Neutrinos are produced whenever particles undergo weak-interaction processes, especially beta decay, electron or positron capture, pion and muon decay, and nuclear fusion. Their sources can be organized into the following major categories.

1. Cosmological and Relic Neutrino Sources

1. Cosmic Neutrino Background

   • Relic neutrinos produced during the first second after the Big Bang

   • Sometimes called the cosmic relic neutrino background

   • Expected temperature today: approximately 1.95 K

   • Not yet directly detected

2. Primordial Nucleosynthesis Neutrinos

   • Produced during weak-interaction reactions in Big Bang nucleosynthesis

3. Diffuse Supernova Neutrino Background

   • The accumulated neutrino emission from all past core-collapse supernovae

   • Also called the supernova relic neutrino background

4. Early-Universe Particle Decays

   • Possible neutrinos from the decay of unstable primordial particles

5. Cosmic-String or Topological-Defect Neutrinos

   • Hypothetical ultra-high-energy neutrinos from cosmic strings, monopoles, or other defects

6. Neutrinos from Primordial Black Holes

   • Possible Hawking-radiation neutrinos from evaporating primordial black holes

2. Stellar Neutrino Sources

2.1 Solar Neutrinos

Produced by nuclear fusion reactions in the Sun:

1. Proton–proton neutrinos

2. Pep neutrinos

3. Helium-3–proton, or hep, neutrinos

4. Beryllium-7 neutrinos

5. Boron-8 neutrinos

6. Nitrogen-13 neutrinos

7. Oxygen-15 neutrinos

8. Fluorine-17 neutrinos

The first five primarily originate from the proton–proton chain, while nitrogen-13, oxygen-15, and fluorine-17 neutrinos arise from the CNO cycle.

2.2 Neutrinos from Other Main-Sequence Stars

• Hydrogen-fusion neutrinos

• CNO-cycle neutrinos

• Neutrinos from massive stellar cores

• Thermal neutrinos from hot stellar interiors

2.3 Red-Giant and Supergiant Stars

• Plasma-process neutrinos

• Pair-annihilation neutrinos

• Photoneutrinos

• Bremsstrahlung neutrinos

• Nuclear weak-decay neutrinos

2.4 White Dwarfs

• Plasmon-decay neutrinos

• Pair-annihilation neutrinos

• Bremsstrahlung neutrinos

• Neutrinos from accreting white dwarfs

• Neutrinos from white-dwarf mergers

2.5 Neutron Stars

• Beta-equilibrium neutrinos

• Direct and modified Urca-process neutrinos

• Neutrinos from neutron-star cooling

• Neutrinos from neutron-star crust reactions

• Neutrinos from superfluid-pair formation and breaking

3. Explosive Stellar Sources

3.1 Core-Collapse Supernovae

Supernovae are intense sources of all neutrino flavors:

• Electron neutrinos from neutronization

• Electron antineutrinos from positron capture

• Muon neutrinos and antineutrinos

• Tau neutrinos and antineutrinos

• Thermal neutrinos from the proto-neutron star

• Neutrinos from shock propagation and stellar collapse

3.2 Failed Supernovae

• Neutrinos from direct stellar collapse into a black hole

• Short-duration, high-luminosity neutrino bursts

3.3 Hypernovae

• Neutrinos from exceptionally energetic core-collapse explosions

• Possible high-energy neutrinos from associated relativistic jets

3.4 Type Ia Supernovae

• Weak-interaction neutrinos from thermonuclear burning

• Electron-capture neutrinos

• Lower neutrino output than core-collapse supernovae

3.5 Classical Novae

• Neutrinos from radioactive nuclei and nuclear reactions

• Possible high-energy neutrinos from shock-accelerated particles

3.6 Superluminous Supernovae

• Possible neutrinos from powerful shocks, dense circumstellar environments, or central engines

3.7 Pair-Instability Supernovae

• Thermal and nuclear-process neutrinos from massive stellar explosions

4. Compact-Object Merger Sources

1. Binary neutron-star mergers

2. Neutron-star–black-hole mergers

3. Binary black-hole mergers with surrounding matter

4. White-dwarf mergers

5. Neutron-star–white-dwarf mergers

6. Post-merger hypermassive neutron stars

7. Black-hole accretion disks formed after mergers

These events may produce thermal MeV neutrinos and, where relativistic jets form, high-energy neutrinos.

5. Galactic High-Energy Neutrino Sources

1. Supernova remnants

2. Pulsars

3. Pulsar wind nebulae

4. Magnetars

5. Soft gamma repeaters

6. Binary-star systems

7. X-ray binaries

8. High-mass X-ray binaries

9. Low-mass X-ray binaries

10. Microquasars

11. Accreting neutron stars

12. Accreting stellar-mass black holes

13. Colliding-wind binary systems

14. Young stellar objects

15. Stellar clusters

16. Massive star-forming regions

17. Molecular clouds illuminated by cosmic rays

18. Galactic Centre

19. Fermi bubbles

20. Galactic cosmic-ray interactions

21. Nova remnants

22. Galactic PeVatrons

High-energy neutrinos are generally produced when accelerated hadrons interact through:

• Proton–proton interactions

• Proton–photon interactions

• Nucleus–nucleus interactions

• Photodisintegration and subsequent particle decay

6. Extragalactic High-Energy Neutrino Sources

6.1 Active Galactic Nuclei

1. Blazars

2. Flat-spectrum radio quasars

3. BL Lacertae objects

4. Radio galaxies

5. Seyfert galaxies

6. Quasars

7. Low-luminosity active galactic nuclei

8. AGN jets

9. AGN cores

10. AGN coronae

11. AGN accretion disks

6.2 Gamma-Ray Bursts

• Long-duration gamma-ray bursts

• Short-duration gamma-ray bursts

• Low-luminosity gamma-ray bursts

• Choked-jet gamma-ray bursts

• Shock-breakout gamma-ray bursts

• Failed gamma-ray bursts

6.3 Tidal Disruption Events

• Stars disrupted by supermassive black holes

• Jetted tidal disruption events

• Non-jetted tidal disruption events

6.4 Star-Forming Systems

• Starburst galaxies

• Ultra-luminous infrared galaxies

• Luminous infrared galaxies

• Galaxy clusters

• Galaxy groups

• Intergalactic shocks

6.5 Other Extragalactic Transients

• Fast blue optical transients

• Luminous fast blue optical transients

• Fast radio burst environments

• Extragalactic magnetar flares

• Changing-look active galactic nuclei

• Black-hole accretion transients

7. Atmospheric Neutrino Sources

Atmospheric neutrinos are produced when cosmic rays strike nuclei in Earth’s atmosphere.

Parent particles include:

1. Charged pions

2. Charged kaons

3. Muons

4. Charmed mesons

5. Other short-lived hadrons

Main atmospheric components

• Conventional atmospheric neutrinos

• Prompt atmospheric neutrinos

• Electron neutrinos and antineutrinos

• Muon neutrinos and antineutrinos

• Tau neutrinos produced through oscillations

• Neutrinos from cosmic-ray air showers

Atmospheric neutrinos cover energies from hundreds of MeV to beyond the PeV scale.

8. Solar-System Neutrino Sources

1. The Sun

2. Cosmic-ray interactions in the solar atmosphere

3. Solar flares

4. Coronal mass-ejection environments

5. Cosmic-ray interactions with the Moon

6. Cosmic-ray interactions with planetary atmospheres

7. Cosmic-ray interactions with asteroids or other solid bodies

8. Possible neutrinos from dark-matter annihilation inside the Sun

9. Possible neutrinos from dark-matter annihilation inside planets

Solar-atmospheric neutrinos differ from ordinary solar-fusion neutrinos because they originate from cosmic-ray collisions near the solar surface.

9. Terrestrial Natural Neutrino Sources

9.1 Geoneutrinos

Electron antineutrinos produced by radioactive decay inside Earth, principally from:

1. Uranium-238 decay chain

2. Thorium-232 decay chain

3. Potassium-40 decay

4. Uranium-235 decay chain

5. Rubidium-87 decay

Major terrestrial regions include:

• Continental crust

• Oceanic crust

• Upper mantle

• Lower mantle

• Possible deep-Earth reservoirs

9.2 Natural Radioactive Materials

• Rocks

• Soil

• Minerals

• Ocean water

• Groundwater

• Atmospheric radionuclides

• Radioactive isotopes in living organisms

9.3 Neutrinos from Earth’s Atmosphere

• Cosmic-ray-induced atmospheric neutrinos

• Neutrinos from radioactive atmospheric isotopes

9.4 Lightning and Thunderstorms

• Possible low-intensity neutrinos associated with photonuclear reactions

• Beta-decay neutrinos from unstable isotopes generated by lightning

10. Biological Neutrino Sources

Living organisms contain naturally radioactive isotopes.

Sources include:

1. Potassium-40 beta decay

2. Carbon-14 beta decay

3. Other trace radioactive isotopes

The human body therefore emits a small number of electron antineutrinos, mainly from potassium-40 beta decay.

11. Nuclear Reactor Neutrino Sources

Nuclear reactors are powerful sources of electron antineutrinos.

Reactor types

1. Commercial power reactors

2. Research reactors

3. Naval propulsion reactors

4. Small modular reactors

5. Fast breeder reactors

6. Molten-salt reactors

7. Experimental reactors

8. Isotope-production reactors

9. Natural nuclear reactors, such as the ancient Oklo reactors

Main production process

Neutron-rich fission fragments undergo beta-minus decay:

[
(A,Z)\rightarrow(A,Z+1)+e^-+\bar{\nu}_e
]

Major fission parents include:

• Uranium-235

• Uranium-238

• Plutonium-239

• Plutonium-241

12. Particle-Accelerator Neutrino Sources

12.1 Conventional Accelerator Neutrino Beams

Produced when proton beams strike a target and generate pions and kaons.

Sources include:

1. Pion-decay beams

2. Kaon-decay beams

3. Muon-decay beams

4. On-axis neutrino beams

5. Off-axis neutrino beams

6. Wide-band beams

7. Narrow-band beams

12.2 Specialized Accelerator Sources

1. Neutrino factories

2. Muon-storage rings

3. Beta beams

4. Decay-at-rest sources

5. Decay-in-flight sources

6. Stopped-pion sources

7. Spallation neutron sources

8. Beam-dump facilities

9. Collider-generated neutrinos

10. Forward neutrinos at particle colliders

12.3 Collider Sources

• Proton–proton collisions

• Proton–antiproton collisions

• Heavy-ion collisions

• Electron–positron collisions

• Decays of heavy-flavor hadrons

• Decays of W and Z bosons

• Tau-lepton decays

• Top-quark decays

13. Laboratory Radioactive Sources

1. Beta-minus decay sources

2. Beta-plus decay sources

3. Electron-capture sources

4. Neutron-activated isotopes

5. Radioisotope neutrino generators

6. Artificial calibration sources

7. Intense electron-capture sources

8. Intense beta-decay antineutrino sources

Examples of useful source isotopes include:

• Chromium-51

• Argon-37

• Cerium-144

• Praseodymium-144

• Strontium-90

• Yttrium-90

• Lithium-8

• Helium-6

14. Nuclear Explosion Sources

1. Atmospheric nuclear explosions

2. Underground nuclear explosions

3. Underwater nuclear explosions

4. Nuclear weapon tests

5. Fission explosions

6. Thermonuclear fusion explosions

They generate intense, short-duration electron-antineutrino emissions, mainly from radioactive fission products.

15. Spontaneous and Induced Nuclear-Process Sources

1. Nuclear beta decay

2. Electron capture

3. Positron emission

4. Nuclear fission

5. Neutron activation

6. Muon capture

7. Pion capture

8. Fusion reactions

9. Spallation reactions

10. Photodisintegration products

11. Heavy-ion collisions

12. Radioactive decay chains

16. Cosmic-Ray Interaction Sources

Cosmic rays can produce neutrinos through interactions with:

1. Earth’s atmosphere

2. The solar atmosphere

3. Interstellar gas

4. Molecular clouds

5. Stellar winds

6. Supernova ejecta

7. Accretion disks

8. Radiation fields

9. Cosmic microwave background photons

10. Extragalactic background light

11. Galaxy-cluster gas

12. Planetary surfaces

13. The Moon

14. Asteroids

15. Intergalactic matter

17. Cosmogenic Neutrino Sources

Cosmogenic neutrinos are produced when ultra-high-energy cosmic rays interact with background radiation.

Principal processes

1. Interactions with the cosmic microwave background

2. Interactions with extragalactic background light

3. Photopion production

4. Neutron beta decay

5. Nuclear photodisintegration

They are also called:

• GZK neutrinos

• Ultra-high-energy cosmogenic neutrinos

Their expected energies may extend from PeV to EeV and beyond.

18. Dark-Matter-Related Neutrino Sources

These remain hypothetical or unconfirmed.

1. Dark-matter annihilation in the Sun

2. Dark-matter annihilation in Earth

3. Dark-matter annihilation in the Galactic Centre

4. Dark-matter annihilation in dwarf spheroidal galaxies

5. Dark-matter annihilation in galaxy clusters

6. Dark-matter particle decay

7. Sterile-neutrino dark-matter decay

8. Dark-matter interactions with stars

9. Dark-matter capture in neutron stars

10. Dark-matter decay throughout the Universe

19. Exotic and Beyond-Standard-Model Sources

1. Sterile-neutrino decay

2. Heavy neutral lepton decay

3. Axion-like particle conversion or decay chains

4. Supersymmetric particle decay

5. Leptoquark decay

6. Grand-unified-theory particle decay

7. Topological defects

8. Cosmic strings

9. Magnetic monopole annihilation

10. Superheavy dark-matter decay

11. Evaporating black holes

12. Extra-dimensional particle processes

13. Lorentz-violation-induced particle decay

14. Secret neutrino interactions

15. Neutrinos from exotic compact objects

Classification by Production Mechanism

Nuclear processes

• Beta-minus decay → electron antineutrinos

• Beta-plus decay → electron neutrinos

• Electron capture → electron neutrinos

• Nuclear fusion → mainly electron neutrinos

• Nuclear fission → mainly electron antineutrinos

Meson and lepton decays

• Charged-pion decay

• Charged-kaon decay

• Muon decay

• Tau-lepton decay

• Charm-hadron decay

• Bottom-hadron decay

High-energy hadronic interactions

• Proton–proton interactions

• Proton–photon interactions

• Nucleus–nucleus interactions

• Nucleus–photon interactions

Thermal plasma processes

• Electron–positron pair annihilation

• Plasmon decay

• Photoneutrino emission

• Bremsstrahlung

• Recombination neutrino emission

• Urca processes

Simplified Master Classification

The principal sources of neutrinos can therefore be summarized as:

1. Big Bang and cosmological relics

2. The Sun and other stars

3. Supernovae and stellar explosions

4. Neutron stars and compact-object mergers

5. Galactic cosmic accelerators

6. Extragalactic cosmic accelerators

7. Cosmic-ray interactions

8. Earth’s atmosphere

9. Radioactive decay inside Earth

10. Nuclear reactors

11. Particle accelerators

12. Laboratory radioactive sources

13. Nuclear explosions

14. Living organisms

15. Cosmogenic interactions

16. Dark matter and other hypothetical exotic phenomena