day. For solar neutrinos on Super-K, about 15 per day.

Fermi National Accelerator Laboratory, in Batavia, Illinois. Hosts the NuMI, BNB, and (future) LBNF neutrino beams. Currently the home of NOvA, MicroBooNE, ICARUS, SBND, and the upcoming DUNE near detector.

The effective target mass of a detector — the inner volume used for analysis, excluding outer regions where backgrounds are too high. For Super-Kamiokande the fiducial mass is 22.5 kt out of 50 kt total water; for JUNO it is 20 kt of the 20 kt total.

The baseline-to-energy ratio L/E at which the oscillation phase reaches $\pi/2$, giving maximum flavor conversion. For atmospheric splittings, the first maximum sits near L/E ~ 500 km/GeV. DUNE's beam is tuned to this maximum at 1300 km / 2.5 GeV.

A neutron-rich nucleus produced when a heavy nucleus (typically U-235 or Pu-239) splits in a reactor. Fission fragments beta-minus decay multiple times on their way to stability, producing the reactor antineutrino flux.

One of the three types of neutrino: electron neutrino, muon neutrino, and tau neutrino. Each flavor is paired with a charged lepton of the same name. Neutrinos can change flavor in flight — see oscillation.

A neutrino with a definite flavor (electron, muon, or tau), defined by which charged lepton it couples to in weak interactions. Flavor eigenstates are not the same as mass eigenstates — this is the basis of neutrino oscillation.

A chemical element with a very large neutron-capture cross section. Doping a water Cherenkov detector with gadolinium (as Super-K has done since 2020) lets the detector tag inverse-beta-decay antineutrino events via the delayed neutron-capture gamma signal.

An unexplained ~20% deficit in the calibration runs of the GALLEX and SAGE solar-neutrino experiments using chromium-51 sources. Recently strengthened to ~5σ by the BEST experiment. Often interpreted as a possible signature of sterile neutrinos.

• → GALLEX and SAGE

Antineutrinos emitted by the radioactive decay of uranium, thorium, and potassium inside the Earth. Detecting them lets scientists measure how much radioactive heat drives plate tectonics.

• → Listening to Earth's radioactive heart

A predicted peak in the electron-antineutrino-electron scattering cross-section at exactly 6.3 PeV, where the kinematics allow direct W-boson production. Predicted by Sheldon Glashow in 1960; first observed by IceCube in 2021.

• → The Glashow resonance

A sharp drop in the cosmic-ray spectrum above about 5 × 10¹⁹ eV, predicted by Greisen, Zatsepin and Kuzmin in 1966. Caused by cosmic-ray protons losing energy through interactions with cosmic microwave background photons over intergalactic distances.

A hypothetical heavy partner of the ordinary light neutrinos, predicted by see-saw models. Mass ranges from GeV to far above TeV depending on the specific model. The target of collider searches at the LHC and proposed experiments like SHiP.

• → Heavy neutral leptons

The projection of a particle's spin along its direction of motion. For massless particles, helicity equals chirality. For massive particles, a Lorentz boost can flip the apparent helicity.

The reaction ν̄_e + p → e⁺ + n, in which an electron antineutrino converts a proton into a neutron, producing a positron and a neutron. The workhorse detection channel for reactor antineutrinos and the basis of the 1956 Cowan-Reines discovery.

The Japan Proton Accelerator Research Complex in Tokai. Produces the muon-neutrino beam that travels 295 km to Super-Kamiokande (T2K) and will continue to Hyper-Kamiokande. One of the two leading accelerator neutrino facilities worldwide.

Liquid Argon Time-Projection Chamber. A detector technology using a volume of pure liquid argon at 87 K with a uniform electric field; charged particles ionize the argon and the electrons drift to a wire-plane readout for 3D event imaging. Used by MicroBooNE, ICARUS, SBND, and DUNE.

The Long-Baseline Neutrino Facility — the 1.2 MW (eventually 2.4 MW) proton beam at Fermilab that will produce the DUNE neutrino beam, sent 1300 km through the Earth to the Sanford Underground Research Facility in South Dakota.

A proposed early-universe process in which decays of heavy right-handed neutrinos generated a small excess of leptons over antileptons, which later turned into the matter-antimatter asymmetry of the cosmos.

• → Leptogenesis: why anything exists

A time-projection chamber filled with cold liquid argon. Charged particles produced by neutrino interactions ionize the argon, and the resulting electrons drift to readout planes — producing a fully three-dimensional image of each event. DUNE is the largest example.

A liquid that emits a brief flash of light when a charged particle passes through it. Used by detectors like KamLAND, Borexino, and JUNO to catch low-energy neutrinos with good energy resolution.

An experiment in which neutrinos travel hundreds of kilometers between source (accelerator) and detector. The baseline is chosen to maximize sensitivity to specific oscillation parameters. T2K, NOvA, DUNE, and Hyper-Kamiokande are the major current and future programs.

The symmetry of special relativity: physical laws are the same in all inertial reference frames. Neutrino experiments routinely test it through arrival-time and oscillation-pattern measurements. The 2011 OPERA superluminal claim was one such test that turned out to be a systematic error.

An excess of electron antineutrino events seen by the LSND experiment in 1996 in a muon-antineutrino beam at short baseline. The standard three-flavor framework cannot accommodate it; the most common explanation is an eV-scale sterile neutrino.

A mass term in the Standard Model Lagrangian that violates lepton number by two units. If neutrinos have Majorana mass, they are their own antiparticles. The seesaw mechanism naturally generates light Majorana masses for active neutrinos via heavy right-handed partners.

Two possibilities for the nature of the neutrino. A Dirac neutrino is distinct from its antiparticle; a Majorana neutrino is its own antiparticle. The search for neutrinoless double-beta decay is designed to tell them apart.

• → Majorana or Dirac?

A neutrino with a definite mass, denoted $\nu_1, \nu_2, \nu_3$. The mass eigenstates are the states that propagate through space with well-defined energies. Each flavor state is a quantum superposition of mass states, related by the PMNS matrix.

The yet-unknown question of which of the three neutrino mass states is the heaviest. Two scenarios remain: normal ordering (one state much heavier than the other two) or inverted ordering. Experiments like JUNO and DUNE aim to settle it.

• → Normal or inverted?

The difference of the squares of two neutrino masses, denoted $\Delta m^2_{ij} = m_i^2 - m_j^2$. The two independent splittings are $\Delta m^2_{21} \approx 7.4 \times 10^{-5}$ eV² (solar) and $|\Delta m^2_{31}| \approx 2.5 \times 10^{-3}$ eV² (atmospheric).

The observation that the Universe contains vastly more matter than antimatter — roughly a one-part-in-10⁹ excess in the early Universe. CP violation in neutrino oscillation, combined with leptogenesis, is one of the leading explanations.

The three angles that parametrize the PMNS matrix. θ₂₃ ≈ 49° (atmospheric), θ₁₂ ≈ 33° (solar), θ₁₃ ≈ 8.6° (the smallest, measured by Daya Bay and RENO). Their values are inputs to every oscillation prediction.

The unitary matrix that relates flavor states to mass states. For neutrinos this is the PMNS matrix; for quarks it is the CKM matrix. Each contains mixing angles and CP-violating phases that must be measured experimentally.

Modification of neutrino oscillation by matter, named after Mikheyev, Smirnov, and Wolfenstein. Electron neutrinos couple slightly differently to matter than other flavors, producing a resonance that enhances or suppresses oscillation. Critical to interpreting solar neutrinos.

• → The MSW effect

Coordinated observation of astrophysical events using multiple kinds of messenger — photons, neutrinos, gravitational waves, cosmic rays. Established as routine by SN 1987A, GW170817, and the 2017 TXS 0506+056 detection.

• → Multi-messenger astronomy

The effective number of relativistic neutrino species in the early universe. The Standard Model predicts N_eff = 3.044. Cosmological measurements from Big Bang nucleosynthesis and the CMB constrain it to be consistent with this value.

A neutrino interaction mediated by a neutral Z boson. The neutrino remains a neutrino but transfers energy and momentum to the target. NC interactions are flavor-blind, which is what made SNO's measurement of total solar-neutrino flux possible.

An elementary particle with no electric charge and almost no mass. Neutrinos interact only via the weak force and gravity, making them extremely hard to detect — about 100 trillion pass through your body every second.

• → What is a neutrino, anyway?

An irreducible background to dark-matter direct-detection experiments. Solar and atmospheric neutrinos coherently scatter off detector nuclei (CEvNS), producing nuclear recoils identical to dark-matter signals. Limits how far dark-matter searches can go.

• → The neutrino fog

The phenomenon in which a neutrino of one flavor changes into another flavor as it travels. Discovered by Super-Kamiokande and SNO around 1998-2002, and the reason we know neutrinos have mass.

• → How neutrino oscillation works

A hypothetical decay in which two neutrons in a nucleus turn into protons and emit two electrons, with no neutrinos. Observing it would prove that neutrinos are their own antiparticles (Majorana) — and likely bring another Nobel Prize.

• → The hunt for 0νββ

A term coined by the Neutrino Energy Group for a technology that converts the kinetic energy of neutrinos and other non-visible radiation into electrical power. The company is developing the approach through a multilayer graphene-and-silicon material platform.

• → Neutrino Energy Group prototype phase

A long-baseline experiment design where the far detector is positioned slightly off the beam axis. The geometry narrows the neutrino energy spectrum, improving sensitivity to oscillation effects. Pioneered by T2K, now standard for NOvA, DUNE, and Hyper-K.

A pressure-resistant glass sphere housing a photomultiplier tube and associated electronics. The basic unit of IceCube and KM3NeT. IceCube has 5160 optical modules; the IceCube-Gen2 design will have ~10,000.

The distance over which a neutrino's flavor composition completes one full oscillation cycle. Formula: $L_{osc} = 4\pi E / \Delta m^2 c^4$. For solar splittings at ~1 MeV: a few hundred km. For atmospheric splittings: a few thousand km.

The discovery — by Wu in 1957, following Lee and Yang's 1956 proposal — that the weak interaction does not respect mirror symmetry. Implies that neutrinos are produced in specific helicity states. One of the foundational asymmetries of the Standard Model.

• → The Wu experiment of 1957

An electronic device that converts a single photon into a measurable electrical pulse through photoemission and dynode-chain amplification. The workhorse light detector in essentially every major water-Cherenkov and scintillator neutrino experiment.

• → Photomultiplier tubes

The 3×3 unitary matrix relating neutrino flavor states to mass eigenstates. Named after Pontecorvo, Maki, Nakagawa, and Sakata. Parametrized by three mixing angles and at least one CP-violating phase. Central to all oscillation predictions.

• → The PMNS matrix

The dominant nuclear fusion sequence in the Sun's core. Four protons fuse in stages to form helium-4, releasing two positrons, two neutrinos, and 26.7 MeV of energy. The pp neutrinos are the lowest-energy and most abundant component of the solar neutrino spectrum.

The probability that an incoming photon is converted to a detectable photoelectron at the photocathode of a photomultiplier tube. Modern PMTs achieve 20–40 %; the Hyper-K Box-and-Line tubes reach above 30 % at peak wavelength.

An electron antineutrino produced in a nuclear reactor by beta-minus decay of neutron-rich fission fragments. A typical 1 GW thermal reactor emits about $6 \times 10^{20}$ antineutrinos per second, all of electron flavor.

A ~6% deficit between measured reactor antineutrino rates and theoretical predictions, first identified in 2011. Originally interpreted as evidence for sterile neutrinos; most current evidence points instead to errors in the predicted flux from forbidden beta-decay transitions.

• → The reactor antineutrino anomaly

A theoretical idea that explains why neutrinos are so much lighter than other particles. If a very heavy, undiscovered neutrino partner exists, ordinary neutrinos are pushed to tiny mass — see-saw balanced.

• → The see-saw mechanism

A neutrino experiment in which the detector sits within a few tens of meters of the source. Used for very-short-distance oscillation searches (PROSPECT, STEREO) or CEvNS detection (COHERENT, NUCLEUS).

The SuperNova Early Warning System. A coincidence network of neutrino detectors (Super-K, IceCube, KamLAND, JUNO, Hyper-K, others) that issues an automated alert to optical observatories within minutes if a galactic supernova produces a coincident neutrino burst.

Electron neutrinos produced in fusion reactions in the Sun's core. The first to be detected (Davis, 1968), and central to the historic 'solar neutrino problem' resolved by the SNO experiment in 2001.

• → The solar neutrino problem

The breakup of a nucleus by a high-energy cosmic-ray muon, producing a shower of secondary particles that can mimic neutrino interactions. A major background in underground detectors; veto systems and timing cuts suppress it.

The energy distribution of neutrinos from a given source. Each neutrino-producing process has a characteristic spectrum: reactor antineutrinos peak at ~3 MeV; solar pp neutrinos extend to ~420 keV; atmospheric neutrinos span GeV to TeV; cosmic neutrinos extend to PeV.

A non-perturbative configuration of the electroweak gauge field that can change baryon and lepton numbers by three units each at high temperatures. Essential for leptogenesis: it converts a primordial lepton asymmetry into the observed baryon asymmetry of the Universe.

The current framework of particle physics, describing quarks, leptons (including neutrinos), and the forces between them. Famously incomplete — most prominently because it does not naturally explain neutrino mass.

A hypothetical fourth neutrino flavor that would not feel the weak force, interacting only through mixing with the three known flavors. Several experimental anomalies hint at its existence; many others rule it out.

• → Sterile neutrinos: a stubborn maybe

A hypothetical neutrino that does not interact via the weak force — only via mixing with the active flavors. The Standard Model has only three active species, but additional sterile flavors are allowed and several experimental anomalies hint at one at the eV mass scale.

A massive burst of neutrinos released when a massive star's core collapses. They carry roughly 99% of the explosion's energy and arrive at Earth hours before the light. SN 1987A produced the first and so far only such burst that was directly detected.

• → SN 1987A

The probability that a neutrino produced as one flavor will be detected as the same flavor. Formula in two-flavor approximation: $P_{\alpha\alpha} = 1 - \sin^2(2\theta) \sin^2(\Delta m^2 L / 4E)$.

Hydrogen-3, an unstable isotope that beta-decays to helium-3 with a Q-value of 18.6 keV. The shape of the resulting electron energy spectrum near its endpoint depends on the neutrino mass — the basis of KATRIN and Project 8.

A laboratory placed deep below the Earth's surface to shield experiments from cosmic-ray backgrounds. Major examples include SNOLAB (Canada, 2 km deep), SURF (US, 1.5 km), Gran Sasso (Italy, 1.4 km), Kamioka (Japan, 1 km), and Jinping (China, 2.4 km).

• → Deep underground laboratories

Weakly Interacting Massive Particles. A class of hypothetical dark-matter candidates with masses of 1 GeV to 10 TeV and weak-scale interactions. Searched for by XENONnT, LZ, and other direct-detection experiments. Distinct from neutrinos but similar in some experimental signatures.

The dimensionless strength with which a fermion couples to the Higgs field. After electroweak symmetry breaking, this coupling becomes the fermion's mass. For Dirac neutrinos, the Yukawa coupling would have to be ~10⁻¹², a million times smaller than for the electron.

The sum of the three neutrino mass eigenvalues. Constrained by cosmological measurements of structure formation (currently below ~0.12 eV) and lower-bounded by oscillation data (above 0.058 eV for normal ordering, 0.098 eV for inverted).