Nuclear Physics The Core of Matter, The Fuel of Stars (1999) / Chapter Skim
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Pages 19-46
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From page 19... ...
THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 19 19 2 The Structure of the Nuclear Building Blocks INTRODUCTION It is a remarkable fact of nature that the average distance between protons and neutrons inside an atomic nucleus is only half again as large as the individual protons and neutrons themselves. This was first revealed by applying the same experimental technique -- the scattering of electrons -- to probe the distribution of electric charge within both nuclei and protons, using electron beams of higher energy to resolve the finer spatial details inside protons.
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20 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS within neutrons, protons, or other strongly interacting particles built from quarks and gluons. All these particles, as a class, are called hadrons.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 21 THE INTERNAL STRUCTURE OF PROTONS AND NEUTRONS The strength of the QCD confining interactions leads to the picture of a nucleon, illustrated in Figure 2.1, as a seething ensemble of a large and everchanging number of constituents. A major aim of nuclear experiments through the next decade is to take detailed "snapshots" of this structure at various levels of resolution.
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22 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS can map the probability for finding the various constituents as a function of the fraction they carry of the nucleon's overall momentum. Such detailed maps provide crucial tests for QCD calculations of the nucleon structure; a number of basic features have yet to be delineated or understood.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 23 FIGURE 2.2 Two ways of probing the distribution of quarks and antiquarks inside nucleons.
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24 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS Spin polarization refers to the preferential orientation, along some chosen direction in space, of the intrinsic spins of the particles that make up a beam or a target. With techniques now available, the spins can be aligned in the same direction for as many as 90 percent of the electrons in a beam or the protons in a target.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 25 FIGURE 2.3 Among recent technological advances that facilitate dramatic improvements in measurements of neutron substructure is the development of spin-polarized 3He targets of usable density. The photograph shows a cell filled with 3He gas, which is exposed to laser light of appropriate frequency to "pump" most of the gas atoms into a state with the desired orientation of the nuclear spin.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 27 FI G U RE 2 .4 P ar ts of th e CE BA F La rg e A cc ep ta nc e Sp ec tro m et er .
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28 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS associated with the primary (valence) quarks versus the induced "sea" of quarkantiquark pairs.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 29 BOX 2.1 Sampling the Proton's Flavors How, exactly, are protons and neutrons made? Nature has guarded her secret recipes as vigilantly as the Coca-Cola Company, even to the extent of forbidding the ingredients from ever being used alone.
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30 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS determined in precise measurements of a tiny expected difference in the scattering rates when the electron beam is prepared in two different ways: with most of the electrons spinning clockwise versus counterclockwise, as seen from the electron's direction of motion. Many more questions remain to be addressed to discern nature's complete recipes for protons and neutrons.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 31 FIGURE 2.1.2 A part of the detector constructed at the University of Illinois, which is currently in use at the Bates accelerator to study small violations of mirror symmetry in electron-proton scattering. One of the collaborators is seen among the large reflectors used to focus radiation emitted by the scattered electrons.
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32 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS the proton's spin from its various constituents is a major goal of nuclear physicists. The first information has been provided by recent, polarized, deep-inelastic-scattering experiments.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 33 FIGURE 2.5 How do the proton's various constituents contribute to its overall spin? As illustrated by the upper diagram, the quarks, antiquarks, and gluons are all believed to have their own intrinsic spins, and these must contribute.
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34 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS which the real photon beams are replaced by virtual photons, emitted and absorbed when an electron interacts inelastically with a target nucleon. The proton and neutron represent the lowest energy states of a family of hadrons that can be constructed from up and down valence quarks taken in combinations of three.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 35 with the apparatus pictured in Figure 2.4, will, over the next several years, allow a dramatic increase in detailed information on the properties of these excited states. These states are referred to as resonances, rather than as particles, because they exist for only a tiny fraction of a second; one detects their excitation by looking at the particles emitted when the resonances decay or de-excite.
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36 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS mining parameter values, and in testing the self-consistency of these calculations. In particular, experimental searches for new mesons and for new excited states of the proton and neutron have the potential to unveil predicted missing links, to reveal elusive quark-gluon properties omitted from model calculations, or to suggest unforeseen symmetries in the complex QCD interactions inside hadrons.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 37 approach is devoted to finding more efficient ways of formulating the QCD problem, to ease the computing crunch. Nuclear theorists are deeply involved in these developments.
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38 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS BOX 2.2 Where's the Glue? Nuclear physicists study matter whose behavior is governed by the strongest of nature's forces.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 39 The data in the energy-sharing diagram here reveal the existence of a previously unknown meson, with properties strongly suggestive of the predicted lightest glueball. However, its not quite "democratic" decay probabilities suggest a possible Jekyll and Hyde particle, which spends part of its time as a glueball and the other part as a conventional meson.
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40 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS FIGURE 2.2.2 Data from the Crystal Barrel experiment reveal the existence of a previously undiscovered meson with mass and quantum properties near those predicted by QCD for the lightest particle made from gluons alone. The different colors indicate the relative probabilities for different ways of sharing the total energy from the proton-antiproton annihilation among three detected pi mesons.
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From page 41... ...
THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 41 The special role of the pion within QCD is that of primary agent for the breaking of chiral symmetry in nucleons and nuclei. An up or down quark can change from left-handed to right-handed (or vice versa)
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From page 42... ...
42 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS near-Goldstone bosons, pions interact with one another, and with nucleons, fairly weakly when they have low momentum. This observation suggests that the more traditional methods for solving quantum field theories when the interactions are weak may be applicable to QCD at low energies, if the theory can be expressed in terms of nucleons and pions as the main players, rather than quarks and gluons.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 43 correspondingly. It is an important goal of future experiments to search for direct evidence of such modifications of hadron properties.
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44 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS FIGURE 2.8 Results of deep-inelastic-scattering experiments carried out at CERN and SLAC that probe the distribution of quarks and antiquarks in nuclei. The results are plotted as the ratio of the probabilities for finding quarks and antiquarks with a given fraction of the average nucleon's momentum in a medium-mass nucleus versus deuterium.
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THE STRUCTURE OF THE NUCLEAR BUILDING BLOCKS 45 them (e.g., a combination of a proton and its various excited states, which may have a much smaller size than a free proton)
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From page 46... ...
46 NUCLEAR PHYSICS: THE CORE OF MATTER, THE FUEL OF STARS theory of quark confinement within hadrons. The next decade should bring enormous progress toward a meaningful confrontation of theory and experiment.
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