Elementary-Particle Physics (1986) / Chapter Skim
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4 Elementary-Particle Physics: What We want to Know
4 Elementary-Particle Physics: What We want to Know
Pages 81-97
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From page 81... ...
Although the standard model provides a framework for describing elementary particles and their fundamental interactions, it is incomplete and inadequate in many respects. As usual, the attainment of a new level of understanding refocuses attention on many old problems that have refused to go away and raises new questions that could not have been asked before.
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From page 82... ...
Thus any explanation of this equality would be welcome to astrophysicists and cosmologists. They would also welcome the new and exceedingly weak forces expected in some grand unified theories that violate previously sacred physical laws, enabling baryons like the proton to decay.
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From page 83... ...
Just as our predecessors discovered that the atoms of previous generations can be subdivided into more elementary physical objects—culminating in the recent discovery that protons, neutrons, and other strongly interacting particles are actually made out of quarks so perhaps we too may discover that quarks and leptons are themselves divisible. - It is possible that free magnetic monopoles (particles containing an unpaired north or south magnetic pole)
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From page 84... ...
~ 1 GeV * ~ 1 MeV ~—1 keV Photon is Far Below levy FIGURE 4.1 Some examples of the range of particle masses.
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From page 85... ...
The symmetry of a gauge theory will be spontaneously broken if some gauge noninvariant scalar quantity is nonzero in the theory lowest energy state. Quarks, leptons, and intermediate bosons can then acquire masses in proportion to their couplings to this nonzero scalar quantity.
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From page 86... ...
However, the standard model is not completely unified and has three independent gauge couplings. Nevertheless, the underlying gauge principle provides hope that one might be able to find a truly unified theory.
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From page 87... ...
These interactions include the dependence of the total interaction probabilities, or cross sections, of hadrons on one another as functions of energy; the elastic scattering of hadrons, in particular at large values of angle or exchanged momentum; the detailed study of lifetimes and decay processes; and the specific production probabilities of hadrons in collision processes as functions of energy and other parameters. One particular class of strong-interaction experiments studies the ejects of the spin (intrinsic angular momentum)
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From page 88... ...
Although the simplest model provides an elegant example of how unification might occur, no preferred unified theory has yet been selected by experiment. Many specific experiments at our existing accelerators will address these issues.
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From page 89... ...
Specific studies of the decays of the Zt' into heavy quarks will determine the neutral-current interactions of the heavy quarks and also make available a rich source of heavy quarks for the study of their spectroscopy and decays. Some aspects of the strong interactions, including the reliability of QCD calculations and the way in which quarks and gluons materialize into hadrons, will also be explored at the SLC and LEP.
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From page 90... ...
are called Z° factories. LEP, there should be some systematic advantages to studying both charged and neutral intermediate bosons in the same detector, under similar production conditions.
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From page 91... ...
· The most serious structural problem is associated with the Higgs sector of the theory. In the standard electroweak theory, the interactions of the Higgs boson are not prescribed by the gauge symmetry as are those of the intermediate bosons.
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From page 92... ...
In a unified theory, the problem of the ambiguity of the Higgs sector is heightened by the requirement that there be a dozen orders of magnitude between the masses of W and Zt' and those of the leptoquark bosons that would mediate proton decay. · Gravitation is omitted from the quantum theory, although the unification scale for the strong, weak, and electromagnetic interactions is only four orders of magnitude removed from the Planck mass at which gravitational effects become strong.
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From page 93... ...
What energy scale must we reach, and what sorts of new instruments do we require? - The mystery of symmetry breaking in the electroweak theory, which is to say the nature of the Higgs sector of the theory, presents an especially important and exciting challenge to experimental highenergy physics.
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From page 94... ...
The higher beam energy required for protons simply reflects the fact that the proton's energy is shared among its quark and gluon constituents. The partitioning of energy among the constituents has been thoroughly studied in experiments on deeply inelastic scattering, so the rate of collisions among constituents of various energies may be calculated with some confidence.
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From page 95... ...
These parameters define a reasonable target for the next major facility for the study of particle physics in the United States. Whatever the physics of the TeV energy regime turns out to be, its exploration will provide sorely needed guidance for the attempts at a deeper theoretical description of nature that is now necessarily highly conjectural.
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From page 96... ...
A little over a decade ago' there was no such unanimity that quantum field theory was appropriate for describing elementaryparticle physics, and many rival approaches were being considered. These have been abandoned since gauge theories have provided such a successful description of the fundamental particles and their interactions.
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From page 97... ...
Familiar symmetries such as the equivalence of the laws of nature at all times and places and timehonored conservation laws like the conservation of electric charge may break down in the presence of intense gravitational fields. Perhaps the quantum field theory itself must be rethought or abandoned.
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