By 1960, the neutrino had been confirmed to exist. Cowan and Reines had detected it in 1956 from a nuclear reactor. But a strange theoretical wrinkle had appeared. Certain processes involving muons — heavier cousins of the electron — appeared to behave as if the neutrinos involved in muon physics were somehow different from the neutrinos involved in electron physics.
If true, this would mean there was not one kind of neutrino but at least two. The question was important enough that three physicists at Columbia University — Leon Lederman, Melvin Schwartz, and Jack Steinberger — set out to settle it experimentally.
What they built at the Brookhaven National Laboratory in 1962 is now remembered as one of the most decisive experiments in the history of particle physics: the two-neutrino experiment.
The argument they were testing
Pion decay produces a muon and a neutrino. If that neutrino is the same kind of particle as the one produced in beta decay (an electron neutrino), then when it later interacts in a detector it should sometimes produce an electron, sometimes a muon, with comparable probabilities.
If, on the other hand, pion decay produces a distinct muon neutrino, then the neutrino remembers which family it belongs to. When it interacts, it should produce a muon almost every time and never an electron.
The way to tell the two scenarios apart was simple in principle: build a detector that could distinguish muons from electrons, fire a beam of muon-flavored neutrinos at it, and count.
Simple in principle. Very hard in practice.
The experimental challenge
The first problem was making a neutrino beam. The Brookhaven AGS was the highest-energy proton synchrotron in the world in 1960. The three physicists routed its proton beam onto a beryllium target, producing pions, which were then allowed to decay in flight to muons and neutrinos. The muons came out with the neutrinos, in the same direction.
The second problem was filtering out everything except neutrinos. The team built a 13.5-meter wall of steel armor salvaged from the battleship USS Missouri — about 4,000 tons of it. Anything that wasn’t a neutrino would interact in the steel and stop. Only the neutrinos would punch through.
The third problem was the detector itself. Lederman, Schwartz, and Steinberger built a 10-ton spark chamber — at the time the largest particle detector ever constructed. It consisted of 90 aluminum plates with thin gas-filled gaps between them. When a charged particle passed through, the gas in each gap would briefly ionize and produce a visible spark, leaving a photographable trail.
The team ran for eight months. After all that running, the world’s largest particle detector had recorded 56 neutrino events.
What the events showed
Of those 56 events, all of them produced muons. Not a single one produced an electron in the way that would have been expected if the neutrinos were the same as Cowan and Reines’s electron-flavored variety.
The conclusion was unavoidable. The neutrinos coming out of pion decay were not electron neutrinos. They were a distinct particle, paired with the muon. The muon neutrino existed as its own flavor.
The result was published in Physical Review Letters in July 1962.
Why this matters in hindsight
The two-neutrino experiment did several things at once.
It confirmed that neutrinos come in distinct flavors — a result that turned out to be more important than anyone realized at the time. The discovery of neutrino oscillation thirty-six years later only makes sense in the context of distinct flavors that can mix.
It demonstrated that lepton family number is conserved, at least at the level of single neutrino interactions. Electron neutrinos make electrons; muon neutrinos make muons; tau neutrinos (discovered in 2000) make taus.
It also pioneered the technique of accelerator-produced neutrino beams, which became the foundation of most subsequent neutrino experiments. Every major modern accelerator-based neutrino program — T2K, NOvA, MINOS, MicroBooNE, and most importantly DUNE — uses essentially the same trick the Brookhaven team invented: aim a proton beam at a target, let pions decay in flight, shield against the muons, point what’s left at a faraway detector.
The Nobel Prize
Lederman, Schwartz, and Steinberger were awarded the 1988 Nobel Prize in Physics “for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino.” Twenty-six years after the experiment, six years before SN 1987A had its own Nobel-prizewinning aftermath.
Lederman, who would later coin the phrase “God particle” for the Higgs boson and direct Fermilab for a decade, called the 1962 result his best work. Schwartz had moved to Stanford by the time the prize was awarded. Steinberger had moved to CERN, where he had helped lead other major experiments.
The chain that followed
The two-neutrino experiment is one of the foundational results that the rest of the field built on. Three flavors are now confirmed (electron, muon, tau). Oscillations between them are measured. CP violation in the neutrino sector is being hunted by DUNE and Hyper-Kamiokande. The possibility of a fourth, “sterile” flavor remains under investigation.
Every one of those programs traces its method back to Brookhaven, 1962 — to a steel wall made of battleship armor, a 10-ton spark chamber, and 56 events that quietly rewrote particle physics.
For the wider history of neutrino discoveries, see our full timeline. For the contemporary picture of what flavor mixing looks like, see How neutrino oscillation works.
Frequently asked
What was the two-neutrino experiment?
A 1962 experiment at Brookhaven National Laboratory in which Leon Lederman, Melvin Schwartz, and Jack Steinberger proved that the neutrino produced in pion decay (a 'muon neutrino') is different from the neutrino emitted in beta decay (an 'electron neutrino'). The result established that neutrinos come in distinct flavors paired with the charged leptons.
How did the experiment work?
The team produced pions at the Brookhaven Alternating Gradient Synchrotron, let them decay to produce muon neutrinos, and shielded the beam with 13.4 meters of steel armor plate to filter out everything except the neutrinos. A spark chamber detector then recorded the rare neutrino interactions. The muon neutrinos produced muons in the detector — but never electrons, confirming that they were distinct from the electron neutrinos that produce electrons in beta-decay-style reactions.
Why did it matter?
Because it established the structure of the leptonic sector of what would become the Standard Model: three generations of leptons, each pairing a charged lepton with its own neutrino flavor. It also opened the door to studying neutrino oscillation, which only makes sense in a multi-flavor picture.
Did they win the Nobel Prize?
Yes. Lederman, Schwartz, and Steinberger shared the 1988 Nobel Prize in Physics 'for the neutrino beam method and the demonstration of the doublet structure of the leptons through the discovery of the muon neutrino.' The 26-year delay was typical for foundational experimental discoveries of that era.
When was the tau neutrino discovered?
The tau lepton was discovered in 1975 by Martin Perl at SLAC, implying the existence of a third neutrino flavor. The tau neutrino itself was directly observed only in 2000, by the DONUT (Direct Observation of the NU Tau) experiment at Fermilab, completing the experimental confirmation of all three Standard Model neutrino flavors.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2025, June 12). The two-neutrino experiment: how 1962 proved a second flavor exists. Neutrino Times. https://neutrino-times.com/articles/muon-neutrino-discovery-1962-lederman-schwartz-steinberger/
Chicago
Neutrino Times Editorial Team. "The two-neutrino experiment: how 1962 proved a second flavor exists." Neutrino Times, June 12, 2025. https://neutrino-times.com/articles/muon-neutrino-discovery-1962-lederman-schwartz-steinberger/.
MLA
Neutrino Times Editorial Team. "The two-neutrino experiment: how 1962 proved a second flavor exists." Neutrino Times, 12 Jun. 2025, https://neutrino-times.com/articles/muon-neutrino-discovery-1962-lederman-schwartz-steinberger/.
BibTeX
@misc{neutrino-times-muon-neutrino-discovery-1962-lederman-schwartz-steinberger,
author = {Neutrino Times Editorial Team},
title = {The two-neutrino experiment: how 1962 proved a second flavor exists},
howpublished = {Neutrino Times},
year = {2025},
month = {jun},
url = {https://neutrino-times.com/articles/muon-neutrino-discovery-1962-lederman-schwartz-steinberger/},
note = {Accessed: 2025-06-12}
} RIS
TY - GEN TI - The two-neutrino experiment: how 1962 proved a second flavor exists AU - Neutrino Times Editorial Team PY - 2025 DA - 2025-06-12 PB - Neutrino Times UR - https://neutrino-times.com/articles/muon-neutrino-discovery-1962-lederman-schwartz-steinberger/ ER -