A Desperate
Remedy
A hundred trillion of them pass through you every second. Over a lifetime, perhaps one will notice.
01The letter
4 Dec 1930 Pauli letter collection; English translation, Physics Today, 1978Dear Radioactive Ladies and Gentlemen
“Dear Radioactive Ladies and Gentlemen,”
Tübingen · 4 December 1930Beta decay had a problem. When a nucleus emitted an electron, the electron came out with a smeared range of energies instead of the single sharp value conservation of energy demanded. Niels Bohr was willing to consider that energy simply was not conserved in individual nuclear events. Wolfgang Pauli was not.
On 4 December 1930 he wrote to a physics meeting in Tübingen and proposed a “desperate remedy”: an electrically neutral, spin-1/2, very light particle emitted alongside the electron, carrying off the missing energy. He called it the neutron — James Chadwick had not yet taken that name, and would find the real neutron in 1932.
He also understood what he had done: proposed a particle so weakly interacting it might never be seen. A line attributed to him afterwards puts it plainly.
Enrico Fermi built the theory. In 1933–34 he wrote a quantitative account of beta decay with Pauli’s particle in it, and the Italian diminutive neutrino — “little neutral one”, coined by Edoardo Amaldi in conversation with Fermi — stuck. Nature rejected Fermi’s paper as too speculative; it appeared in Nuovo Cimento and Zeitschrift für Physik in 1934.
02What it is
Nothing to Hold On To
A neutrino has no electric charge and no colour charge, so electromagnetism and the strong force do nothing to it. Only the weak force and gravity are left. That is the entire reason neutrinos are hard to catch: the weak interaction is short-ranged and feeble, and everything else the particle simply passes through.
They are also peculiarly one-handed. Every neutrino ever observed in a weak interaction is left-handed — spin pointing against its direction of motion — and every antineutrino right-handed. Maurice Goldhaber’s 1958 experiment measured that handedness directly, a year after Chien-Shiung Wu showed the weak force violates mirror symmetry. Whether right-handed neutrinos exist at all, and whether the neutrino is its own antiparticle — a Majorana particle — remains open.
Source: fundamentals.md — neutral, nearly massless, barely there. “—” means the figure is not in this page’s sources; relative strengths are not given there either, so no column claims them.
Proposed by Pauli in 1930, named in Fermi’s circle in the early 1930s, and finally detected by Clyde Cowan and Frederick Reines in 1956 at the Savannah River reactor. The first one took twenty-six years.
Lederman, Schwartz and Steinberger fired a beam from the Brookhaven AGS into a 10-ton spark chamber behind roughly 13 m of steel. Muons came out, and essentially no electrons: a second, distinct flavour. Nobel Prize, 1988.
Fermilab’s DONUT experiment announced direct observation on 21 July 2000: four candidate events in emulsion film, later extended to nine.
03The numbers
A Light-Year of Lead
The weak force is weak enough that the numbers stop feeling physical. Five of them, with their hedges attached.
Solid lead would stop a typical solar neutrino only after something on the order of a light-year of travel — about 9.5 trillion kilometres of metal.
Order-of-magnitude only · depends strongly on energy fundamentals.md — a light-year of leadSummed over source components and flavours, at the Earth. Hold up a thumbnail: that is your share, every second.
Standard Solar Model; Bahcall–Serenelli flux tablesLeft over from the first second and cooled to 1.95 K — the second most abundant known particle in the universe, behind the photon by a nose, ahead of protons and electrons by a factor of about a billion. Never directly detected; PTOLEMY is at demonstrator stage.
curiosities.md — relics from second oneNeutrinos stopped interacting and flew free about one second after the Big Bang — some 380 000 years before photons did the same and became the cosmic microwave background.
Standard cosmologyThey leave at essentially light speed and arrive in 8 minutes 20 seconds. The energy released beside them random-walks out through the radiative zone and takes on the order of 100 000 years to reach the surface. Neutrinos are the only live view of the core — which is how Borexino detected the Sun’s CNO fusion cycle in 2020.
Estimates range ~10 000–200 000 years · “100 000” is a round number, not a measurement curiosities.md — the Sun’s eight-minute shortcut04The catch
26 years Pauli’s letter to the telegram: 1930 → 1956. This section is 12 m undergroundProject Poltergeist
Frederick Reines and Clyde Cowan took Bethe and Peierls as a challenge. Their insight: you do not need a better detector, you need an absurdly intense source. After a first attempt at the Hanford reactor in 1953 they moved to the Savannah River Plant in South Carolina, about 11 metres from a reactor core and some 12 metres underground.
The trick was the signature. An antineutrino hitting a proton makes a positron and a neutron; the positron annihilates instantly, and the neutron is captured microseconds later by cadmium dissolved in the water target, giving a delayed gamma flash. Two pulses, correct spacing, or it does not count.
Two pulses, correct spacing, or it does not count.Savannah River, 1956 · results in Science, 20 July 1956
On 14 June 1956 they sent Pauli a telegram: they had detected his particle. The results appeared in Science on 20 July 1956. Reines received a share of the Nobel Prize in 1995; Cowan had died in 1974 and could not be named.
05The deficit
~1/3 2.56 SNU observed against roughly 8 SNU predicted (Homestake final result, 1998; Bahcall standard solar model).The Missing Two-Thirds
From the late 1960s Ray Davis ran a tank holding 615 tonnes of dry-cleaning fluid 1 478 m underground in the Homestake gold mine, South Dakota. Solar neutrinos occasionally converted a chlorine atom to argon-37; Davis flushed out the argon and counted individual atoms, a dozen or so per run.
John Bahcall’s standard solar model predicted the rate. Davis measured about one third of it — 2.56 SNU against a prediction of roughly 8 SNU, a figure that shifted with successive model versions. The gap held for three decades.
Most people assumed either Davis or Bahcall was wrong. Both were right.Homestake final result, 1998 · Bahcall standard solar model
06Oscillation
1998 2001 Super-Kamiokande, atmospheric νμ. Sudbury Neutrino Observatory, solar νe.The Particle That Changes Its Mind
Fire a beam of muon neutrinos and some of them arrive as electron or tau neutrinos. This is neutrino oscillation, and it is the only laboratory phenomenon so far that does not fit the original Standard Model.
The three flavour states are not the same as the three states of definite mass. Each flavour is a quantum superposition of the mass states ν₁, ν₂, ν₃, and the recipe for mixing them is the PMNS matrix — three mixing angles and one phase that may encode CP violation.
Because the mass states have slightly different masses they travel at slightly different rates and drift out of phase, so the flavour you measure depends on how far the neutrino has flown. If all three masses were zero there would be no phase difference and no oscillation at all.
Which resolves §05 in two beats. On 5 June 1998, at Neutrino ’98 in Takayama, Takaaki Kajita reported that muon neutrinos made by cosmic rays arrived in expected numbers from above but were depleted by roughly half after crossing the whole Earth. Then the Sudbury Neutrino Observatory, 1 000 tonnes of heavy water 2 km down in Ontario, counted electron neutrinos separately from all flavours combined: in 2001 and 2002 the electron-neutrino flux came out at a third of the total, while the total matched Bahcall’s prediction within its uncertainties. Kajita and McDonald shared the 2015 Nobel Prize.
- νμ survival — upper band
- νμ → ντ — lower band
Two-flavour approximation · θ₂₃ = 45° · Δm²₃₁ = 2.5 × 10⁻³ eV²
Scroll to change the beam energy E. With the figure focused: ←/→ move the detector line by 50 km, Home/End jump to 0 and 1 300 km, Page Up/Page Down step E by 0.2 GeV between 0.6 and 4.0 GeV.
Oscillation measures mass differences, not masses. JUNO in China, taking data since August 2025, DUNE in the United States and Hyper-Kamiokande in Japan are the instruments built to settle which rung is on the bottom.
07Mass
× 1 000 000 At least a million times lighter than the electron — ten million if the oscillation minimum, ~0.05 eV, is near the truth.How Light Is Light?
The best direct limit comes from KATRIN at Karlsruhe, a 200-tonne spectrometer that weighs the neutrino by measuring the last few electronvolts of the tritium beta-decay spectrum. Its 2024 result: the effective electron-antineutrino mass is below 0.45 eV at 90% confidence, with a design sensitivity near 0.3 eV.
Cosmology tightens the screw further. DESI baryon-acoustic-oscillation data combined with the CMB constrain the sum of the three masses to ≲ 0.06–0.07 eV at 95% confidence — a bound that assumes standard ΛCDM, loosens considerably with other dataset combinations and models, and sits awkwardly close to the ~0.05 eV minimum that oscillation already requires.
08The method
41° Opening angle of the Cherenkov cone in water, refractive index ≈ 1.33.Look for the Wake, Not the Particle
No detector sees a neutrino. What detectors see is the charged particle a neutrino occasionally kicks out of a nucleus or an electron — a muon, an electron, a tau — and the light that particle leaves behind.
The workhorse signal is Cherenkov radiation. When a charged particle moves through water faster than light does in water — light slows to about 0.75 c there, since water’s refractive index is roughly 1.33 — it drags a shock front of light behind it, a blue cone opening at about 41 degrees. Photomultiplier tubes lining the tank record the ring where that cone lands. The ring’s brightness gives the energy, its centre gives the direction. A sharp ring means a muon; a fuzzy one means an electron.
Why everything is buried
At sea level roughly 170 muons from cosmic-ray air showers cross every square metre each second — about one per square centimetre per minute. A neutrino detector might record a handful of genuine events per day. Rock is the filter.
Super-Kamiokande sits under 1 000 m of mountain in the Mozumi mine near Kamioka, Japan — about 2 700 metres of water equivalent, which cuts the muon rate by roughly a factor of 100 000. SNOLAB, JUNO, Gran Sasso and the Sanford Underground Research Facility follow the same logic. IceCube uses 1.5 km of Antarctic ice instead of rock; KM3NeT uses 2.5–3.5 km of Mediterranean seawater.
The alternative to water is liquid scintillator: an organic liquid that emits light when anything charged passes through it. It is far more sensitive at low energies, which is why reactor and solar experiments favour it, but it discards most directional information.
09The fleet
11 129 Twenty-inch photomultipliers in Super-Kamiokande’s inner volume (Kamioka Observatory, ICRR).The Current Fleet
Because neutrinos almost never interact, the only way to see them is to build something enormous, bury it, and wait. Open a row for the one fact that does not fit the table.
| Instrument | Where | Medium | Scale | Depth | Status |
|---|---|---|---|---|---|
| Kamioka, Japan | Ultrapure water | 50 000 t · 11 129 inner 20-inch PMTs | 1 000 m | Running since 1996 · Gd-doped since 2020 | |
Gadolinium captures neutrons and lets Super-K tag antineutrinos individually — a decisive upgrade for spotting the neutrino burst from the next galactic supernova. | |||||
| South Pole | Glacial ice | 1 km³ · 5 160 sensors on 86 strings | 1 450–2 450 m | Complete since December 2010 | |
The ice is the detector; the drill was the hard part. | |||||
| Mediterranean Sea | Seawater | Two sites: ARCA off Sicily, ORCA off Toulon | 2.5–3.5 km | Taking data while under construction | |
ARCA does high-energy astronomy, ORCA does oscillations. KM3-230213A was recorded while ARCA was still only partly built. | |||||
| Lake Baikal | Lake water | ~0.5 km³ effective, as of 2025 | — | Running | |
A third medium after ice and seawater, with an effective volume of roughly half a cubic kilometre as of 2025. | |||||
| Guangdong, China | Liquid scintillator | 20 kt · 35.4 m sphere · 17 612 large + 25 600 small PMTs | — | Data since August 2025 | |
It sits about 52.5 km from two reactor complexes — the distance chosen to read the fine ripple that reveals which neutrino mass is heaviest. | |||||
| Illinois → South Dakota | Liquid argon | 1 300 km beamline | 1.5 km | Under construction | |
No tunnel required: the beam is fired straight through the Earth from Fermilab to the Sanford Underground Research Facility. | |||||
| Japan | Ultrapure water | ~8× Super-K fiducial volume | — | Planned to begin 2028 | |
About eight times Super-K’s fiducial volume, roughly five times its total mass. | |||||
| Karlsruhe, Germany | Tritium beta spectrum | 200-tonne spectrometer | — | 2024 result: < 0.45 eV (90% CL) | |
Not size but precision: it weighs the neutrino off the last few electronvolts of the tritium beta-decay spectrum. | |||||
| Gran Sasso, Italy | Liquid scintillator | ~300 t | — | pp neutrinos 2014 · CNO cycle 2020 | |
Radio-purity, not size: only about 300 tons, but astonishingly clean — which is what made the Sun’s low-energy neutrinos visible. | |||||
| China | Liquid scintillator | Reactor antineutrinos | — | θ₁₃ measured, 2012 | |
The measurement that made the whole DUNE and Hyper-Kamiokande programme worth building. | |||||
Depth bars are drawn proportional to the logarithm of the depth. “—” means the figure is not in this page’s sources. Source: detectors.md — the current fleet.
Twenty-two decades of energy
10SN 1987A
~25 events ~13 s Kamiokande-II, IMB and Baksan, 23 February 1987.Thirteen Seconds
Two dozen particles. That is the entire observational record of a star dying nearby.Kamiokande-II, IMB and Baksan · 23 February 1987
On 23 February 1987 three detectors recorded about 25 neutrino events within about 13 seconds from SN 1987A in the Large Magellanic Cloud, some 168 000 light-years away. The neutrinos led the earliest optical record by about two to three hours, depending on which observation is counted first; Ian Shelton’s independent discovery at Las Campanas that same night is what alerted astronomers.
When a massive star’s iron core collapses, about 99% of the energy released — of order 3 × 10⁴⁶ joules on standard theoretical estimates — escapes as neutrinos. The supernova that outshines its host galaxy for weeks is powered by the leftover one percent. SN 1987A remains the only supernova seen in neutrinos.
Neutrino astronomy grows up
Three detections, three firsts, in eight years. The field has gone from proving it can work to doing astronomy.
11Open questions
What Nobody Knows Yet
01Is the neutrino its own antiparticle?
If neutrinos are Majorana particles, a nucleus could undergo neutrinoless double beta decay. No one has seen it. KamLAND-Zen, using xenon-136, holds the leading half-life limit above 3.8 × 10²⁶ years (90% CL, 2024); LEGEND-200 is running with germanium-76; nEXO is proposed with five tonnes of enriched xenon.
02Which way do the masses go?
The “normal” and “inverted” orderings are both still alive. JUNO, a 20-kilotonne liquid scintillator detector in southern China, began taking data in August 2025 and expects to settle the ordering after roughly six to seven years of running.
03Do neutrinos violate CP symmetry?
T2K’s data favour a CP-violating phase near −π/2. The first joint T2K–NOvA analysis, published in 2025, finds evidence for CP violation only if the mass ordering turns out to be inverted, and nothing near discovery either way. DUNE and Hyper-Kamiokande are built to settle it. If the answer is yes, leptogenesis becomes a serious candidate for why the universe contains matter and almost no antimatter.
04Is there a fourth, sterile neutrino?
LSND in the 1990s and MiniBooNE pushed the idea; MicroBooNE found no matching electron excess in 2021–2022, and the reactor antineutrino anomaly largely dissolved once flux predictions were recalculated. The gallium anomaly, sharpened by BEST in 2022, is the one that refuses to go away.
05How heavy are they?
KATRIN puts the effective electron-antineutrino mass below 0.45 eV at 90% confidence — the tightest direct laboratory bound. Cosmology is stricter still, and awkwardly close to the minimum that oscillations require.
Source: curiosities.md — what nobody knows yet. Hovering or focusing one row dims the other four.
Interstitial
You
100 000 000 000 000 solar neutrinos cross your body every second, day and night. At night they arrive through the Earth, which barely notices them.
4 000–5 000 of your own potassium-40 decays per second. About 0.012% of your potassium is radioactive, and an adult carries around 140 g of it. Every decay emits a neutrino or an antineutrino. You are a neutrino source.
~1
interaction with you, in a lifetime.
curiosities.md — you are a neutrino source; fundamentals.md — a light-year of lead.