Elementary-Particle Physics Revealing the Secrets of Energy and Matter (1998) / Chapter Skim
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4 The Past 25 Years: Establishing the Standard Model
4 The Past 25 Years: Establishing the Standard Model
Pages 52-67
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From page 52... ...
THE WORLD OF ELEMENTARY-PARTICLE PHYSICS CIRCA 1972 In 1972, both theoretical and experimental particle physics were unknowingly on the verge of vast and exciting changes. A crude but effective model existed to explain how hadrons were held together by strong interactions, and there were rules to allow the calculation of weak interaction processes.
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From page 53... ...
The Electroweak Force A central idea of the Standard Model is that the electromagnetic force and the weak force are different manifestations of a single unified force called electroweak. This was not evident for many years because the weak force acts over only very short distances and is completely negligible at the atomic distance scales at which the electromagnetic force acts to bind electrons to the nucleus.
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From page 54... ...
The number of events observed was consistent with the prediction of the electroweak theory, and this discovery provided the first real evidence that theorists were on the right path. Theorists had successfully predicted the existence of new particles, but this was the first time that a fundamental particle physics interaction had first been predicted by theory and then discovered experimentally.
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From page 55... ...
Evidence that the electromagnetic and weak forces were unified was overwhelming; the next task was to actually discover the predicted gauge bosons of the weak interaction. Discovery of W and Z Bosons The second major prediction of electroweak theory was the existence of W and Z bosons.
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From page 56... ...
The traditional calculational tools developed for QED and electroweak interactions are often inadequate for QCD. Nevertheless, QCD has been verified experimentally, and there is continuing progress in developing the calculational tools necessary for precision tests of the theory.
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From page 57... ...
, electron-positron colliders with about 35 GeV energy were built at the German laboratory DESY and at SLAC. Most of the events were of the type shown in Figure 4.2(a)
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From page 58... ...
The strength of the strong force has been very difficult to measure, however, in part because quarks and gluons are confined. Nevertheless, a series of measurements of the strength of the strong interaction (g3)
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From page 59... ...
The breathtaking discovery of the charm quark in 1974 was hailed as the beginning of "the new physics." The experimental evidence was dramatic, and in an astounding quirk of fate, the charm quark was discovered simultaneously at two different laboratories with very different experiments. The Mark I collaboration at SLAC observed the charm quark using the electron-positron collider SPEAR.
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From page 60... ...
It served as the prototype for the next generation of experiments. (Courtesy of the Stanford Linear Accelerator Center.)
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From page 61... ...
After years of subsequent research, the properties of the tau lepton have been measured to be precisely as predicted for a heavier repetition of the electron, leading to the conclusion that the three generations of charged leptons are distinguished only by the large differences in mass. Discovery of the Bottom Quark The charm quark completed the second quark generation.
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From page 62... ...
The top quark was a necessary component of the Standard Model of electroweak interactions, but there was no consistent theoretical guidance as to what its mass should be. By 1988, the search had extended to the lofty mass of 41 GeV, almost 10 times the mass of the bottom quark, with no success.
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From page 63... ...
Finally, in 1994, the Collider Detector at Fermilab (CDF) experiment announced the first evidence of top quark production.
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From page 64... ...
If there were a perfect symmetry between matter and antimatter, equal amounts of each would have been produced in the very early universe and would have largely been annihilated, leaving little raw material on which to build structure in the universe. Description of the transitions of quarks from one generation to another provided a framework for understanding CP violation.
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From page 65... ...
In a truly groundbreaking experiment located deep in the Homestake mine in South Dakota, physicists have collected the first evidence of neutrinos originating from the Sun. However, the number of neutrino interactions they see is well below the calculated rate of solar neutrinos that should reach Earth.
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From page 66... ...
After years of searching, no proton decays were seen, thus eliminating the simplest grand unified theories from contention. In this case, successful experiments shaped particle physics by discovering that something did not happen.
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From page 67... ...
One measure of the breakthroughs recognized during this 25-year period is the Nobel Prizes that have been awarded for experimental or theoretical work in this field. These are listed here: bosons; · Burton Richter and Samuel Ting for the discovery of the charm quark; · Martin Pert for the discovery of the tau lepton; · Carlo Rubbia and Simon Van der Meer for the discovery of W and Z · James Cronin and Val Fitch for the discovery of particle-antiparticle asymmetry; · Sheldon Glashow, Abdus Salam, and Steven Weinberg for development of the unified electroweak theory; · Jerome Friedman, Henry Kendall, and Richard Taylor for the first observation of quarks inside the proton; · Frederick Reines for the first observation of the electron neutrino; · Leon Lederman, Melvin Schwartz, and Jack Steinberger for the experiment establishing that the muon neutrino and the electron neutrino are separate particles; and · Georges Charpak for the development of particle detectors.
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