In Part 1 we met the neutrino — a particle so weakly interacting that 100 trillion of them pass through you every second and you never notice. A reasonable question is: how do we know they exist at all? If they don’t interact with anything, how could anyone have ever proven they were there?
The answer is a 26-year detective story spanning two continents and the birth of nuclear physics.
A puzzle in radioactivity
By the late 1920s, physicists had been studying radioactive decay for about 30 years. They knew that certain unstable nuclei emit a beta particle — what we now call an electron — and transform into a different element. Energy conservation, one of the foundational principles of physics, predicted that every beta particle from a given decay should come out with exactly the same energy. The reaction was simple: one nucleus turns into another, the electron carries away the energy difference.
The data said otherwise. Measurements of beta-decay electron energies showed a continuous spectrum — every electron came out with a different energy, sometimes much less than the maximum allowed. The energy difference was simply gone.
This was a serious problem. Either energy conservation was wrong at the nuclear level — an idea Niels Bohr was prepared to entertain — or something invisible was carrying away the missing energy.
Pauli’s letter
On December 4, 1930, Wolfgang Pauli wrote what is now one of the most famous letters in the history of physics. Pauli was 30 years old and already one of the most influential theorists in Europe. He had been invited to a small radioactivity conference in Tübingen, Germany. He did not want to go — by his own admission, he was planning to attend a ball in Zürich that same weekend.
So he wrote an open letter to the conference attendees. It opened with: “Dear radioactive ladies and gentlemen, I have hit upon a desperate remedy to save…”
Pauli’s “desperate remedy” was to postulate a new particle. The particle would be:
- Electrically neutral (so it would not have been detected as charged radiation).
- Almost massless (or very light).
- Spin-½ (to balance the angular momentum).
- Emitted along with the electron in beta decay, carrying away the missing energy.
He apologized for the suggestion. He thought the particle would be too weakly interacting to ever be detected. He wrote to a friend: “I have done a terrible thing today: I have postulated a particle that cannot be detected.”
For details on the letter and its context, see our dedicated article on Pauli’s 1930 proposal or the Wolfgang Pauli profile.
Naming the particle
Pauli’s original 1930 name was the “neutron.” That became inconvenient in 1932 when James Chadwick discovered the actual neutron — the much heavier neutral particle inside the nucleus. The community needed a new name. Enrico Fermi, then leading a remarkable physics group in Rome, suggested “neutrino” — Italian for “little neutral one.”
Fermi then took the next big step. In 1933, he wrote a complete theoretical description of beta decay that incorporated the neutrino as a real particle. His “tentative theory of beta rays” introduced the weak nuclear force as a distinct interaction (separate from electromagnetism, the strong nuclear force, and gravity) and predicted in detail how beta decay should look. The theory was rejected by Nature as too speculative but published in Italian and German journals. It became one of the foundational papers of twentieth-century physics.
The detection challenge
Now physicists had a theory of beta decay that involved the neutrino, and they could calculate roughly how often a neutrino should interact with matter if you sent one at a target. The answer was dispiriting: the cross-section was something like 10⁻⁴⁴ cm² per neutrino. That meant a single neutrino could pass through a light-year of solid lead with only about a 50% chance of bumping into anything.
For decades, physicists agreed with Pauli that the particle was probably undetectable in any practical sense.
What changed was the development of nuclear reactors during and after World War II. A power reactor produces about 10²⁰ antineutrinos per second — a flux trillions of times higher than any natural source. With a flux that high, even a tiny cross-section becomes potentially observable, as long as you have a big enough detector and clever enough background-rejection.
Cowan and Reines
Frederick Reines and Clyde Cowan, two physicists at Los Alamos National Laboratory, took on the detection challenge in the early 1950s. Their first proposal, slightly seriously, was to detonate a nuclear bomb in the desert and put a detector down a deep mine shaft nearby. Cooler heads pointed out that nuclear reactors would do nearly as well without the explosions.
They built a “sandwich” detector: two large tanks of liquid scintillator (a mineral-oil-based fluid that flashes when energetic charged particles pass through), with a water tank containing cadmium chloride sandwiched between them. The detection chain works like this:
- An antineutrino from the reactor occasionally hits a proton in the water, converting it to a neutron and emitting a positron (the antimatter version of an electron). This is called inverse beta decay.
- The positron immediately annihilates with a nearby electron, producing two gamma rays that flash in the scintillator (the “prompt” signal).
- The neutron wanders around for a few microseconds before being captured by a cadmium nucleus, producing another flash of gamma rays (the “delayed” signal).
The combination — a prompt double flash followed by a delayed flash, with the right timing and energies — is a specific enough signature that no background process could plausibly mimic it. If you saw events with that pattern, you had caught an antineutrino.
The 1956 telegram
By June 1956, after several years of refinement at the Savannah River Plant in South Carolina, Reines and Cowan’s detector had accumulated enough events to make a confident announcement. They sent a telegram to Pauli at the Swiss Federal Institute of Technology in Zürich:
We are happy to inform you that we have definitely detected neutrinos from fission fragments by observing inverse beta decay of protons. Observed cross-section agrees well with expected six times ten to the minus forty-four square centimeters.
The legend is that Pauli received the telegram during a conference and read it aloud to his colleagues, who then opened champagne. He later wrote back: “Thanks for message. Everything comes to him who knows how to wait.”
Pauli’s “desperate remedy” was now confirmed. The full story of the 1956 detection is covered in our dedicated article on Reines and Cowan.
The aftermath
The 1956 detection opened the door to everything else in neutrino physics. Within a few years, physicists had begun to explore questions like: How many kinds of neutrino are there? Do they have mass? Can they change from one kind into another?
The answers, it turns out, are: three, yes, and yes.
In Part 3, we’ll look at how the field discovered that neutrinos come in three “flavors” and that they spontaneously change from one flavor to another in flight — an effect that took until 1998 to confirm and won the 2015 Nobel Prize.
Frequently asked
Who proposed the neutrino?
Wolfgang Pauli, in 1930, in an open letter to a physics conference. He suggested that a new, electrically neutral, nearly massless particle must be carried away in beta decay to balance the energy that otherwise appeared to be missing. He called it 'a desperate remedy.'
Who first detected one?
Frederick Reines and Clyde Cowan, in 1956, using a detector parked next to the Savannah River nuclear reactor in South Carolina. They sent a telegram to Pauli announcing the result. Reines won the Nobel Prize for it in 1995 — 39 years later.
Why did it take 26 years to detect?
Because neutrinos interact so weakly that nobody knew how to catch one. Pauli himself thought they were undetectable. What eventually worked was a combination of intense reactor antineutrino sources, large detector volumes, and a clever 'coincidence' detection technique that distinguished neutrino events from background.
Why did Cowan and Reines use a nuclear reactor?
Because reactors produce enormous fluxes of antineutrinos as a byproduct of fission — about 10²⁰ per second from a single power reactor. Earlier neutrino sources (like natural radioactivity) produced far too few neutrinos for a reasonable chance of detection. The reactor program at the Savannah River Site was the first source intense enough to make the experiment feasible.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, January 12). Neutrinos 101 — Part 2: How we know they exist. Neutrino Times. https://neutrino-times.com/articles/neutrinos-101-part-2-how-we-know-they-exist/
Chicago
Neutrino Times Editorial Team. "Neutrinos 101 — Part 2: How we know they exist." Neutrino Times, January 12, 2026. https://neutrino-times.com/articles/neutrinos-101-part-2-how-we-know-they-exist/.
MLA
Neutrino Times Editorial Team. "Neutrinos 101 — Part 2: How we know they exist." Neutrino Times, 12 Jan. 2026, https://neutrino-times.com/articles/neutrinos-101-part-2-how-we-know-they-exist/.
BibTeX
@misc{neutrino-times-neutrinos-101-part-2-how-we-know-they-exist,
author = {Neutrino Times Editorial Team},
title = {Neutrinos 101 — Part 2: How we know they exist},
howpublished = {Neutrino Times},
year = {2026},
month = {jan},
url = {https://neutrino-times.com/articles/neutrinos-101-part-2-how-we-know-they-exist/},
note = {Accessed: 2026-01-12}
} RIS
TY - GEN TI - Neutrinos 101 — Part 2: How we know they exist AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-01-12 PB - Neutrino Times UR - https://neutrino-times.com/articles/neutrinos-101-part-2-how-we-know-they-exist/ ER -