Nuclear Physics (1986) / Chapter Skim
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3 Fundamental Forces in the Nucleus
3 Fundamental Forces in the Nucleus
Pages 67-86
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From page 67... ...
Another kind of experiment is to measure violations of symmetry in nuclear transitions. Nuclear states have symmetries that are easy to classify and measure, and any violations can be attributed to fundamental particles that mediate the nuclear forces.
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From page 68... ...
In a grazing collision, the projectile nucleon hardly disturbs the target except for the fleeting effect of the pionic cloud of the projectile, as well as the effects of the other forces. Measurement of the scattering and absorption of pions by nuclei has provided knowledge of the hadronic interactions, supporting the idea that the symmetries embodied in the quark physics apply to the pions in the nuclear medium.
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From page 69... ...
Because the internal motions in the hypernucleus are closely related to known motions in the original nucleus, properties of the nucleonhyperon interactions can be inferred from the measured hypernuclear structure. A new class of experiments still being developed uses protonantiproton collisions at moderate energies to bridge the gap between nuclear physics and particle physics.
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From page 70... ...
The experimental results of scattering high-energy electrons from the very light nucleus helium-3 cannot be explained satisfactorily using theoretical models that take into account only the ejects of the charge and magnetism of the two protons and one neutron; one must also include the electromagnetic effects arising from the exchange of a pion or rho meson between nucleons. The meson-exchange model gives a strikingly better account of the data (see Figure 3.1~.
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From page 71... ...
To describe this unique situation, quark models are based on the assumption that the constituent quarks of a hadron are confined in an impenetrable bag or tied together by unbreakable strings, so that they cannot escape. This aspect of quark behavior is based on an astonishing characteristic of the strength of their color interaction: it is nearly zero when they are very close together (a condition called asymptotic freedom)
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From page 72... ...
The existence of a free fractional electric charge has never been convincingly demonstrated for any macroscopic object; this is explained on the basis of quark confinement. However, electron scattering from hydrogen and deuterium at the Stanford Linear Accelerator Center and neutrino scattering from a fluorinated hydrocarbon at CERN in Geneva have both produced results consistent with those predicted by a quark model based on pointlike particles having charges of -1/3 and + 2/3 (in units of the electron charge)
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From page 73... ...
An alternative explanation is that the additional quarks are part of the virtual pions in the nucleus; the lepton scattering, in eject, provides a "snapshot" of the nuclear constituents. The progress of these experiments is being closely watched by nuclear physicists and elementary-particle physicists, all of whom have much to gain from a deeper understanding of the role of quarks in nuclear structure.
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From page 74... ...
A lambda hyperon implanted in a nucleus does not modify the nucleus drastically, because a lambda is very much like a neutron: it has zero charge, about 20 percent greater mass, and only somewhat weaker interactions with nucleons. Thus a lambda hypernucleus is different from the original nucleus, but not so different as to preclude understanding.
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From page 75... ...
Quantum Chromodynamics at Low Energies It is now widely believed that quantum chromodynamics will become established as the correct theory of the strong interaction. For the region of asymptotic freedom, where the quarks are close together and interact very weakly, QCD calculations produce results in good agreement with experiment.
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From page 76... ...
According to the quark model, a proton has the quark structure uad (two up quarks and one down quark)
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From page 77... ...
The study of nuclear systems has also revealed new symmetries and conservation laws not apparent in the behavior of macroscopic objects. As theory pushes on to examine the nature of the fundamental forces at energies far beyond the reach of the largest manmade accelerators, searches for symmetry violations in the precisely calibrated environment of the nucleus may be the only viable approach for seeing the subtle residual effects predicted to occur at energies that are accessible.
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From page 78... ...
In terms of symmetry, this result is described by saying that the weak force does not behave symmetrically under reflection; in terms of conservation laws, it is described by saying that weak-force interactions do not conserve parity. The strong, electromagnetic, and gravitational forces do not appear to violate parity; why the weak force does is not understood.
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From page 79... ...
It does not explain (but does allow) the violations of parity and time-reversal invariance, it does not unify the strong force or the gravitational force with the electroweak force, and it does not predict, a priori, the observed relative strengths of the electromagnetic and weak forces.
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From page 80... ...
The theory of relativity shows that particles moving with the speed of light must have zero mass. The Standard Model admits only massless neutrinos, but in most proposed Grand Unified Theories, electron neutrinos, for example, can have a very small mass, typically between 10-8 and 1 eV.
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From page 81... ...
. The Standard Model strictly maintains the separate identities of electron neutrinos, muon neutrinos, and tauon neutrinos, in accord with the currently accepted lepton-family-number conservation laws: the total number of electrons and electron neutrinos in the universe minus the total number of antielectrons (positrons)
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From page 82... ...
The rate of change as the quantum-mechanical beats ebb and swell depends on the mass differences between the various neutrinos; equal-mass or zero-mass neutrinos retain their identities. If neutrino oscillations were observed experimentally, it would imply that at least one kind of neutrino has nonzero mass.
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From page 83... ...
Certain conditions in addition to the violation of lepton-number conservation must also be satisfied to allow neutrinoless double beta decay to occur. The neutrinoless mode is described as a two-step process: the decaying nucleus first emits one electron and a virtual antineutrino, a reaction analogous to ordinary beta decay.
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From page 84... ...
Although the necessary conditions described above stack the cards heavily against the neutrinoless mode, a single observed instance would shatter many currently held ideas. Meanwhile, considerable effort has been put into the search for two-neutrino double beta decay, despite the experimental difficulties imposed by the very long half-lives and the consequent low rates of decay.
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From page 85... ...
is believed to conserve parity strictly, but quarks also take part in the parity-nonconserving weak force, in which charged W+ or W- bosons or neutral Z° bosons are exchanged. The quark model predicts that the exchange of charged W+ or W- bosons will add to the nucleon-nucleon force a small weak-force component that does not conserve parity and that chiefly causes the isospin of a pair of interacting nucleons either to remain the same or to change by two units.
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From page 86... ...
86 NUCLEAR PHYSICS tend to cancel. Higher sensitivity should soon allow the pure neutralcomponent contribution in a nearby nucleus, Huorine-18, to be measured.
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