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8 Changing Descriptions of Nuclear Matter In the preceding chapter, we discussed the exciting opportunity of using relativistic nuclear collisions to produce in the laboratory a previously unobserved form of matter-one whose properties are of fundamental importance in understanding the basic forces of nature and the early moments in the evolution of the universe. While pursuing this goal, it is essential to remember that many properties of nuclear matter under more conventional conditions are not yet well under- stood. An improved description of nuclear matter would represent a critical advance in addressing one of the most difficult and important questions in physics: how does nature build stable structures from smaller, more elementary building blocks? We now understand that the most elementary building blocks of nuclei are quarks and gluons. However, the problem of describing nuclear matter completely in terms of quarks and gluons is at present intractable. The fundamental theory of the strong interaction, quantum chromodynamics (QCD), cannot yet be solved for the force between quarks when they are separated by distances comparable with the size of a nucleon. Thus, QCD indicates the existence of but does not provide a practical treatment for the crucial transition region between the short-distance regime, where the color force between quarks and gluons is in evidence, and the confinement region, where it is hidden within the exchange of mesons between baryons. 150
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CHANGING DESCRIPTIONS OF NUCLEAR MATTER 151 Is the short-distance quark-gluon regime important in the description of ordinary nuclear matter, or do the neutrons and protons stay far enough apart that they neither significantly affect their internal sub- structure nor are affected by it? If the latter is true, can we develop a suitable quantum field theory of the baryon-meson, i.e., hadron, interactions-"quantum hadrodynamics" (QHD)-that can accurately describe the substantial influence of meson exchange within the nuclear many-body system? These are central questions to be ad- dressed by the next generation of nuclear-physics experiments and theories. An important part of the experimental program will be carried out at the 4-GeV Continuous Electron Beam Accelerator Facility (CEBAF) proposed by the Southeastern Universities Research Association. Energetic electrons will interact in well-understood ways with the particles relevant to each possible level of description of nuclei and should thus help to reveal the relative roles of nucleons, mesons, and quarks. Experiments at other accelerators will utilize beams of several- GeV protons to probe the short-distance aspects of nucleon-nucleon interactions inside and outside nuclei. Intense intermediate-energy beams of mesons will be used to implant unusual baryons in nuclei, and low-energy proton-antiproton collisions will study the short-distance phenomenon of particle annihilation under the influence of the strong force. Theoretical progress will hinge on finding a prescription for a smooth transition from a hadronic to a quark-gluon description of nuclear matter. A successful many-body theory must, of course, improve on existing theoretical accounts of the detailed properties of ordinary nuclear matter that have been inferred from many years of investigation of nuclear structure. In addition, however, we expect it to provide a framework for understanding how the properties of nuclear levels evolve under more and more extreme conditions of excitation, angular momentum, or ratio of proton and neutron numbers. It is thus important to extend current studies of nuclear reactions that produce such unusual conditions even when the experiments are not directly sensitive to the presence of particles other than nucleons in the nucleus. QUARKS IN NUCLEI The most fundamental building blocks of atomic nuclei, the quarks, interact with each other via the exchange of gluons, thereby creating mesons, baryons, and, ultimately, nuclei. Very little is known about
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152 NUCLEAR PHYSICS the role of quarks in whole nuclei. Apart from the fact that quarks are asymptotically free when very close to each other, and totally confined from escaping as individual quarks to large distances, almost nothing is known about their behavior. Thus far, our best information about quarks in nuclei has come from studies using photons, electrons, and muons. Recently, studies of electron scattering with precise, intense beams of particles at Stanford, MIT, and several European and Japanese laboratories have revealed much about the nature of the quark structure of nuclei, as has the recent work of the European Muon Collaboration, discussed in Chap- ters 2 and 3. Work done at Stanford over a decade ago showed that the proton is in fact composed of three fractionally charged quarks that, surprisingly, interact weakly when they are close together inside their confining bag. Later work at other laboratories uncovered peculiar structural anomalies in the nucleus of helium-3, which apparently has a dip in the central region of its matter distribution. This finding, as well as similar ones in other light nuclear systems, will likely be explainable only after mesons and quarks are fully incorporated into our descrip- tions of nuclear matter. These issues will be explored in the future at CEBAF. If nucleons inside ordinary nuclei spend enough time sufficiently close together that they have an appreciable probability of merging into bags containing six or more quarks, then the description of associated nuclear properties will require explicit treatment of the quarks and gluons. To probe this possibility, it is important to study systematically the correlations in the motions of pairs of nucleons within nuclei. An effective way of carrying out such investigations makes use of electron beams to knock nucleon pairs out of the nucleus. By detecting the scattered electron and the ejected nucleons in time coincidence, one can study short-distance two-body correlations in the nucleus. Exper- iments of this sort require electron beams of high energy to transfer the requisite momentum to the target nucleus and high duty factor for clean and efficient identification of events in time coincidence; they are thus ideally suited for CEBAF. Additional quark aspects of the strong interaction can be probed by studying other selected features of it. For example, it is now known that parity is not strictly conserved in proton-proton scattering. The tiny but measurable deviations arise from the very-short-range weak force between nucleons. In order to account quantitatively for the parity violations, one must understand both the strong force and the weak force at very short distances, because the interplay of the two produces the effect. Recent experiments have suggested that the
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CHANGING DESCRIPTIONS OF NUCLEAR MATTER 153 observed parity violation is 10 times larger at 5-GeV than at 50-MeV proton energies. Exploration and theoretical treatment of the interme- diate-energy region should stringently test models based on QCD, in which the forces between hadrons are built up from the forces among their constituent quarks. The experiments would require high- intensity, high-quality beams of spin-polarized protons throughout the few-GeV energy region. Another quark-related program of experiments using such proton beams would involve searches for so-called dibaryon resonances. The normally occurring hadrons fall into two classes: the baryons and the mesons, consisting, respectively, of three quarks and of a quark- antiquark pair confined inside a bag. Quark models, however, also predict the existence of more exotic combinations, for example, six-quark bags, which do not for reasons related to the distribution of quark colors inside the bag readily separate into two normal baryons. Such six-quark objects, or dibaryons, might be manifested as reso- nances in nucleon-nucleon scattering experiments at energies above 1 GeV, that is, as sharp variations with energy in the probability of scattering or in its dependence on the spin orientations of the two nucleons. Excellent opportunities for the further study of six-quark physics will be afforded by the new Low-Energy Antiproton Ring (LEAR) recently constructed at CERN. In antiproton-proton collisions, one will have the chance to study the interaction between quarks and antiquarks in a rather uncomplicated way. A collision between matter and antimatter can lead to an intermediate state of pure energy, which can subsequently form many interesting and varied final states, few of which have been extensively investigated. Of special interest is the proton-antiproton "atom," in which the positively charged proton captures a slowly moving, negatively charged antiproton, pulling it into an atomic orbit. Here one would search for transitions between the atomic bound states (due to the Coulomb force) and the very deeply bound states (due to the strong interaction), which would signify for the first time the existence, however fleeting, of the so-called baryonium states. These states are formed very rarely, if ever, because matter-antimatter collisions at close range almost always lead to total annihilation. The confirmation of such events would open an exciting new field of study. Along similar lines, other atomic systems never before seen could be prepared at the LEAR facility. At the proper energy, the antiproton- proton collision can lead to production of other particle-antiparticle final states. Since these objects are oppositely charged, at the threshold
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154 NUCLEAR PHYSICS for the reaction they will be bound together in an atomic state because of the electrical attraction between them. Some of the completely new systems formed in this way can be used to check the most detailed predictions of quantum electrodynamics. As the system decays, the particles come closer together, until the strong force takes over and the system is annihilated. Here too, the opportunity for studying unknown details of the reaction is presented. Intense beams of kaons can also be very useful in the study of dibaryons, because they permit systems with one or even two strange quarks to be formed. One of the most exciting predictions of the bag models of hadrons is the existence of a stable, doubly strange dibaryon called the H particle, with a predicted mass around 2.15 GeV. Even if it is not stable, the relatively low mass of this dibaryon means that it should be fairly easy to separate it from other events that would confuse its identification. The experiments would still be difficult, however, because they typically involve two steps: the production of a very-short-lived hyperon, the cascade particle, followed by the inter- action of the hyperon with a nucleon in the target. Many other strange dibaryons have been predicted; observation of these objects would be an important confirmation of the dynamics of the quark model. Finally, based on our experience with other quantum many-body systems, we can expect great opportunities for discovery in physics arising from the underlying quark-gluon nature of nuclear matter. Even when the forces in question are better understood and more tractable (as in quantum electrodynamics) than the strong force, unpredicted phenomena can still appear. Had it not been for the experimental discovery of superconductivity, for example, this phenomenon would not have emerged from our theoretical understanding of the electro- magnetic force in the form of QED. MESONS AND BARYON RESONANCES IN NUCLEI It has been known for many years that neutrons and protons interact via the exchange of virtual mesons. On even the simplest level, therefore, the nucleus must contain, in addition to nucleons, the force-carrying mesons. Searching for direct evidence of their presence has nevertheless been an elusive chase, because seeing them requires particle beams of very short wavelength. One of the oldest and least ambiguous ways of examining nuclei is to irradiate them with beams of light of extremely short wavelength (gamma radiation); this interaction can result in the photodisintegration
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CHANGING DESCRIPTIONS OF NUCLEAR MATTER 155 of the nucleus, the mechanism of which is then studied. When applied to the deuteron near the threshold for its breakup, such studies gave the first experimental results that required the presence of mesons in nuclei for the data to be understandable. Within the last decade, much progress along these lines has been made using high-energy electron beams. As mentioned in Chapters 2 and 3, these studies have produced results in light nuclei that can be explained only by introducing the electric currents and distributions of magnetism due to the exchanged mesons themselves. Work at several laboratories is progressing on this subject, and the eagerly anticipated 4-GeV electron accelerator (CEBAF) will greatly extend our knowl- edge of it. As might be expected, most of our knowledge of nuclear properties comes from experiments using electrons, protons, and pions to probe the most probable configurations of nucleons in a nucleus; these are the configurations that predominate under ordinary conditions. Recent experimental and theoretical advances have now also made it possible to perform (and understand) experiments designed to examine highly improbable configurations in which, for example, two nucleons are very close together, several nucleons are clustered together as a unit, or one nucleon is moving much faster than the average speed of the others. Most such experiments, which include electron and proton scattering as well as the production of exotic particles from the nucleus, take advantage of processes that could not occur if the nucleus were composed only of relatively isolated nucleons. These studies are expected to reveal much about the quark structure of the nucleus, the nature of the nucleon-nucleon interaction at short distances, and the ways in which the motions of several nucleons might be correlated in the nuclear environment. Using selective reactions to probe and identify correlations will help us understand the degree to which certain states of nuclear excitation can be characterized as nuclear molecules or as relatively unexcited clusters of nucleons, rather than as a nucleon gas in which all the particles move rapidly and independently of one another. To understand better how the nuclear many-body system is con- structed, physicists have devised methods for implanting particle impurities inside nuclei and studying the effects of such changes on the nuclear system. The usual way of implanting an impurity in a nucleus is to bombard the latter with a beam of pions or kaons. When these particles interact with neutrons or protons, a baryon resonance can be formed inside the nucleus. Examples of such excited baryon species
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156 NUCLEAR PHYSICS are the N* and the delta, which are made in pion-nucleon interactions, and the Y*, which is made in kaon-nucleon interactions. Although the lifetimes of these species inside the nucleus are very short (even by nuclear standards), they are long enough to allow modifications of the nuclear medium and of the baryon resonances themselves to be examined. Under the right (gentler) conditions, bombardment of nuclei- with negative kaons can produce lambda hypernuclei, in which a relatively long-lived lambda hyperon is formed within a nucleus rather than within a baryon resonance. Here too, it is not just the nucleus that is modified; the properties of the hyperon itself (such as its lifetime) may change substantially from the free-particle values. Measurement of such modifications will help us understand more about the detailed nature of the interactions taking place. Plans for the future include the study of the properties of exotic nuclei made with other kinds of strange-particle implantations, as well as the creation of such rare objects as double hypernuclei, which contain two imbedded hyperon . impurities. NUCLEAR PROPERTIES UNDER EXTREME CONDITIONS Nuclear spectroscopic measurements using elastic and inelastic scattering reactions as well as a variety of single-particle and multiparticle transfer reactions to study the properties of nuclear energy levels and their decays have provided most of our knowledge about the behavior of nuclear systems. While some nuclear physicists are trying to understand the roles of mesons and quarks in nuclei, others are pursuing the study of the properties of nuclear levels (nuclear wave functions) under more and more extreme conditions of such parameters as excitation, angular momentum, and proton/neutron number. The use of more powerful accelerators and more sophisticated detectors will continue to extend our knowledge of the nuclear many-body systems, so that we can refine our nuclear models by testing them under more extreme conditions. While it is clear that we are just now opening up an exciting frontier in the study of the role of subnucleonic constituents in nuclei (as discussed above), a critical test of these new, more microscopic descriptions will have to be their ability to describe accurately the properties of real nuclei and their energy levels. Some of these conditions can be explored by inducing collisions between nuclei at speeds greater than the speed of sound in nuclear
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CHANGING DESCRIPTIONS OF NUCLEAR MATTER 157 matter. As illustrated by the sonic boom of an airplane, dramatic phenomena can occur when the sound barrier is exceeded. In nuclei, however, the speed of sound is 105 times greater than it is in air! It is therefore gratifying that nuclear accelerators now allow studies of collisions between heavy nuclei at such speeds, which correspond to energies intermediate between those used to study nuclear spectros- copy and those that will be required to induce the transition to a quark-gluon plasma. Under such conditions, we hope to investigate such phenomena as nuclear shock waves, compression of nuclear material, and the complete disintegration of a nucleus into lighter fragments or even its constituent nucleons. Nuclear properties are expected to change drastically in this region, from the fluidlike coop- erative behavior of many nucleons at very low energies to a succession of many individual nucleon-nucleon collisions at high energies. The experimental problems posed by studies of this transition region are challenging. New accelerators at Michigan State University, the Chalk River Nuclear Laboratories in Canada, and GANIL in Caen, France, will provide the necessary beams. Sophisticated instruments capable of detecting and analyzing the many particles (of the order of 100) in the debris of such collisions must be designed and built, and we must learn how to process and interpret the flood of data from such experiments to reveal the underlying physical phenomena (see Figure 8.11. The theoretical challenges arejust as great, since a conceptual and computational framework must be developed for describing a region in which simplifying assumptions present at very low or very high energies are not valid. Related subjects for future research will include such topics as the properties of nuclear systems with very high angular momentum, up to values beyond which the nuclei are torn apart by centrifugal forces. Another extreme condition is a large excess in the proton number or neutron number of a nucleus, which will cause marked instability. Very proton-rich or neutron-rich nuclei are typically produced in reactions between two heavy elements in which many nucleons are transferred from one nucleus to the other. The study of such nuclei at or near the limits of stability against proton or neutron decay may reveal interest- ing new radioactive decay modes. A number of astrophysically important reactions, for example, the rapid capture of neutrons in supernova explosions and the rapid capture of protons on the surfaces of white dwarfs and accreting neutron stars, also depend critically on the properties of nuclei at the
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158 NUCLEAR PHYSICS _15- __ BY __ __ / Protons: /;rojectile \ ;~, / | fragment \ \ \ \ \ ~ FIGURE 8.1 The tracks left by particles emitted in high-energy nuclear collisions can be recorded photographically in a gas-filled detector called a streamer chamber (top panel; see also the cover of this book). Here an argon-40 projectile with an energy of 1.8 GeV per nucleon collided with a lead target nucleus. A charge-coupled device (in effect, a computer-controlled TV camera) reconstructed the event (middle panel). The diagram at the bottom identifies some of the charged particles produced in the collision. The length shown corresponds to about 1 meter. (After W. C. McHarris and J. O. Rasmussen, Scientific American, January 1984, p. 58.)
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CHANGING DESCRIPTIONS OF NUCLEAR MATTER 159 limits of stability. Many such nuclei can most readily be created through the use of short-lived radioactive beams as projectiles; these are produced in an initial nuclear reaction and then selected and accelerated to cause a second reaction. Several different approaches are currently being studied for producing such beams, which promise to open up completely new areas of nuclear spectroscopy.
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