1964
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Murray Gell-Mann and
George Zweig tentatively put forth the idea of
quarks. They suggested that mesons and
baryons are composites of three quarks or antiquarks,
called up, down, or strange (u, d, s) with spin 0.5
and electric charges 2/3, -1/3, -1/3, respectively
(it turns out that this theory is not completely accurate).
Since the charges had never been observed, the introduction of
quarks was treated more as a mathematical explanation of
flavor patterns of particle masses than as a postulate of
actual physical object.
Later theoretical and experimental developments allow
us to now regard the quarks as real physical objects,
even though they cannot be isolated.
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1964
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Since leptons had a certain pattern, several papers suggested a
fourth quark carrying another flavor to give a similar
repeated pattern for the quarks, now seen as the generations of matter.
Very few physicists took this suggestion seriously at the time.
Sheldon Glashow and
James Bjorken coin the term "charm" for the fourth
(c) quark.
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1965
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O.W. Greenberg, M.Y. Han, and Yoichiro Nambu
introduce the quark property of color charge.
All observed hadrons are color neutral.
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...1966...
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The quark model is accepted rather slowly because
quarks hadn't been observed.
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1967
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Steven Weinberg and Abdus Salam separately
propose a theory that unifies electromagnetic and
weak interactions into the electroweak interaction.
Their theory requires the existence of a neutral,
weakly interacting boson (now called the Z0) that mediates a
weak interaction that had not been observed at that time.
They also predict an additional massive boson called the
Higgs Boson that has not yet been observed.
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1968-69
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At the Stanford Linear Accelerator, in an experiment in
which electrons are scattered off protons,
the electrons appear to be bouncing off small
hard cores inside the proton.
James Bjorken and
Richard Feynman analyze this data in terms of a
model of constituent particles inside the proton
(they didn't use the name "quark" for the constituents,
even though this experiment provided evidence for quarks.)
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1970
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Sheldon Glashow,
John Iliopoulos, and Luciano Maiani recognize
the critical importance of a fourth type of quark in the context of the
Standard Model. A fourth quark allows a theory that has flavor-changing
Z0-mediated weak interactions but no flavor-changing ones.
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1973
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Donald Perkins, spurred by a prediction of the Standard Model,
re-analyzes some old data from CERN and finds indications of
weak interactions with no charge exchange
(those due to a Z0 exchange.)
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1973
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A quantum field theory of strong interaction is formulated.
This theory of quarks and gluons (now part of the Standard Model)
is similar in structure to quantum electrodynamics (QED),
but since strong interaction deals with color charge this theory is called
quantum chromodynamics (QCD). Quarks are determined to be real
particles, carrying a color charge. Gluons are
massless quanta of the strong-interaction field.
This strong interaction theory was first suggested by
Harald Fritzsch and Murray Gell-Mann.
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1973
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David Politzer, David Gross, and Frank Wilczek
discover that the color theory of the strong interaction
has a special property, now called "asymptotic freedom."
The property is necessary to describe the 1968-69
data on the substrate of the proton.
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1974
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In a summary talk for a conference,
John Iliopoulos presents,
for the first time in a single report,
the view of physics now called the Standard Model.
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1974 (Nov.)
|
Burton Richter and Samuel Ting,
leading independent experiments,
announce on the same day that they discovered the same new particle.
Ting and his collaborators at Brookhaven called this particle the
"J" particle, whereas Richter and his collaborators at
SLAC called this particle the psi particle.
Since the discoveries are given equal weight,
the particle is commonly known as the J/psi particle.
The J/psi particle is a charm-anticharm meson.
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1976
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Gerson Goldhaber and Francois Pierre find the
D0 meson (anti-up and charm quarks).
The theoretical predictions agreed
dramatically with the experimental results,
offering support for the Standard Model.
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1976
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The tau lepton is discovered by Martin Perl
and collaborators at SLAC.
Since this lepton is the first recorded particle of the third generation,
it is completely unexpected.
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1977
|
Leon Lederman and his collaborators at Fermilab discover
yet another quark (and its antiquark).
This quark was called the "bottom" quark.
Since physicists figured that quarks came in pairs,
this discovery adds impetus to search for the sixth quark -- "top."
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1978
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Charles Prescott and Richard Taylor observe a
Z0 mediated weak interaction in the scattering of
polarized electrons from deuterium which shows a violation of
parity conservation, as predicted by the Standard Model,
confirming the theory's prediction.
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1979
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Strong evidence for a gluon radiated by the initial quark or antiquark
if found at
PETRA, a colliding beam facility at the DESY laboratory in Hamburg,
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1983
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The W± and Z0 intermediate bosons demanded by
the electroweak theory are observed by two experiments using the
CERN synchrotron using techniques developed by
Carlo Rubbia and Simon Van der Meer
to collide protons and antiprotons.
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1989
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Experiments carried out in SLAC and CERN strongly
suggest that there are three and only three generations of
fundamental particles. This is inferred by showing that the
Z0-boson lifetime is consistent only with the existence of
exactly three very light (or massless) neutrinos.
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1995
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After eighteen years of searching at many accelerators,
the CDF and D0 experiments at Fermilab discover the top
quark at the unexpected mass of 175 GeV.
No one understands why the mass is so
different from the other five quarks.
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