A history of the evidence
Neutrinos: a worldwide search
From reactor coincidences to the Sun, supernovae and Antarctic ice: follow the instruments, collaborations and arguments that made neutrinos observable.
Follow the argument
Choose a question to follow
Start with the award or with a scientific question. Each path runs from an early puzzle to the experiments that tested it.
Start with the prize
2026 Nobel Prize in Physics: why look for neutrinos in Antarctic ice?
How can a block of ice work as a telescope?
02What can a neutrino tell us that light cannot?
If we already have telescopes, why build a neutrino observatory?
03A huge detector, a few events: reading neutrino scales
How can one neutrino be energetic while its detector still needs years of data?
04How does a faint signal become a neutrino discovery?
What must be checked between a sensor flash and a claim about the universe?
05Read IceCube’s discovery paper, one figure at a time
What does each figure actually establish?
06Five ways a neutrino headline can lead us astray
What should “seeing a neutrino” mean in a report about this year’s prize?
07Francis Halzen and the team that turned Antarctic ice into a telescope
How do you turn an idea for a telescope into an instrument people can trust?
08Why do neutrinos keep appearing in the Nobel Prize?
What was different about each discovery?
From missing energy to a new particle
Where did the energy go? The puzzle that led to the neutrino
Why did a spread of electron energies put energy conservation in doubt?
10Pauli’s letter: proposing a particle no one had seen
What did Pauli actually propose, and what still had to be established?
11Fermi’s next question: how often should the unseen particle appear?
How does an unseen particle become part of a theory that experiments can test?
12Cowan and Reines: finding the neutrino in two flashes
How do you recognize a neutrino interaction among all the other signals in a detector?
Tell the particle families apart
Why did physicists need a second kind of neutrino?
How can the particle left in a detector tell us which neutrino arrived?
14Gargamelle: what if a neutrino interacted and stayed a neutrino?
How do you identify an interaction when the outgoing neutrino also goes unseen?
15LEP: counting particles that leave no trace
How can the shape of a collision-energy peak count invisible decay channels?
16DONUT: catching a tau before it disappears
What made four events persuasive evidence for a third neutrino flavor?
What happened to the Sun’s neutrinos?
Which reactions in the Sun make neutrinos?
Why can a rare solar reaction dominate the events in a detector?
18The Sun’s missing neutrinos: a tank, a calculation and a long disagreement
When a detector counts fewer particles than a solar model predicts, which part of the explanation should change?
19Gallium experiments: opening the Sun’s low-energy window
What can an experiment learn when it counts atoms instead of individual light flashes?
20How do you know those neutrinos came from the Sun?
What changed when solar neutrinos became individual, timed events?
21SNO: where did the Sun’s missing neutrinos go?
Was something wrong with our picture of the Sun, or with what we expected neutrinos to do?
22Why does passing through the Sun change a neutrino’s flavor?
How can matter change a particle that usually passes straight through it?
23Borexino: how do you hear the Sun inside a radioactive world?
What has to become quiet before low-energy solar neutrinos become visible?
24After the missing neutrinos were explained, what could they tell us about the Sun?
Can a neutrino measurement tell us what the solar core is made of?
Understand flavor change and mass
Before anyone saw oscillations, how did the mixing idea take shape?
Why does the modern mixing matrix carry four names?
26Why can a neutrino change flavor?
What changes during the journey if a neutrino does not need to collide with anything?
27How does the atmosphere make neutrinos all around us?
Why do atmospheric neutrinos offer both a useful flavor comparison and many travel distances?
28Super-Kamiokande: the neutrinos that took the long way
Why did the detector see fewer muon neutrinos coming from below?
29Is finding a missing flavor the same as finding a new one?
What extra evidence do we gain by looking for the flavor that arrives?
302015: how did two very different experiments establish flavor change?
Why were the atmospheric and solar results stronger together?
31Neutrinos have mass. Why does oscillation not tell us how much?
How can an experiment establish that masses differ without weighing each state?
32Can the Earth help tell which neutrino mass comes first?
Why does a journey through rock contain information about mass ordering?
Read reactors and the Earth
Where does a reactor’s antineutrino spectrum come from?
Why can an accurately measured reactor power still leave an uncertain neutrino spectrum?
34KamLAND: testing a solar mystery with reactors on Earth
Why were reactor antineutrinos arriving from roughly 180 kilometres away useful for understanding solar neutrinos?
35Daya Bay: why compare a near detector with a far one?
How can comparing detectors reduce uncertainty without making it disappear?
36RENO and Double Chooz: why repeat the mixing-angle measurement?
What makes an independent measurement more than another number on the same plot?
37JUNO: why must a huge detector resolve tiny spectral features?
How can the spacing of spectral features say something that a total count cannot?
38The reactor anomaly and the spectrum bump ask different questions
How can a measured spectrum contain both a deficit and an excess?
39Short reactor baselines: look for a change with distance
How do you test the sterile explanation without trusting one absolute flux prediction?
40Geoneutrinos: what can particles tell us about the Earth’s heat?
Why is detecting the Earth’s antineutrinos easier than uniquely measuring its mantle heat?
Make a source and test its journey
How do you make a beam of particles you cannot steer?
If neutrinos have no electric charge, what are the beamline magnets focusing?
42K2K and MINOS: test flavor change with a distance you choose
What does a manufactured source add to evidence already found in the atmosphere?
43T2K: why look slightly away from the beam axis?
How can a small angular offset make a long-baseline measurement more informative?
44NOvA: how appearance, disappearance and the Earth fit together
Why can more electron-flavor events have more than one explanation?
45OPERA: tau appearance and a corrected clock anomaly
How can a valid appearance discovery coexist with a rejected timing claim?
46DUNE: what detailed argon images and a long baseline can answer
Why does a sharper interaction image still need a near detector and an uncertainty model?
47Hyper-K: a larger target still needs a precise comparison
What does a much larger water detector improve, and what must still be calibrated?
48Neutrino CP violation: what comparing neutrinos with antineutrinos can show
When do different neutrino and antineutrino samples imply a symmetry is broken?
Weigh the particle and test its nature
KATRIN: limiting neutrino mass at the end of the beta spectrum
How can electrons constrain the mass of an unobserved neutrino?
50Dirac or Majorana: is the neutrino its own antiparticle?
Which observation could distinguish two theories that ordinary oscillations leave open?
51Double-beta searches: why use several nuclei?
What does a longer half-life limit tell us across different instruments?
52Weigh neutrinos with the universe: what is assumed?
Why can two cosmological analyses return different mass limits?
53Sterile neutrinos: an explanation must pass several tests
Why does an unexplained excess not establish a new neutrino?
54Neutrinos in 2026: which questions remain open?
How can you read a research frontier without mistaking a preference for a discovery?
Follow the measurement chain
Coherent nuclear scattering: a small detector, a tiny recoil
How can a more likely interaction leave a signal that is harder to measure?
56Neutrino energy: the nucleus is part of the measurement
Why is the energy of an incoming neutrino not simply the energy we see?
57Why put a neutrino experiment under rock, sea or ice?
What does a remote site remove, and which backgrounds remain?
58Water, scintillator and argon: what does each detector see?
Does a brighter event always contain more information about the neutrino?
Build a new way to observe the sky
SN 1987A: a supernova seen from underground
What could a few signals in underground tanks tell us about a distant star?
60Before IceCube: learning to build telescopes in water and ice
What had to work before a vast volume of water or ice could become a telescope?
61IceCube: how do you turn a cubic kilometre of ice into an instrument?
What has to work for a neutrino interaction to become trustworthy data?
62IceCube’s 28 events: finding high-energy neutrinos from space
How could researchers tell that the atmosphere was not the whole story?
63TXS 0506+056: one alert, two separate lines of evidence
How can a single event and an excess in older data both support a link to one source while remaining separate results?
64NGC 1068: what does a neutrino excess establish?
How do many uncertain directions become evidence for one galaxy?
65Sea, lake and ice: telescopes with complementary skies
Why is another neutrino telescope more than extra volume?
66One neutrino sky, three different questions
Does detecting cosmic neutrinos mean we have already mapped their individual sources?
Read evidence, data and worldwide contributions
Read the world through neutrino experiments
What should a dot on an experiment map tell us?
68The engineering inside a neutrino measurement
Which parts of a discovery happen before a physics fit?
69When can a few neutrino events become evidence?
Does seeing five events mean the same thing in every experiment?
70Open a real neutrino event file and read it carefully
What can a public event table establish on its own?
71How an international experiment keeps its knowledge
How does knowledge survive after the original builders leave?
721930–2026: a timeline of neutrino evidence
What changed at each historical milestone?
Change a condition
Build the evidence yourself
15 teaching models. Each Lab generates its own data, and none of it is original collaboration data.
ACCELERATOR · CP PHASE
Same journey, different odds
Record spectra from a neutrino beam and an antineutrino beam, then pull apart the effects of the CP phase, matter and detector assumptions.
Open LabNEUTRINO ASTRONOMY · SKY SEARCH
A hotspot, or a lucky sky?
Look for a hotspot among recorded neutrino directions. Paired background skies show what each extra candidate costs.
Open LabBETA DECAY · ORIGINS
One electron, a whole spectrum
Replay a decay and compare a continuum with a two-body line.
Open LabRARE DECAY · NATURE
Two electrons, one forbidden peak
Open a cryogenic detector array and run a rare-decay search on one recorded set of counts.
Open LabENDPOINT · MASS
A giant filter for a tiny missing energy
Scan the end of the beta spectrum, then see how calibration changes the fit to the same counts.
Open LabREACTOR · SPECTRA
Two distances, one subtle dip
Fit spectra from a near and a far detector, then refit the same counts with a different energy calibration.
Open LabMATTER · RESONANCE
Matter turns the quantum compass
Send a neutrino through slow and sudden changes in density, and follow its coherent state numerically.
Open LabSUPERNOVA · TIMING
A star speaks before it shines
Record a burst, test a trigger on it, and follow one flash of light through a spherical detector.
Open LabARGON · IONIZATION
A clock becomes a dimension
Drift the electrons from one ionization deposit onto three wire planes, then recover the center of charge in each time slice.
Open LabSUN · HEAVY WATER
Recover the missing flavor
Replay a heavy-water detector. Its three reactions respond differently to each flavor; find where their flux limits cross for one recorded exposure.
Open LabATMOSPHERE · DIRECTION
Use Earth as a baseline
Compare upward and downward event counts in each energy band, with the source model and detector acceptance stated.
Open LabREACTOR · TIME
Find a pair in the noise
Record detector pulses, change the coincidence window and check the background against a time-shifted copy of the data.
Open LabWATER · GEOMETRY
Make a ring from light
Change a charged particle’s speed and direction, and trace its photons onto a flat detector plane.
Open LabICE · INFERENCE
Reconstruct an unseen track
Generate sensor times and charges, hide the true track, then fit its direction from the data.
Open LabVACUUM · MIXING
Follow a changing flavor
Scan the distance traveled in vacuum, watch the mass-state phases, then compare the probabilities with counts from ideal measurements.
Open LabDifferent places, connected questions
The world behind the discovery
Selected milestones to start reading from, not a complete history. The country that hosts an experiment does not tell you everyone who contributed.
- 1930
Wolfgang Pauli
A neutral particle is proposed
- 1933–34
Enrico Fermi
A calculable theory of beta decay
- 1956
Savannah River
Reactor antineutrinos detected directly
- 1957–62
Pontecorvo · Maki · Nakagawa · Sakata
Different routes towards neutrino mixing
- 1962
Brookhaven · Columbia
A beam shows that muon neutrinos are a separate flavor
- 1968
Homestake
A persistent solar-neutrino shortfall
- 1973
Gargamelle · CERN
Weak neutral currents survive background tests
- 1978–85
Wolfenstein · Mikheyev · Smirnov
Matter changes how flavor evolves
- 1987
Kamiokande-II · IMB · Baksan
Three detectors catch one supernova burst
- 1988–89
Kamiokande-II
Solar neutrinos gain a direction and an arrival time
- 1989
LEP · CERN
Count light neutrinos through Z decay
- 1990–92
SAGE · GALLEX
Open a lower-energy solar window
- 1998
Super-Kamiokande
Atmospheric flavor and travel distance
- 1999–2006
K2K · KEK → Kamioka
A timed beam tests a chosen 250-kilometer journey
- 2000–01
DONUT · Fermilab
Direct evidence for tau-neutrino interactions
- 2002
SNO
Solar electron neutrinos and the total of all active flavors
- 2002–03
KamLAND
Reactor antineutrinos test the solar explanation
- 2005–22
KamLAND · Borexino
Antineutrinos from Earth constrain its radioactive heat
- 2006
MINOS · Fermilab → Soudan
Rate and spectrum test muon-flavor loss
- 2011–20
Double Chooz · RENO
Independent sites cross-check reactor antineutrino disappearance
- 2012
Daya Bay
Near and far detectors reveal a small deficit
- 2013
IceCube
Evidence for a high-energy cosmic population
- 2014
T2K · J-PARC → Kamioka
Electron-flavor appearance in an off-axis beam
- 2015–18
OPERA · CERN → Gran Sasso
Tau appearance identified from tracks in emulsion
- 2015
Nobel Prize · Kajita · McDonald
Two independent paths to flavor change
- 2017
COHERENT · Oak Ridge
A pulsed source reveals coherent nuclear recoils
- 2017–18
TXS 0506+056 · IceCube · partner telescopes
An alert and an earlier neutrino excess
- 2018–22
Borexino · Gran Sasso
Separate the Sun’s individual reactions at low energy
- 2022
NGC 1068 · IceCube
Testing a list of candidate sources finds an excess at an active galaxy
- 2023
Milky Way · IceCube
The Galactic plane adds a spread-out neutrino signal
- 2025
T2K · NOvA
A joint fit whose CP limits depend on the mass ordering
- 2025
CONUS+ · Leibstadt
Reactor on/off data test tiny nuclear recoils
- 2025
DESI · Planck · ACT
Cosmological mass bounds depend on the model
- 2025
KATRIN · Karlsruhe
A beta-endpoint mass limit from 259 measurement days
- 2025–26
JUNO · Jiangmen
Fine reactor spectra measure oscillation parameters
- 2026
DUNE · Fermilab → Sanford
Far-detector construction and near-detector prototypes
- 2026
Hyper-K · Kamioka
The tank lining is finished; observations are still to come
- 2026
KM3NeT · ARCA / ORCA
Two complementary deep-sea detectors keep expanding
- 2026
IceTracks-DR2 · Public data
An updated track sample and detector response for source searches
- 2026
Nobel Prize · Francis Halzen
Recognition of a new observing window
The same terms in two languages
A small vocabulary for a large story
- Neutrino · 中微子
- An electrically neutral lepton detected through weak interactions.
- Antineutrino · 反中微子
- The antiparticle partner; different charged-current reactions distinguish it.
- Flavor · 味道
- Electron, muon or tau character defined by the associated charged lepton in a weak interaction.
- Mass state · 质量态
- A propagation state with a definite mass; a flavor state is a mixture of mass states.
- Oscillation · 振荡
- A change in flavor probabilities caused by relative quantum phases during propagation.
- Flux · 通量
- The particle flow per area and time, before detector response is applied.
- Cross section · 截面
- A measure of interaction probability, depending on the particle, energy and target.
- Baseline · 基线
- The distance from source to detector, with geometry specified for the experiment.
- Charged current / CC · 带电流 / CC
- A weak interaction mediated by a charged W boson, relating a neutrino to a charged lepton.
- Neutral current / NC · 中性流 / NC
- A weak interaction mediated by the neutral Z boson; active neutrino flavor is not tagged by an outgoing charged lepton.
- Background · 本底
- Other processes that can enter the selected sample; estimated or constrained separately.
- Exposure · 曝光
- The source or target and useful observation time relevant to a specified measurement.
- Reconstructed direction · 重建方向
- An estimate inferred from detector signals, with finite resolution and assumptions.
- SNU · 太阳中微子单位
- A capture-rate unit per target isotope atom; it is not an event count. Its exact definition is given in the radiochemical comparison.
Definitions follow the PDG review and the source-linked articles in this collection. PDG ↗