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A brief history of particle experimentation

A set of bubble chamber tracks
Bubble chamber tracks of charged particles
Photo courtesy of Fermilab.

. The photo above shows the tracks left in a bubble chamber by tiny electrically charged subatomic particles as they travel through a special fluid that makes bubbles in the presence of electric charge. The animated neutral particle, marked N, represents a neutral elementary particle such as a neutrino colliding with one of the nuclei of the atoms in the fluid, producing a cascade of charged particles that then decay into other charged particles.
. Particle physicists have to decode tracks like those above in order to deduce basic information about the observed particles, such as the electric charge, particle spin, mass, lepton number, baryon number, parity and other quantum numbers that turn out to be useful in describing the elementary particle side of Nature.
. Now here's a short lesson in the history of elementary particle observations, which began with the discovery of the electron in 1895:

A Timeline of Particle Discoveries

1895 The electron is discovered, except electrons are called cathode rays by their discoverer.
1896 X rays and other forms of radioactivity are observed
1899 Alpha particles are discovered, and later shown to be helium nuclei consisting of two neutrons and two protons.
1911 Nuclear model of atom with heavy nucleus in the middle and light electrons orbiting around it, is proposed, and becomes accepted.
1911 Electron charge measured in an oil drop experiment indicates that all electrons carry the same electric charge.
1932 The neutron directly observed in an experiment for first time.
1932 The positron, predicted by a theorist in 1928, is discovered.
1934 Radioactive nuclei produced in the laboratory.
1937 The muon, a charged lepton like the electron only heavier and hence unstable, is discovered.
1947 Two charged pi mesons, with positive and negative charge, are discovered.
1950 The neutral pi meson is discovered.
1953 The lambda baryon and K meson are discovered.
1956 The electron neutrino, predicted by theory in 1930, is confirmed to exist.
Lots of baryons and mesons being discovered, and their properties occur in regular patterns that look as if baryons and mesons are made of smaller building blocks. Physicists exhibit a tendency to name new particles after letters in the Greek alphabet.
1961 The muon neutrino is discovered and shown to be a different particle from the electron neutrino..
1963 Quark theory postulates that protons are made of smaller particles that carry charges that come in thirds of the electron charge. The three flavors of quarks are given names: up, down and strange.
1970s Deep inelastic scattering and other experiments reveal more of the quark structure inside protons and other hadrons.
1974 A fourth flavor of quark, named charm, is detected in a newly discovered meson called the J (aka the yor Psi).
1975 The tau lepton is discovered, making a triplet of charged leptons with the electron and muon, leading to predictions of a tau neutrino to accompany the electron neutrino and the muon neutrino.
1979 A fifth flavor of quark, named bottom, is detected in the newly discovered Upsilon meson. This pattern leads particle physicists to believe they will find a sixth and final flavor of quark some day. This predicted last flavor of quark is called top.
1983 The massive gauge bosons that carry the weak nuclear force, called the W+,W- and Z0, are discovered and the Standard Model of Particle Physics is confirmed.
1989 The lifetime of the Z0 weak nuclear gauge boson is measured, and agrees precisely with there being exactly three kinds of neutrinos, and no more.
1995 The top quark is finally directly observed and measured, confirming the predictions of theorists that there are six flavors of quarks, as described in the Standard Model.
Future The search goes on for the Higgs boson (the only particle predicted by the Standard Model that hasn't been seen yet), for supersymmetric particles predicted by string theory, for proton decay and for magnetic monopoles predicted by Grand Unified Theories, and new kinds of exotic unpredicted particles is ongoing. Perhaps in a few years there will be some more interesting entries for this page. Come back later and see.




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Books from

Marie Curie: A Life (Radcliffe Biography Series)

Quantum Generations: A History of Physics in the Twentieth

Strange Beauty: Murray Gell-Mann and the
Revolution in Twentieth-Century Physics

Supersymmetry: Unveiling the Ultimate Laws
of Nature

A brief history // The Standard Model // Unification // Supersymmetry to the rescue? // Extra dimensions

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