| | {{infobox |
| | | title = Neutrino |
| | | image = superk-dome-tokyo.jpg |
| | | caption = The photomultiplier dome of a giant water-Cherenkov neutrino detector — the replica of Super-Kamiokande's tank interior (CC0, National Museum of Nature and Science, Tokyo) |
| | | type = Elementary particle — lepton; three flavors: electron, muon, tau |
| - | | mass = Below ~0.45 eV/c² — at least 200,000× lighter than an electron; exact mass unknown |
| + | | mass = Below ~0.45 eV/c² — over a million times lighter than an electron; exact mass unknown |
| | | interactions = Weak nuclear force + gravity only — no charge, no strong force, no electromagnetic response |
| | | abundance = ~65 billion pass through every cm² of you, every second — from the Sun alone |
| | | discovered = Predicted 1930 (Pauli); detected 1956 (Reines & Cowan) |
| | }} |
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| | The **neutrino** is the ghost of the particle world: the second most abundant particle in the universe after the photon, and the least interactive thing known to have mass. Roughly **sixty-five billion solar neutrinos pass through every square centimetre of your body every second** — through you, through the Earth, through light-years of lead — and almost none of them notice. Catching one requires kilotons of water, cubic kilometres of Antarctic ice, or a tank of liquid chlorine the size of a house. Yet this barely-there particle keeps remaking physics: it forced the Standard Model's first real amendment (it has mass), confirmed how stars die (SN 1987A), and may hold the key to [[why does matter exist|why anything exists at all]].[[cite: solar neutrino flux, standard solar model; neutrino literature]] |
| | |
| | ## Invented to save physics — Pauli's "desperate remedy" |
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| | The neutrino began as bookkeeping. In 1930, **Wolfgang Pauli** faced an apparent violation of energy conservation: in beta decay, the electron's energy came out in a spread rather than a sharp value — energy seemed to go missing. Rather than abandon conservation laws, Pauli proposed an invisible neutral particle carrying the missing energy away — "a desperate remedy," he called it, for "a particle that cannot be detected." **Enrico Fermi** named it the *neutrino* ("little neutral one" in Italian) and built it into his 1934 theory of beta decay. For twenty-six years it was pure mathematics.[[cite: Pauli 1930 letter; Fermi beta-decay theory]] |
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| | Then **Frederick Reines and Clyde Cowan** proved it real. In 1956, beside a reactor at the Savannah River plant — which floods out ~10¹³ antineutrinos per square centimetre per second — they caught the unmistakable double flash of an antineutrino triggering inverse beta decay. Reines telegrammed Pauli: *"We are happy to inform you that we have definitely detected neutrinos."* The ghost had a body. The work won the 1995 Nobel Prize — Reines alone, Cowan having died two decades earlier.[[cite: Reines & Cowan 1956; Nobel Prize 1995]] |
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| | ## Three flavors — and the discovery that broke the Standard Model |
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| | Neutrinos come in **three flavors** — electron, muon, and tau — matching the three charged leptons. For decades, experiments kept finding fewer neutrinos than expected: Ray Davis's famous chlorine tank in a South Dakota gold mine caught only about a third of the predicted **solar neutrinos**, a "deficit" that stood for thirty years as either broken astrophysics or broken particle physics.[[cite: Homestake experiment; solar neutrino problem]] |
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| | The resolution was one of the great discoveries of the late twentieth century: **neutrinos change flavor in flight.** An electron neutrino leaving the Sun's core can arrive at Earth as a muon or tau neutrino — **neutrino oscillation**, proven definitively by Super-Kamiokande (atmospheric neutrinos, 1998) and Canada's SNO experiment (solar neutrinos, 2001). The catch: oscillation is only possible if neutrinos have **mass** — and the Standard Model had always assumed them massless. It remains the first and clearest experimental crack in the Standard Model itself. Takaaki Kajita and Arthur McDonald shared the 2015 Nobel Prize for it.[[cite: Super-Kamiokande 1998; SNO 2001-02; Nobel Prize 2015]] |
| | |
| | ## How much does a ghost weigh? |
| | |
| - | Nobody knows exactly — and the answer matters cosmically. The **KATRIN** experiment in Germany weighs the neutrino by watching the extreme endpoint of tritium beta decay, where the electron's energy is stolen by the departing neutrino; its best limits put the mass below roughly **0.45 electronvolts** — at least 200,000 times lighter than the next-lightest particle, the electron. Why such a tiny mass exists at all — and whether neutrinos are their own antiparticles (a **Majorana** particle, which would make them unique) — are among the deepest open questions in physics, with neutrinoless double-beta-decay experiments hunting the answer underground worldwide.[[cite: KATRIN collaboration mass limits; Majorana/double-beta-decay program]] |
| + | Nobody knows exactly — and the answer matters cosmically. The **KATRIN** experiment in Germany weighs the neutrino by watching the extreme endpoint of tritium beta decay, where the electron's energy is stolen by the departing neutrino; its best limits put the mass below roughly **0.45 electronvolts** — over a million times lighter than the next-lightest particle, the electron. Why such a tiny mass exists at all — and whether neutrinos are their own antiparticles (a **Majorana** particle, which would make them unique) — are among the deepest open questions in physics, with neutrinoless double-beta-decay experiments hunting the answer underground worldwide.[[cite: KATRIN collaboration mass limits; Majorana/double-beta-decay program]] |
| | |
| | ## The day 24 neutrinos rewrote astronomy |
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| | On 23 February 1987, three underground detectors — **Kamiokande-II** (Japan), **IMB** (Ohio), and **Baksan** (Caucasus) — caught roughly two dozen neutrinos in thirteen seconds. They had traveled 168,000 years from the collapsing core of a star in the Large Magellanic Cloud, and they arrived **hours before the supernova's light** — neutrinos escape the dying core instantly, while the shock wave still fights its way out. [[File:sn1987a.jpg|right|300px|SN 1987A — the supernova whose collapsing core announced itself in ~24 neutrinos, hours before its light (NASA/ESA, public domain)]] Twenty-four particles, from one explosion 168,000 light-years away — and they confirmed the core-collapse mechanism, proved neutrinos carry ~99% of a dying star's energy, and launched **neutrino astronomy** as a field. Masatoshi Koshiba shared the 2002 Nobel Prize for it.[[cite: Kamiokande-II, IMB, Baksan SN 1987A detections; Nobel Prize 2002]] |
| | |
| | ## Why ghosts shape galaxies |
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| | For all their elusiveness, neutrinos are cosmological giants. They are so numerous and so light that their combined mass once influenced how matter clumped in the early universe — because they stream freely through space, they smooth out structure on small scales: **the precise value of the neutrino mass measurably shapes the map of the galaxies**, which is why cosmologists extract neutrino-mass limits from the cosmic microwave background and galaxy surveys. The smallest signal leaves its handwriting on the largest structure — the recurring lesson of the [[zeptoverse]] program.[[cite: neutrino free-streaming; Planck cosmological neutrino-mass bounds]] |
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| | Neutrinos may also be why we're here: leading theories of the matter–antimatter asymmetry (leptogenesis, CP violation in the neutrino sector) run through neutrino physics. The lightest particle may carry the heaviest answer.[[cite: leptogenesis literature]] |
| | |
| | ## Hunting them today |
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| - | The instruments are cathedrals built for ghosts: **Super-Kamiokande** (a 50,000-ton water tank under Mount Ikeno, Japan), **IceCube** (a cubic kilometre of Antarctic ice at the South Pole), **KATRIN** (a 200-ton spectrometer in Karlsruhe), **DUNE** (the giant long-baseline experiment, Fermilab→South Dakota), and **JUNO** (China). Each catches a handful of events to answer questions about everything.[[cite: experiment documentation]] |
| + | The instruments are cathedrals built for ghosts: **Super-Kamiokande** (a 50,000-ton water tank a kilometre underground in Kamioka, Japan), **IceCube** (a cubic kilometre of Antarctic ice at the South Pole), **KATRIN** (a 200-ton spectrometer in Karlsruhe), **DUNE** (the giant long-baseline experiment, Fermilab→South Dakota), and **JUNO** (China). Each catches a handful of events to answer questions about everything.[[cite: experiment documentation]] |
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| | ## See also |
| | |
| | * [[zeptoverse]] — the realm of the smallest signals |
| | * [[smallest things ever measured]] — the neutrino's place on the list |
| | * [[proton decay]] — the other thing the giant underground tanks hunt |
| | * [[why does matter exist]] |
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| | ## References |
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| | - Reines & Cowan — first neutrino detection, Savannah River (1956); Nobel 1995 |
| | - Kamiokande-II / IMB / Baksan — SN 1987A neutrino burst (1987); Nobel 2002 |
| | - Super-Kamiokande — atmospheric neutrino oscillation (1998); SNO — solar neutrino solution (2002); Nobel 2015 |
| | - KATRIN collaboration — neutrino mass upper limits (2022–2025) |
| | - Pauli 1930 letter; Fermi 1934 beta-decay theory |
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| | [[Category:Science]] |
| | [[Category:Physics]] |
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