Timeline of Particle Physics
Subatomic particles, accelerators and the Standard Model, from J. J. Thomson's identification of the electron in 1897 to the discovery of the Higgs boson at the Large Hadron Collider in 2012.
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- 32 events 1897–2012
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1897: J. J. Thomson identifies the electron
At Cambridge's Cavendish Laboratory, J. J. Thomson deflected cathode rays with electric and magnetic fields and found a mass-to-charge ratio over 1,000 times smaller than a hydrogen ion's. Unlike Emil Wiechert and Walter Kaufmann, who measured similar ratios, he concluded that the rays were "corpuscles", a new particle common to all atoms; the name "electron" came from George Johnstone Stoney's 1891 term for the unit of charge.
Wikipedia1919: Rutherford finds the proton in atomic nuclei
Ernest Rutherford, who in 1911 had explained Hans Geiger and Ernest Marsden's alpha-particle scattering at Manchester by concentrating most of an atom's mass and a large charge in a tiny central region, later called the nucleus, found that alpha particles striking nitrogen released hydrogen nuclei. He concluded that the hydrogen nucleus was a constituent of all heavier nuclei and in 1920 named it the proton.
Wikipedia1928–1931: Dirac's electron theory predicts antimatter
Paul Dirac's relativistic wave equation for the electron explained its spin but also allowed states of negative energy. After first suggesting that the resulting "holes" were protons, Dirac accepted objections from Robert Oppenheimer and Hermann Weyl and in 1931 predicted an "anti-electron" with the electron's mass and opposite charge, adding that the proton should likewise have an "anti-proton".
Wikipedia1930: Pauli proposes the neutrino
In a letter to Lise Meitner and other colleagues that opened "Dear radioactive ladies and gentlemen", Wolfgang Pauli proposed that beta decay also emits an unseen light, neutral particle, so that energy and angular momentum would be conserved. Enrico Fermi, who popularized the name neutrino, made the particle part of his 1933–1934 theory of beta decay, the forerunner of the modern theory of the weak interaction.
Wikipedia1931: Lawrence and Livingston build the first cyclotron
At the University of California, Berkeley, Ernest Lawrence and his student M. Stanley Livingston ran the first working cyclotron, in which a magnet holds charged particles on a spiral while an alternating voltage accelerates them again and again. Lawrence won the 1939 Nobel Prize; in 1932 John Cockcroft and Ernest Walton at Cambridge used a high-voltage accelerator to split lithium nuclei, the first accelerator-induced nuclear disintegration.
Wikipedia1932: Chadwick discovers the neutron
At the Cavendish Laboratory in Cambridge, James Chadwick showed that the penetrating radiation that Walther Bothe and Herbert Becker, and then Irène Joliot-Curie and Frédéric Joliot, had produced by striking beryllium with alpha particles consisted of neutral particles about as heavy as the proton. Within months, Dmitri Ivanenko and Werner Heisenberg described nuclei as protons and neutrons; Chadwick won the 1935 Nobel Prize.
Wikipedia1932: Anderson discovers the positron
In a magnetized cloud chamber at the California Institute of Technology, Carl Anderson photographed a cosmic-ray particle with the electron's mass but positive charge: the positron, the first known antiparticle, matching Paul Dirac's predicted anti-electron. Patrick Blackett and Giuseppe Occhialini confirmed it in Cambridge in 1933; Anderson shared the 1936 Nobel Prize with Victor Hess, who discovered cosmic rays on balloon flights in 1912.
Wikipedia1935: Yukawa predicts the meson
In a paper submitted in 1934, Hideki Yukawa of Osaka Imperial University proposed that protons and neutrons were bound by exchanging a new particle, and from the short range of the nuclear force estimated its mass at about 200 times the electron's. He suggested that the particle, later called a meson, might be found in cosmic rays; Yukawa received the 1949 Nobel Prize in Physics after the pion's discovery.
Wikipedia1936–1937: Muon discovered in cosmic rays
Carl Anderson and Seth Neddermeyer at the California Institute of Technology, and J. Curry Street and Edward Stevenson at Harvard, found cosmic-ray particles with a mass between the electron's and the proton's. Taken at first for Hideki Yukawa's nuclear-force particle, the muon was shown in Rome in 1946 to barely interact with nuclei: it proved to be a heavier relative of the electron, the first known particle of a second generation of matter.
Wikipedia1944: Veksler and McMillan conceive the synchrotron
Vladimir Veksler in Moscow and, independently in 1945, Edwin McMillan of the University of California found the principle of phase stability, by which particles stay in step with an accelerating voltage while its frequency or the magnetic field is varied. It lifted the cyclotron's relativistic energy limit and led to the synchrotron, used from the Bevatron to the Large Hadron Collider; the two shared the 1963 Atoms for Peace Award.
Wikipedia1947: Bristol physicists discover the pion in cosmic rays
César Lattes, Giuseppe Occhialini, Cecil Powell and colleagues at the University of Bristol found the pion, which decays into the muon, in photographic emulsions exposed to cosmic rays in the Pyrenees and the Andes; it was the strongly interacting particle Hideki Yukawa had predicted. In Manchester, George Rochester and Clifford Butler saw the first "strange" particles, now called kaons, in a cloud chamber.
Wikipedia1947–1949: Quantum electrodynamics made finite by renormalization
After Willis Lamb and Robert Retherford measured a tiny shift in hydrogen's energy levels and Hans Bethe calculated it by absorbing infinities into the electron's measured mass, Shin'ichirō Tomonaga, Julian Schwinger and Richard Feynman independently recast quantum electrodynamics in covariant form with finite predictions at every order. Freeman Dyson showed their methods equivalent; Tomonaga, Schwinger and Feynman shared the 1965 Nobel Prize.
Wikipedia1954: CERN founded by 12 European states
On September 29, the convention establishing the European Organization for Nuclear Research (CERN), signed in 1953 by 12 European states, came into force, creating a shared particle physics laboratory near Geneva. CERN's Proton Synchrotron, started in 1959, was briefly the world's highest-energy accelerator, and CERN later hosted the discoveries of weak neutral currents, the W and Z bosons and the Higgs boson.
Wikipedia1954: Yang and Mills generalize gauge theory
Working at Brookhaven National Laboratory, Chen Ning Yang and Robert Mills extended the gauge symmetry underlying electromagnetism to non-commuting symmetry groups, describing force-carrying fields that interact with one another. The theory's massless force carriers seemed unphysical at first, but Yang–Mills theories became the basis of the electroweak theory and quantum chromodynamics; Ronald Shaw reached similar equations independently.
Wikipedia1955: Antiproton discovered at Berkeley's Bevatron
Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Thomas Ypsilantis identified antiprotons among the particles produced by the Bevatron, a proton synchrotron at Berkeley, California, built to be energetic enough to create them. The discovery showed that the proton, like the electron, has an antiparticle; Segrè and Chamberlain shared the 1959 Nobel Prize in Physics.
Wikipedia1956: Cowan and Reines detect the neutrino
Clyde Cowan and Frederick Reines of Los Alamos confirmed Wolfgang Pauli's neutrino by detecting antineutrinos from a reactor at the Savannah River Plant in South Carolina, through the paired signals of a positron and a neutron produced when one struck a proton in water. Reines shared the 1995 Nobel Prize, Cowan having died in 1974; in 1962 Leon Lederman, Melvin Schwartz and Jack Steinberger found a second, muon-type neutrino.
Wikipedia1956–1957: Weak interaction found to violate parity
Tsung-Dao Lee and Chen Ning Yang pointed out that mirror symmetry, or parity, had never been tested in weak interactions. Chien-Shiung Wu, with a low-temperature team at the National Bureau of Standards, found that cobalt-60 nuclei aligned near absolute zero emitted beta-decay electrons asymmetrically, showing that the weak force distinguishes left from right; Lee and Yang won the 1957 Nobel Prize, which Wu did not share.
Wikipedia1961: Gell-Mann and Ne'eman devise the Eightfold Way
Murray Gell-Mann and, independently, Yuval Ne'eman sorted the growing zoo of strongly interacting particles into families linked by an approximate symmetry, a scheme Gell-Mann called the "Eightfold Way". Gell-Mann predicted in 1962 a missing member with strangeness minus three, the Omega-minus, and a 33-physicist team at Brookhaven National Laboratory found it in 1964 with an 80-inch bubble chamber.
Wikipedia1964: Gell-Mann and Zweig introduce quarks
Murray Gell-Mann and, independently, George Zweig proposed that strongly interacting particles are built from three kinds of constituents with fractional electric charge, now called the up, down and strange quarks. Gell-Mann took the name "quark" from James Joyce's "Finnegans Wake", while Zweig called them aces; many physicists at first doubted that quarks were physical objects rather than a mathematical device.
Wikipedia1964: Cronin and Fitch discover CP violation
At Brookhaven National Laboratory, James Cronin, Val Fitch, James Christenson and René Turlay saw long-lived neutral kaons sometimes decay into two pions, a decay forbidden if physics is unchanged when particles are swapped for antiparticles and mirror-reflected, the charge-conjugation–parity (CP) symmetry. Cronin and Fitch shared the 1980 Nobel Prize; Andrei Sakharov later argued that CP violation is needed for the universe's excess of matter.
Wikipedia1964: Three teams propose the Higgs mechanism
Building on work by Yoichiro Nambu and Philip Anderson, papers by François Englert and Robert Brout, by Peter Higgs, and by Gerald Guralnik, C. Richard Hagen and Tom Kibble showed how force carriers in a gauge theory can gain mass through spontaneous symmetry breaking without the massless particles required by Goldstone's theorem. Higgs's paper explicitly pointed to a new massive particle, later called the Higgs boson.
Wikipedia1967: Electroweak theory unifies weak and electromagnetic forces
Steven Weinberg and, independently in a 1968 paper, Abdus Salam used the Higgs mechanism to give mass to the weak force's carriers, the W and Z bosons, in the unified model Sheldon Glashow had proposed in 1961, leaving the photon massless. Gerard 't Hooft, a student of Martinus Veltman, proved in 1971 that such theories can be renormalized, and Glashow, Salam and Weinberg shared the 1979 Nobel Prize in Physics.
Wikipedia1968: Electron scattering reveals point-like particles in protons
Physicists from the Stanford Linear Accelerator Center (SLAC) and the Massachusetts Institute of Technology, led by Jerome Friedman, Henry Kendall and Richard Taylor, fired electrons from SLAC's 3.2-kilometer accelerator at protons and found unexpectedly many scattered at large angles with large energy losses. Published in 1969, the results revealed point-like constituents later identified as quarks; the three leaders shared the 1990 Nobel Prize.
Wikipedia1973: Gross, Wilczek and Politzer discover asymptotic freedom
David Gross and Frank Wilczek, and independently David Politzer, found that in Yang–Mills theories of quarks the strong force weakens at short distances, so quarks inside protons act almost as free particles in high-energy collisions. With the theory of colored quarks and gluons of Harald Fritzsch, Heinrich Leutwyler and Murray Gell-Mann, this established quantum chromodynamics, the strong-force part of the Standard Model; the three shared the 2004 Nobel Prize.
Wikipedia1973: Gargamelle detects weak neutral currents
A seven-laboratory collaboration formed by André Lagarrigue, using the Gargamelle bubble chamber at the European Organization for Nuclear Research (CERN), found neutrino interactions in which the neutrino scattered without becoming a charged lepton, the mark of an exchanged neutral Z boson. Announced on July 19, these weak neutral currents were the first experimental evidence for the Z and a major step toward confirming electroweak theory.
Wikipedia1974: J/ψ discovery reveals the charm quark
Teams led by Burton Richter at the Stanford Linear Accelerator Center and Samuel Ting at Brookhaven National Laboratory announced on November 11 the same narrow, long-lived particle, named ψ and J respectively. A charm quark bound to its antiquark, the fourth quark that the Glashow–Iliopoulos–Maiani mechanism had required in 1970, it convinced the physics community of the quark model's validity; Richter and Ting shared the 1976 Nobel Prize.
Wikipedia1975: Perl's team discovers the tau lepton
Martin Perl's group from the Stanford Linear Accelerator Center (SLAC) and Lawrence Berkeley Laboratory found electron–positron collisions yielding an electron, a muon and undetected particles, the trace of a new charged lepton about 3,500 times heavier than the electron. The tau, the first known particle of a third generation of matter, earned Perl a share of the 1995 Nobel Prize; its partner, the tau neutrino, was observed at Fermilab in 2000.
Wikipedia1979: Gluon observed in three-jet events
At the Positron–Electron Tandem Ring Accelerator (PETRA) of the Deutsches Elektronen-Synchrotron (DESY) laboratory in Hamburg, experiments saw electron–positron collisions produce three jets of particles rather than two, the third coming from a gluon radiated by a quark. Predicted by John Ellis, Mary K. Gaillard and Graham Ross, such three-jet events remain the most direct evidence for gluons, the carriers of the strong force.
Wikipedia1983: W and Z bosons discovered at CERN
Teams at the European Organization for Nuclear Research (CERN) led by Carlo Rubbia and Pierre Darriulat announced the W boson in January and the Z boson in June and July, as electroweak theory predicted, using the Super Proton Synchrotron as a proton–antiproton collider. Rubbia and Simon van der Meer, whose beam cooling made that possible, shared the 1984 Nobel Prize; in 1989, Z measurements at CERN and Stanford found three light neutrino types.
Wikipedia1995: Fermilab experiments discover the top quark
The Collider Detector at Fermilab (CDF) and DØ collaborations at Fermilab's Tevatron collider jointly announced the top quark on March 2. Partner of the bottom quark found at Fermilab by Leon Lederman's team in 1977, it completed the third quark generation that Makoto Kobayashi and Toshihide Maskawa had proposed in 1973 to explain the charge–parity (CP) violation seen in kaon decays; it is the heaviest known elementary particle.
Wikipedia1998: Super-Kamiokande finds that neutrinos oscillate
The Super-Kamiokande detector, about 50,000 metric tons of water 1,000 meters underground in Kamioka, Japan, found a shortfall of atmospheric muon neutrinos that depended on their direction of arrival, the first evidence of atmospheric neutrino oscillation. Oscillation implies that neutrinos have mass, which the Standard Model had omitted; Takaaki Kajita shared the 2015 Nobel Prize with Arthur McDonald of the Sudbury Neutrino Observatory.
Wikipedia2012: Higgs boson discovered at the Large Hadron Collider
On July 4, two collaborations at the Large Hadron Collider (LHC) near Geneva, A Toroidal LHC Apparatus (ATLAS) and the Compact Muon Solenoid (CMS), announced a new boson with a mass of about 125 gigaelectronvolts (GeV), later confirmed as a Higgs boson. Its field gives mass to the W and Z bosons and, through Yukawa couplings, to quarks and charged leptons, while most of the proton's mass comes from the strong force; François Englert and Peter Higgs shared the 2013 Nobel Prize.
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