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3 Symmetries, Forces, and Particles
Pages 33-51

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From page 33...
... A description of how the strong, weak, and electromagnetic forces arise as a consequence of three symmetries can be found below; indeed, the symmetries dictate precisely the form of these three interactions. Note: This chapter presents a picture of the rich theoretical underpinnings to the field of elementary-particle physics.
From page 34...
... Also, one might expect such a theory to generate particle masses that are much heavier than observed. The masses of elementary particles are a crucial clue in deciphering the ultimate theory of nature, and much detective work lies ahead.
From page 35...
... First, there is a direct consequence for the very nature of elementary particles themselves. Rotational symmetry leads to elementary particles' possessing a new attribute, called spin.
From page 36...
... As physicists discovered more elementary particles, they found that patterns in their properties could be understood in terms of mathematical symmetries. These newer symmetries often act in more abstract, so-called internal spaces.
From page 37...
... The circular blob represents the actual interaction between particles and is highly constrained if it arises in a theory with a local internal symmetry. If one could look inside this blob at high magnification, such a theory would dictate that the interaction results from exchange of a force particle, which for an electromagnetic interaction is called the photon, as illustrated in Figure 3.3.
From page 38...
... (In these figures, straight lines represent matter particles and wavy lines represent force particles.) Local symmetries are also called gauge symmetries, and the resulting force particles, such as the photon, are known as gauge bosons.
From page 39...
... The quantum field theory of electromagnetism, describing the electron and the photon, reached its final form in the late 1940s, but theories involving larger local internal symmetries were not fully understood until the early 1970s. Quantum field theories are the basic tool for theoretical particle physicists.
From page 40...
... . For strong and weak forces, the entries represent the size of the multiplet of particles that interacts with the corresponding force particles.
From page 41...
... The only known difference between the three generations is their mass in particular, the force particle vertices of the heavier generations are identical to those of Figures 3.4, 3.5, and 3.6 for the lightest generation. This replication of particles suggests to some that there is a new internal symmetry to be discovered that is responsible for the different generations.
From page 42...
... Spontaneous Symmetry Breaking Whereas interactions of the force particles are restricted by the three local symmetries, the observed masses of the quarks are restricted by the strong symmetry. For example, although ur, ug, and ub have the same mass, members of weak doublets, such as ve and e, do not.
From page 43...
... The Higgs boson is still a matter of speculation, lacking solid experimental support. Nevertheless, something must generate particle masses, and physicists know that this physics is inextricably linked to the mass scale of the W and Z particles.
From page 44...
... This breaking is described by a single extra parameter that enters the flavor-changing vertices of weak interactions. The parameter is capable of describing all of the CP violation observed to date.
From page 45...
... The massless photon can be understood in terms of the electromagnetic symmetry, and the mass of the proton follows from the dynamics generated by the strong symmetry. Hence, it is only the mass scale of weak interactions, which leads to heavy W and Z particles, that is not constrained by a symmetry principle.
From page 46...
... Technicolor, if it exists, would be a new strong force similar in many ways to the known strong, or color, force. In the same way that the strong force is responsible for the masses of the proton and other hadrons, so the strong technicolor force could provide masses for the W and Z particles.
From page 47...
... However, at lower energies, where today's experiments are performed, the grand unified symmetry is broken and the three interaction strengths have different dependencies on particle energy, as shown in Figure 3.9. A combination of gi and g2 (measured at the energy scale of weak interactions)
From page 48...
... This dependence has been observed experimentally and can be calculated theoretically. Values for go, g2, and g3, measured at the energy scale of weak interactions, can be extrapolated theoretically to high energies where, if the theory is supersymmetric, they are found to meet, providing a visual picture of the unification of the three forces.
From page 49...
... PHYSICS OF THE PLANCK SCALE The energy scale at which the strengths of the three forces are predicted (by assuming supersymmetry) to become equal, as shown in Figure 3.9, is very high, about 10~4 times larger than the energy scale of weak interactions.
From page 50...
... String theory says that if we could look at a quark with a microscope that can resolve distances of 10-33 cm, we would not see smaller subobjects, but rather a quark would look to us like a little closed string. String theory is a natural generalization of previous theories of particles but represents a radical departure from the tradition initiated by Thales of Miletus.
From page 51...
... Moreover, string theory requires the existence of both quantum mechanics and gravity, whereas previous theories in physics make it impossible to have both together; other general predictions of string theory are gauge invariance, which has been seen to be the bread and butter of the Standard Model, and supersymmetry, which is one of the main targets in the worldwide enterprise of particle physics. Many deep problems remain to be solved before the theory can be compared directly with experiment.


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