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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.
Representative terms from entire chapter:
changing descriptions
