neutrino‑physics

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Glossary

Alphabetical definitions of technical terms used across neutrino physics, each with links to deeper concept pages.

31 definitions · 12 letters

C 5 terms

CEvNS · Coherent Elastic Neutrino-Nucleus Scattering
A weak-neutral-current process in which a neutrino of any flavor scatters elastically off an entire atomic nucleus, predicted by Freedman in 1974 and first observed by the COHERENT collaboration in 2017. At momentum transfer small compared to the inverse nuclear size, all nucleons scatter in phase and the cross-section scales as N² in the neutron number — making CEvNS the largest neutrino cross-section at sub-50-MeV energies.
Cherenkov radiation · Čerenkov radiation
Electromagnetic radiation emitted by a charged particle traversing a transparent dielectric medium at a velocity greater than the phase velocity of light in that medium. The light is emitted in a cone whose half-angle satisfies cos θ = 1/(βn), fixed for a given velocity and refractive index. Cherenkov imaging is the basis of water and ice neutrino detectors from Kamiokande to IceCube.
Chirality
An intrinsic Lorentz-invariant property of fermions, defined by the eigenvalue of the γ⁵ operator in the Dirac equation. Left- and right-chiral components of a Dirac field are independent. The Standard Model weak interaction couples only to left-chiral fermions — the V − A structure. For massive fermions, a mass term mixes left and right chiralities; propagation thereby continually oscillates between the two.
Cosmic neutrino background · CνB, CNB, relic neutrinos
The thermal bath of relic neutrinos released when weak interactions decoupled from the cosmological plasma, about one second after the Big Bang. The CνB has temperature T_ν = (4/11)^(1/3) T_γ ≈ 1.95 K today and a number density of about 336 neutrinos per cubic centimetre (summed over three flavors). Direct laboratory detection has not been achieved; indirect evidence is provided by its imprint on CMB and large-scale structure.
CP violation · charge-parity violation, leptonic CP violation
The phenomenon by which a physical process and its charge-parity-conjugate process proceed at different rates. In the neutrino sector, CP violation manifests as a difference between P(νμ → νe) and P(ν̄μ → ν̄e) in long-baseline experiments. Observable CP violation requires all three PMNS mixing angles non-zero, non-degenerate neutrino masses, and a non-trivial Dirac phase δ_CP. T2K, NOvA, DUNE, and Hyper-K are the principal experimental probes.

D 2 terms

Delayed coincidence
A detection technique in which two physically correlated signals — typically a prompt positron annihilation followed by a delayed neutron capture — are required within a specific time and spatial window. The delayed-coincidence signature in liquid-scintillator detectors provides a background rejection factor of 10⁵ or better and is the foundation of reactor antineutrino detection since the Cowan-Reines experiment.
Dirac particle · Dirac fermion
A fermion described by a four-component spinor satisfying the Dirac equation, distinct from its antiparticle. All charged leptons (e, μ, τ) and quarks are Dirac fermions. Whether neutrinos are Dirac or Majorana is determined by the nature of their mass term and, experimentally, by searches for lepton-number-violating processes such as neutrinoless double beta decay.

F 3 terms

Fermi coupling constant · G_F
The dimensional coupling that sets the strength of weak interactions at low energies, G_F ≈ 1.166 × 10⁻⁵ GeV⁻². Extracted with high precision from muon lifetime measurements and equivalent to G_F/√2 = g²/(8 M_W²) in the electroweak theory, where g is the SU(2)_L gauge coupling and M_W is the W-boson mass. Originally introduced in Fermi's 1934 four-fermion theory of beta decay.
Flavor eigenstate · weak eigenstate, interaction eigenstate
A neutrino state defined by the charged lepton it partners with in a weak-interaction vertex. The three flavor eigenstates are νe, νμ, and ντ. Flavor eigenstates are not the same as mass eigenstates — they are linear combinations of them, related by the unitary PMNS mixing matrix. Neutrinos are produced and detected as flavor eigenstates but propagate as mass eigenstates, giving rise to oscillation.
Free-streaming length
The characteristic distance a relativistic neutrino travels between significant gravitational interactions in the early universe. For neutrinos with mass m_ν, the free-streaming length today is of order 100 Mpc (m_ν/0.1 eV)⁻¹. Below this scale, neutrinos cannot cluster gravitationally and therefore suppress the growth of structure. This suppression is the principal cosmological signature of neutrino mass.

H 1 term

Helicity
The projection of a particle's spin onto its momentum direction, h = ŝ · p̂. A spin-½ particle has h = +½ (right-handed) or h = −½ (left-handed). Helicity is frame-dependent for massive particles: a boost overtaking the particle flips the sign. For massless particles, helicity coincides with chirality and is Lorentz-invariant.

I 1 term

Inverse beta decay · IBD
The charged-current reaction ν̄e + p → e⁺ + n, the principal detection channel for reactor and supernova antineutrinos. The prompt positron (with energy E_prompt ≈ E_ν - 0.78 MeV) and the delayed neutron capture provide a clean delayed-coincidence signature. Used from Reines-Cowan 1956 through KamLAND, Daya Bay, and JUNO.

J 1 term

Jarlskog invariant
A convention-independent measure of CP violation in a three-flavor mixing matrix, defined for the PMNS matrix as J_CP = Im(U_{e2} U*_{μ2} U*_{e3} U_{μ3}). The quantity vanishes if any mixing angle is zero or if δ_CP = 0 or π, making it the natural parameter against which CP-violation measurements are normalized. Current best estimates give |J_CP| up to about 0.033 depending on δ_CP.

L 1 term

Leptogenesis
A class of scenarios in which the observed cosmic matter-antimatter asymmetry is generated first as a lepton asymmetry through CP-violating decays of heavy right-handed neutrinos, and then partially converted to a baryon asymmetry through electroweak sphaleron processes. Leptogenesis works most naturally if neutrinos are Majorana, providing additional phenomenological motivation for neutrinoless-double-beta-decay searches.

M 5 terms

MAC-E filter · Magnetic Adiabatic Collimation with Electrostatic filtering
A high-resolution electron-energy spectrometer that uses a gradient of magnetic field to collimate electron momenta onto the axis, then an electrostatic retarding potential to filter by total kinetic energy. The MAC-E technique combines the high transmission of a solenoid with the sharp energy cut of an electrostatic filter. KATRIN uses a 10 m × 23 m MAC-E spectrometer to achieve 1 eV resolution at the 18.6 keV tritium endpoint.
Majorana particle · Majorana fermion
A fermion that is identical to its own antiparticle, first proposed by Ettore Majorana in 1937 as an alternative to Dirac fermions. If neutrinos are Majorana, they violate lepton number and permit neutrinoless double beta decay. Whether the neutrino is Dirac or Majorana is one of the central open questions in particle physics; current experiments (KamLAND-Zen, LEGEND, nEXO, CUPID) are searching for the signature 0νββ decay.
Mass eigenstate · propagation eigenstate
A neutrino state with a definite mass mᵢ, which propagates in vacuum with a definite energy-momentum relation Eᵢ² = p² + mᵢ². The three mass eigenstates ν₁, ν₂, ν₃ are distinct from the flavor eigenstates; their mixing angles and squared-mass differences are extracted from oscillation measurements. The mass eigenstates are the natural basis for propagation, while flavor eigenstates are the natural basis for production and detection.
Mass ordering · mass hierarchy
The sign of Δm²₃₁, which is undetermined by vacuum oscillation alone. Normal ordering (m₁ < m₂ < m₃) and inverted ordering (m₃ < m₁ < m₂) are both consistent with the known squared-mass differences. The ordering is extracted from matter effects in long-baseline and atmospheric experiments (DUNE, IceCube, ORCA) and, independently, from reactor spectral analysis (JUNO).
MSW effect · Mikheyev-Smirnov-Wolfenstein effect, matter-enhanced oscillation
Modification of neutrino flavor oscillation by coherent forward-scattering on electrons in dense matter. νe feels an additional charged-current potential that shifts the effective mixing angles and eigenvalues, giving rise to a resonance at a characteristic energy-density combination. The effect is central to the explanation of solar neutrino oscillation (matter in the Sun) and to the determination of mass ordering in long-baseline experiments (matter in the Earth's mantle).

N 3 terms

N_eff · effective number of neutrino species
The parameter describing the radiation energy density in the early universe in units of the photon density, conventionally normalized so that a thermally decoupled neutrino contributes unity. Standard Model prediction with three neutrino flavors is N_eff = 3.044, slightly above 3 because of partial re-heating during e⁺e⁻ annihilation and finite-temperature QED corrections. Planck CMB measurements give N_eff = 2.99 ± 0.17, in agreement with the SM expectation and constraining additional light relics.
Neutrinoless double beta decay · 0νββ, 0ν2β
A hypothetical nuclear transition (A, Z) → (A, Z+2) + 2 e⁻ in which two protons decay with no emitted neutrinos. The process violates lepton number by two units and is possible only if neutrinos are Majorana particles. The decay rate is proportional to the square of the effective Majorana mass ⟨m_ββ⟩, constraining both the absolute mass scale and the Dirac/Majorana nature. Current limits: T_{1/2} > 2 × 10²⁶ years in ¹³⁶Xe (KamLAND-Zen). Next-generation experiments target the inverted-ordering floor at ~15 meV.
Neutrinovoltaic · neutrino energy conversion
An applied-research direction concerned with converting components of the invisible radiation spectrum — including neutrino momentum transfer through coherent neutral-current scattering, cosmic muon flux, electromagnetic field fluctuations, and thermal gradients — into usable electrical output. The governing expression is the Master Equation P(t) = η · ∫V Φ_eff(r,t) · σ_eff(E) dV. Materials research focuses on multilayer graphene-silicon nanostructures.

P 1 term

PMNS matrix · Pontecorvo-Maki-Nakagawa-Sakata matrix, neutrino mixing matrix
The unitary matrix that relates the three neutrino flavor eigenstates (ν_e, ν_μ, ν_τ) to the three mass eigenstates (ν_1, ν_2, ν_3). It is parameterized by three mixing angles (θ_12, θ_13, θ_23), one CP-violating phase δ_CP, and — if neutrinos are Majorana particles — two additional Majorana phases.

S 6 terms

Seesaw mechanism · Type-I seesaw
A theoretical framework that generates small Majorana neutrino masses through the interplay of an electroweak-scale Dirac mass and a much larger right-handed Majorana mass. If the heavy Majorana mass M is much greater than the Dirac mass m_D, the light-neutrino mass scales as m_ν ∼ m_D²/M. For m_D ∼ 100 GeV and M ∼ 10¹⁴ GeV, m_ν naturally falls in the sub-eV range — providing a 'natural' explanation for the smallness of neutrino masses. Seesaw also provides the basis for leptogenesis.
SNEWS · SuperNova Early Warning System
An international network of neutrino detectors (including Super-Kamiokande, IceCube, LVD, KamLAND, Borexino, Daya Bay, NOvA, HALO, and more) that coordinate real-time alerts for galactic supernova neutrino bursts. The neutrino signal precedes the optical flash by hours, enabling early follow-up observation by optical, infrared, X-ray, and gamma-ray telescopes. SNEWS 2.0 went online in 2020 with improved coincidence logic and latency.
Solar Neutrino Unit · SNU
The conventional unit for radiochemical solar-neutrino rates, defined as 10⁻³⁶ neutrino captures per target atom per second. Introduced by John Bahcall to compare predictions and measurements in chlorine, gallium, and other radiochemical experiments. Homestake measured about 2.5 SNU against a Standard Solar Model prediction near 8 SNU — the ratio that defined the solar neutrino problem.
Sphaleron
A non-perturbative electroweak configuration that violates baryon-plus-lepton number (B + L) while conserving B − L. At temperatures above the electroweak phase transition, sphaleron processes are in thermal equilibrium and rapidly interconvert B and L asymmetries. In leptogenesis scenarios, sphalerons convert a primary lepton asymmetry into the observed baryon asymmetry.
Standard Solar Model · SSM
A set of one-dimensional hydrostatic models of the Sun's interior, constrained by observed surface luminosity, radius, age, and heavy-element abundance, and predicting the core temperature, density profile, and individual solar-neutrino flux components. Developed by John Bahcall and collaborators beginning in the 1960s, the SSM predictions were confirmed to high precision by SNO (2001) and Borexino (through 2020), with CNO-cycle neutrinos the last branch to be directly observed.
Sterile neutrino · right-handed neutrino
A hypothetical neutrino that does not participate in the Standard Model weak interactions and is therefore invisible to detectors except through mixing with active neutrinos. Eligible mass ranges include eV-scale (motivated by short-baseline oscillation anomalies) and keV-scale (a warm-dark-matter candidate). LEP bounds on the Z-boson invisible width constrain the number of active neutrinos to exactly three; additional species must be sterile.

W 1 term

Weinberg operator
The unique dimension-five operator consistent with Standard Model gauge symmetries that generates a Majorana mass for neutrinos after electroweak symmetry breaking. In schematic form, (L H)(L H)/Λ, with Λ the scale of new physics. For light-neutrino masses of order 0.1 eV, the implied scale is Λ ∼ 10¹⁴ GeV, close to the grand-unification scale. The Weinberg operator is typically realized by the seesaw mechanism.

# 1 term

Δm² (squared-mass difference) · mass-squared splitting
The difference between the squares of two neutrino mass eigenstates, Δm²ᵢⱼ = mᵢ² − mⱼ². Vacuum oscillation probabilities depend on these differences, not on the individual masses. Two independent values are known: Δm²₂₁ ≈ 7.4 × 10⁻⁵ eV² (solar sector) and |Δm²₃₁| ≈ 2.5 × 10⁻³ eV² (atmospheric sector). The sign of Δm²₃₁ — the mass ordering — remains to be determined.