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1
Overview
INTRODUCTION
This fourth decadal assessment of nuclear physics by the National Research
Council (NRC) comes exactly one century after Ernest Rutherford’s discovery of
the atomic nucleus. His visionary insight marked the beginning of nuclear phys-
ics. At 100 years, nuclear physics is a robust and vital science, with technological
breakthroughs enabling experiments and computations that, in turn, are opening
diverse new frontiers of exploration and discovery and addressing deep and impor-
tant questions about the physical universe. Nuclear physicists today are advancing
the frontiers of human knowledge in ways that are forcing us to revise our view of
the cosmos, its beginnings, and the structure of matter within it. At the same time,
these advances in nuclear physics are yielding applications that address some of the
nation’s challenges in security, health, energy, and education, as well as contribut-
ing innovations in technology and manufacturing that help drive our economy.
There have been stunning accomplishments and major discoveries in nuclear
science since the last decadal assessment. Like Rutherford, today’s nuclear scientists
find that the data from well-crafted experiments often challenge them to revise
their ideas about the structure of matter. Indeed, the matter that makes up all liv-
ing organisms and ecosystems, planets and stars, throughout every galaxy in the
universe, is made of atoms, and 99.9 percent of the mass of all the atoms in the uni-
verse comes from the nuclei at their centers, which are over 10,000 times smaller in
diameter than the atoms themselves (the proton’s radius is about a femtometer, or
10−15 m, a distance scale called the “femtoscale”). Although nuclei are incredibly
9
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10 Nuclear Physics
small and dense, they are far from featureless: They are complex structures made
of protons and neutrons, which themselves are complex structures made out of
(as far as is known) elementary constituents known as quarks and gluons. Beyond
what Rutherford could possibly have imagined, nuclear physics spans an enormous
range of distance scales from well below the femtoscale upward to the scale of the
universe itself.
The United States became a powerhouse in nuclear physics in the decades
following the Manhattan Project. Today, vibrant nuclear physics programs are
found, along with large and sophisticated nuclear physics laboratories, in most of
the technologically advanced countries around the world. U.S. nuclear physicists
often involve themselves in large collaborative efforts with scientists from many
countries, carrying out experiments in the United States or abroad. Such efforts
create new opportunities and optimize the deployment of the resources needed to
germinate and sustain scientific progress and maintain intellectual leadership in
nuclear physics. Managing these resources has become essential. To this end, the
U.S. nuclear physics community has developed processes that build a community-
wide vision, identifying which pathways will be the most effective and direct to
scientific discoveries that open new vistas and drive the field. NRC’s decadal assess-
ments of nuclear physics have become one of the tools by which the field develops
its roadmap. In this report, the Committee on the Assessment of and Outlook for
Nuclear Physics (“the committee”) assesses the state of nuclear physics at a time
when it is rapidly evolving and new frontiers are opening up. It also looks at the
prospects for the field in an international context.
Nuclear physics is broad and diverse in the questions it is answering and the
challenges it faces on its many frontiers, as well as in its techniques and technolo-
gies. The committee frames this introduction with four overarching questions that
span several of the traditional subfields of nuclear physics, that are central to the
field as a whole, that reach out to other areas of science as well, and that together
animate nuclear physics today:
(1) How did visible matter come into being and how does it evolve?
(2) How does subatomic matter organize itself and what phenomena emerge?
(3) Are the fundamental interactions that are basic to the structure of matter
fully understood?
(4) How can the knowledge and technological progress provided by nuclear
physics best be used to benefit society?
Accomplishments since the last NRC decadal assessment have brought us much
closer to answering each of these four questions. In each case, recent research has
revealed new physics discoveries and opened new frontiers for exploration. The
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Overview 11
questions are multifaceted, broad, and deep, and the challenges they pose provoke
intriguing opportunities for the decade to come.
In the remainder of this introduction, these four questions are discussed in
some detail and illustrated by a few vignettes. In Chapter 2 the scientific rationale
and objectives of nuclear physics are articulated more fully. Chapter 2 is organized
according to the main science areas within the field, but the four overarching ques-
tions cross the boundaries between these subfields, linking the discipline together as
an intellectual whole while at the same time advancing on varied frontiers. Nuclear
physics has Janus-like qualities, probing fundamental laws of nature that link it to
particle physics while at the same time looking toward complex phenomena that
emerge from the fundamental laws, as in atomic and condensed matter physics, and
astrophysics and cosmology; zooming in on phenomena happening at the short-
est distance scales that our best microscopes can see and zooming out to the stars
and the cosmos. Because it sits in this liminal position between the fundamental
and the emergent, between the microscopic and the astronomical, nuclear physics
naturally addresses these central questions from varied angles, providing unique
perspectives.
HOW DID VISIBLE MATTER COME INTO
BEING AND HOW DOES IT EVOLVE?
The challenges posed by this question are shared by cosmologists, astronomers, par-
ticle physicists, and nuclear physicists alike. The universe is not entirely made of atoms
and light: It also contains dark matter and dark energy—components that are known
to exist because their gravitational influence on ordinary matter can be observed. But
all the matter that can be seen—visible matter—is made of atoms, consisting of a tiny,
compact nucleus and electrons orbiting around it. Atomic nuclei come in a very broad
range of masses and electric charges. When the charges of the negative electronic cloud
cancel out the positive charge of the nucleus, the atom is neutral. Interactions of the
electronic clouds around nuclei enable the complex chemical processes that are essential
for life and form the basis of our modern technological world. Atomic interactions are
thus dictated by the atomic nuclei, as it is their charge that determines the electronic
structure. Understanding nuclear physics and what goes on within the nuclei at the
core of all visible matter starts with understanding the origins of the nuclei, light and
heavy, and of the protons and neutrons of which they are made. How were the protons
and neutrons created during the big bang? And, how did these protons and neutrons
assemble into such a broad range of nuclei through nuclear transformations inside
stars and stellar explosions? Nuclear science in concert with astrophysics attempts to
answer these questions. The quest to understand how protons, neutrons, and nuclei
form and evolve is fundamental to understanding our origins.
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12 Nuclear Physics
One example of how nuclear physicists are learning how visible matter comes
into being is provided by experiments at two accelerators: the Relativistic Heavy
Ion Collider (RHIC) in Brookhaven, New York, and the Large Hadron Collider
(LHC) in Geneva, Switzerland. By colliding nuclei at enormous energies, scientists
are using these facilities to make little droplets of “big bang matter”: the same stuff
that filled the whole universe a few microseconds after the big bang. Using powerful
detectors, they are seeking answers to questions about the properties of the matter
that filled the microseconds-old universe that cannot be ascertained by any conceiv-
able astronomical observations made with telescopes and satellites. Since the last
decadal assessment of nuclear physics, research has shown that during the micro-
second epoch, when the temperature of the universe was several trillion degrees,
it was filled with a nearly perfect liquid that flowed with little viscous dissipation.
This basic feature of big bang matter could only be discovered by recreating such
matter in the laboratory.
As illustrated in Figure 1.1, sometime when the universe was about 10 micro-
seconds old, this hot liquid cooled enough that it condensed, forming protons and
neutrons (as well as other particles called pions), which, as far as is known, are the
first complex structures ever created. These basic building blocks of all the visible
FIGURE 1.1 Nuclear physics in the universe. Over 99.9 percent of the mass of all the matter in all the
living organisms, planets, and stars in all the galaxies throughout our universe comes from the nuclei
found at the center of every atom. These nuclei are made of protons and neutrons that themselves
formed a few microseconds after the big bang as the primordial liquid known as quark-gluon plasma
cooled and condensed. The lightest nuclei (those at the centers of hydrogen, helium, and lithium
atoms) formed minutes after the big bang. Other elements were formed later in nuclear reactions
occurring deep within the early stars. Cataclysmic explosions of these early stars dispersed these
heavy nuclei throughout the galaxy, so that as the solar system formed it contained nuclei of carbon,
nitrogen, oxygen, silicon, iron, uranium, and many more elements, which ended up forming our planet
and ourselves. SOURCE: Adapted from the Nuclear Science Wall Chart, developed by the Nuclear Sci-
ence Division of the Lawrence Berkeley National Laboratory and the Contemporary Physics Education
Project. Available at http://www.lbl.gov/abc/wallchart/index.html. Last accessed on May 30, 2012. Star
Formation image: NASA/ESA and the Hubble Heritage Team (AURA/STScI/HEIC); Formation of Heavy
Elements image: NASA/ESA/JHU/R. Sankrit and W. Blair; Today image: NASA.
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Overview 13
matter in the universe today are under intense investigation at the Thomas Jefferson
National Accelerator Facility (JLAB) in Newport News, Virginia. The facility there
hosts an accelerator that can be thought of as an electron microscope so powerful
that it can see inside protons and neutrons. Once the universe was a few minutes
old, all the remaining neutrons in the universe paired up with protons to form light
nuclei like those at the centers of helium and lithium atoms today; the remaining
protons became the nuclei of hydrogen atoms. However, a panoply of elements
exist in the world, not just hydrogen, helium, and lithium.
The processes of element synthesis go on today all across the universe, continu-
ously creating new worlds. Nuclei are the fuel that powers the burning of stars and
drives stellar explosions, some of which result in the formation of neutron stars,
which can be thought of as nuclei of giant stellar masses. New nuclei, including
those of which life is composed, are the ashes of stellar burning ejected into space
by violent explosive events and stellar winds. The nuclear reactions that synthesize
elements depend directly on the structure of the nuclei involved. This means that
the element-by-element composition of matter in the universe today depends on
features of thousands of nuclei, both the stable ones that are ordinarily seen and
the unstable ones whose presence is fleeting. Many of these short-lived radioactive
nuclei also play crucial roles in reactions taking place within the cores of nuclear
reactors. Most important, very short-lived nuclei that are close to the limits on
proton or neutron richness, beyond which no nuclei can exist, are thought to hold
the secrets to the structure and formation of many of the stable nuclear species that
surround us. There have been significant advances in the study of neutron-rich,
proton-rich, and super heavy nuclei in the last decade, but the limits of nuclear
existence still have not been demarcated. The characterization of nuclei near these
limits that are so important to understanding the origins of visible matter also
remains a challenge. Here, the Facility for Rare Isotope Beams (FRIB) at Michigan
State University will utilize beams of short-lived nuclei to access the unknown
regions of the nuclear landscape, providing new tools and new opportunities to
address the challenge.
Significant advances in astronomy since the last decadal assessment have led
to the discovery of very rare, very ancient stars whose composition reflects the
production of elements by even earlier generations of stars, in some cases reaching
back to stars formed from the debris of the very first generation of stellar explo-
sions after the big bang. These ancient metal-poor stars are beginning to provide
us with a chemical history of the galaxy, providing detailed information about the
output of element-producing processes and in some cases hinting at previously
unknown cycles of nuclear reactions responsible for making some of the elements
heavier than iron. New facilities like FRIB will allow nuclear physicists to unravel
the unknown properties of the nuclei and reactions that, in stars, are responsible
for the creation of heavy elements.
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14 Nuclear Physics
Exploring the nuclear physics of the cosmos requires a broad range of experi-
mental and theoretical approaches and can push nuclear science to its technical
limits. Two important frontiers have arisen in the last decade and will be explored
in the next decade with accelerators, detectors, and computers: (1) the fabrication
and characterization in the laboratory of unstable nuclei that nature makes in
stellar explosions and (2) the description of extremely slow nuclear reactions that
are important for the understanding of stars, where they occur on astronomical
timescales.
HOW DOES SUBATOMIC MATTER ORGANIZE
ITSELF AND WHAT PHENOMENA EMERGE?
This question has been central to nuclear physics from Day One: Rutherford’s
1911 discovery of the nuclei at the center of every atom framed it and provided the
very first step toward answering it. Rutherford discovered heavy, apparently pointlike
entities at the centers of atoms. He was correct to conclude that nuclei contain most
of the mass of an atom, but little did he know how intricate their composition and
structure would turn out to be. Nuclei are complex structures made of protons and
neutrons. The number of protons in a nucleus determines the chemistry of the atom
in which it is found—for example, all carbon nuclei have six protons, and this is what
distinguishes carbon from oxygen, which possesses eight protons. As of today, nuclei
containing as many as 118 protons have been found in nature or created in laborato-
ries. The number of neutrons in a nucleus with a given number of protons can vary
significantly. For example, although stable carbon nuclei contain either six or seven
neutrons, short-lived variants have been discovered containing anywhere from 2 to
16 neutrons. There are far more isotopes (nuclei with a specified number of neutrons
and protons) than elements (nuclei with a specified number of protons). Indeed, more
than 3,100 different isotopes are known, and many thousands of additional isotopic
species are believed to participate in element production in the stellar cauldrons of the
cosmos. Understanding the patterns and regularities of their structure is one of the
challenges of nuclear physics.
Remarkably, this challenge repeats itself at an even smaller length scale: Each pro-
ton and neutron is itself a complex structure made of (apparently) pointlike quarks,
which are continually exchanging the force-carrying particles called gluons that pro-
vide the strong interactions binding the quarks into protons and neutrons (and pions
and other short-lived complex structures). Unless, that is, one is talking about the
matter that filled the microseconds-old universe, which was so hot that the matter that
would later cool down and form protons, neutrons, and nuclei was a liquid of quarks
and gluons. The complexities of the different structural elements of subatomic matter
result in a plethora of possible states of matter at varying temperatures and densities.
Understanding the structure of nuclei, and of their constituent protons and neutrons,
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Overview 15
as well as understanding the phases and phenomena that emerge when many of them
get together, is among the grand challenges in nuclear physics. These challenges reso-
nate across the many other areas of science in which macroscopic complexity emerges
from large numbers of microscopic constituents obeying elementary rules. Some of the
questions that arise are analogous to questions in other fields: How do large numbers
of atoms organize themselves into materials: crystals, glasses, liquids, superfluids,
and gases? How do large numbers of electrons arising from the atoms that make
up these materials organize themselves to create metals, semiconductors, insulators,
magnets, and superconductors? Just as the rich and varied forms of matter that make
up the world originate in vast numbers of atoms and electrons interacting according
to elementary microscopic laws, both theory and experiments have shown that large
numbers of quarks or neutrons and protons or nuclei can also assemble themselves
into a rich tapestry of possible phases of strongly interacting matter. The question of
how many-body systems that are strongly correlated manifest new phases and new
phenomena is a major intellectual thrust across many areas of physics. Examples of
such bodies include novel superconductors, newly discovered topological patterns of
quantum entanglement and quantum phase transitions in various condensed matter
systems, warm dense plasmas, nuclear matter, quark-gluon plasma, and cold dense
quark matter.
One of the most exciting discoveries since the last decadal assessment is that the
long-assumed periodicities in nuclear structure are, in fact, not always periodic. For
about half a century, nuclei have been understood to be complex structures made
of densely packed protons and neutrons with a structural organization that exhibits
many regularities, analogous to the regularities in the structural organization of
atoms that are manifest in the periodic table (see Figure 1.2). Recent experiments
have shown that this need not always be so and have revealed that the familiar
pattern of regularities occurs only for nuclei in which the numbers of protons and
neutrons are not very different, as is the case for most known nuclei. For example,
the number of neutrons it takes to “fill a shell”—the analogue of starting a new
row in the periodic table, when structure starts to repeat itself—turns out to be
different in short-lived nuclei with many more neutrons than protons than in
stable nuclei with similar numbers of each. These recent discoveries challenge us
to extend our understanding of the structure of matter and further motivate the
study of very exotic nuclei—those that are extremely neutron-rich or extremely
proton-rich or extremely heavy. These short-lived nuclei are but one example of
the diverse patterns or phenomena that emerge as protons and neutrons organize
themselves into nuclei.
In many instances, the quest to understand emergent phenomena connects
nuclear physics directly with other areas of science in which interacting many-
particle structures are central. For example, superconductivity in metals, in which
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16 Nuclear Physics
pairs of electrons move in lockstep, and superfluidity in ultracold trapped atoms,
in which pairs of atoms are created, both have analogues in nuclei (involving pairs
of neutrons, pairs of protons, or possibly even proton-neutron pairs), in nuclear
matter within neutron stars, and in dense quark matter that may exist at the very
centers of neutron stars (where it is the quarks that form pairs). These are examples
of collective quantum mechanical phenomena that can emerge only when many
particles interact with one another. Many equally important emergent collective
phenomena involving protons and neutrons in atomic nuclei will be studied at
FRIB.
Remarkably, the basic story of having new phenomena emerge when ele-
mentary constituents organize themselves into complex structures repeats itself
within single protons and neutrons. Although they are ordinarily thought of as
the elementary constituents of nuclei, when protons and neutrons are looked at on
shorter length scales, they themselves are revealed to be complex structures. Their
CRC Ionization Energy (eV)
He
Regularities and 25 Ne
periodicities in 20 Ar
Kr Xe
atoms and nuclei 15 Rn
10
2
5
0 2
0
8 0 10 20 30 40 50 60 70 80
5 Atomic Number (Z)
0 8
2
126
Ni P
Ca Sn b
O
1-02.eps
bitmaps with some vector type & rules
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Overview 17
elementary constituents, quarks, are glued together by a force that is much stron-
ger than the familiar electric and magnetic forces and that is associated with the
exchange of elementary force-carrying particles called gluons. The experimental
discoveries of quarks and gluons and the discovery of the laws that govern how
they interact—called “quantum chromodynamics” (QCD)—are now more than
30 years old. And yet our understanding of how the properties of protons and
neutrons arise from the interactions between their elementary constituents remains
incomplete because the equations of QCD are simple to state but fiendishly dif-
ficult to solve. The underlying reason for this difficulty is that it is the interactions
themselves that are the key feature. For example, while the electric and magnetic
forces (mediated by massless photons) that bind an electron and a proton to form
a hydrogen atom contribute only a tiny fraction of a percent to the mass of the
atom, which is mostly just the mass of its constituents, it is now understood that
approximately 99 percent of the mass of the protons and neutrons comes from
FIGURE 1.2 Regularities in the patterns of nuclei and of electrons in atoms. Upper panel: The
elements in the periodic table are arranged in order of increasing atomic number, which is to say
increasing numbers of electrons and protons per atom. (Atoms are electrically neutral; the number
of protons in each atomic nucleus is balanced by the number of electrons orbiting the nucleus.)
Elements having similar chemical properties and electronic structures appear in the same groups.
This atomic periodicity, governed by the motion of the electrons in atoms, shows up in the behavior
of the atomic ionization energy measured in electron volts of energy—namely, the energy needed
to remove one electron from an atom. The chemical reactivity of an atom is determined by this
ionization energy. Large jumps in the ionization energy occur in the noble gases (helium, neon,
argon, krypton, xenon, and radon). High ionization energies mean that noble gases have very low
chemical reactivity. Lower panel: Atomic nuclei themselves offer many examples of regularities and
periodic behavior. The two-neutron separation energy (measured in millions of electron volts) is the
energy required to remove a pair of neutrons from a nucleus that contains even numbers of protons
and neutrons. This energy exhibits a sudden decrease immediately after specific “magic” neutron
numbers (2, 8, 20, 28, 50, 82, 126). Nuclei with magic numbers of neutrons are more tightly bound
than their neighbors with one extra neutron, making the former very much like noble gas atoms. (Dif-
ferent colors denote isotopes lying between proton magic numbers.) However, recent experiments
have shown that the regular pattern of separation energies and other nuclear properties does not
hold in sufficiently exotic nuclei, and that the magic numbers are “fragile.” Examples that illustrate
this phenomenon and are causing textbooks to be rewritten include the neutron-rich nuclei around
magnesium-32, marked by a dashed circle, which do not seem to know about the magic neutron
number N = 20, and the neutron-rich nuclei just to their right in the figure, which seem to almost
ignore the magic neutron number N = 28. SOURCES: (Upper right) Data from David R. Lide (ed.),
CRC Handbook of Chemistry and Physics, 84th Edition. Boca Raton, Florida: CRC Press. 2003, Sec-
tion 10, Atomic, Molecular, and Optical Physics, Ionization Potentials of Atoms and Atomic Ions;
(Lower left) E.J. Lingerfelt, M.S. Smith, H. Koura, Nuclear Masses Toolkit, 2012. Available at http://
nuclearmasses.org.
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18 Nuclear Physics
the motion of the quarks inside them and from the mediators of the strong inter-
action: massless gluons interacting with one another. The elementary masses of
the quarks are so small that they contribute only a small fraction of the mass of
the visible matter in the universe. So, the origin of 99 percent of the mass of the
visible matter in the universe can be traced back to the energy of moving quarks
and interacting gluons, according to Einstein’s famous equation, m = E/c2. The last
decade has seen tremendous growth in the development of decisive experimental
and theoretical tools that are for the first time giving us a precise look at the “shape”
of protons and neutrons—for example, at the distribution of electric charge within
them. Since quarks are the underlying charge carriers, such results are essential for
understanding how the complex structure of protons and neutrons emerges from
quarks and their QCD interactions.
One of the great surprises of the most recent decade has been the discovery
that the elementary constituents within a proton or neutron have a significant net
orbital motion, an “orbital angular momentum,” as if the nucleons have hidden
within them a circulating current of quarks and/or gluons. It has long been known
that protons and neutrons have spin, a feature that makes medical diagnoses via
magnetic resonance imaging possible. However, it was long assumed that this spin
was due to the intrinsic spins of the elementary quarks lurking inside protons and
neutrons rather than to their orbital motion. Just as the electron in a hydrogen atom
has no orbital motion (no angular momentum) when it is in its lowest energy state,
it was assumed that since protons and neutrons are the lowest energy states of three
quarks, these quarks must have no orbital motion. Instead, experiments carried out
during the last decade, discussed in more detail in Chapter 2 under “Momentum
and Spin Within the Proton,” have taught us that the spin of the proton and neu-
tron appears to be largely due to orbital motion of the quarks or gluons trapped
within them. This makes the internal structure of a single proton or neutron less
like that of a hydrogen atom and more like that of a large nonspherical nucleus
in which the collective orbital motion of hundreds of constituents is primarily
responsible for the overall spin of the nucleus (see Figure 1.3). However, protons
and neutrons are unique in having constituents within them that are moving at
(ultrarelativistic) speeds very close to the speed of light. This discovery is motivat-
ing a new generation of experiments at JLAB, Brookhaven National Laboratory,
and many nuclear laboratories worldwide that will exploit advances in accelerator
and detector technologies to fully characterize the distribution of mass and orbital
motion within protons and neutrons.
As described above, the formation of the first protons and neutrons about
10 µsec after the big bang represented the earliest instance of the emergence of
complex structures from the previously featureless primordial fluid. Although
featureless in the sense that it was the same everywhere in the universe, the liquid
of quarks and gluons (called the quark-gluon plasma, or QGP) that filled the
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Overview 19
s=1/2 J=2
proton nucleus
FIGURE 1.3 The spin of the proton is the sum of contributions from the spins and motions of all
the quarks (u and d), quark-antiquark pairs 1-03_spin.eps
(little circles), and gluons (connecting lines) within it.
Recent experiments indicate that the sum of the orbital motion contributes more than the sum of all
bitmaps with some vector type & rules
the spins, much as the total angular momentum of a large nonspherical nucleus is primarily the sum
of contributions from the orbital motion of hundreds of protons (red) and neutrons (blue). SOURCE:
(left) Lawrence S. Cardman, JLAB; (right) Witold Nazarewicz, University of Tennessee.
microseconds-old universe—and that is now being created and studied in experi-
ments in which nuclei are collided at extreme energies—turns out itself to have
very interesting properties that are emergent, in the sense that characteristics of
the macroscopic fluid are far from apparent from the fundamental laws that gov-
ern the fluid’s elementary constituents. As discussed in more detail in Chapter 2
under “Exploring Quark-Gluon Plasma,” all observations of the droplets of QGP
made in nuclear collisions over the last decade indicate that QGP acts more like
a pureed soup—a liquid—than a dilute plasma in which particle-like quarks and
gluons would be traveling appreciable distances between interactions, analogous
to particle-like disturbances in ordinary gaseous atomic plasmas. Instead, liquid
QGP responds to disturbances only with hydrodynamic waves, like those in water
reacting to a dropped pebble. QGP is not the only known example of a fluid with
collective properties but no apparent particle description: The challenge of under-
standing such liquids appears in several formerly disparate frontier areas of physics,
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20 Nuclear Physics
including the study of ultracold atomic fluids; condensed matter systems such as
oxide superconductors, which resist all conventional approaches to explaining their
properties; and, perhaps most surprisingly, the fluid of quantum fluctuations found
near black hole horizons. In the coming decade, nuclear physicists have the oppor-
tunity to understand how a complex liquidlike phase of matter emerges from the
underlying elementary quarks and gluons, whose dynamics are well understood at
very short distances. In addition to shedding light on the nature of the QGP that
filled the microseconds-old universe, progress on this frontier could advance our
understanding of phenomena that pose central challenges in many other areas of
contemporary science.
We move into the twenty-first century with confidence that a full understand-
ing of the fundamental theory of the strong interaction is within reach. QCD is a
rich and enormously complex theory that describes complex structures, phases, and
phenomena at the femtoscale. Applying QCD, and the effective nuclear theories
that emerge from it at longer length scales, to develop a full understanding of the
structure and properties of stars, nuclei, protons, and neutrons, and of the liquid
QGP, will be one of the most compelling contributions of nuclear physics to science.
Within the next few years, a new generation of accelerators and detectors
enabling new and perhaps unanticipated experimental discoveries, together with
unprecedented computing power enabling groundbreaking calculations, will yield
myriad new opportunities for advancing our understanding of the organization
and properties of nuclear matter in all of its manifestations (see Figure 1.4). Cur-
rent calculations are able to explain the basic properties of a proton or neutron,
including its mass, in terms of interacting quarks and gluons. Now, such calcula-
tions are being extended to include more than one neutron or proton. Similarly,
while the most advanced calculations done today that describe nuclei in terms
of protons and neutrons and the empirical strong forces between them (without
looking at the quarks and gluons inside the protons and neutrons) can explain the
long lifetime of carbon-14 used for archaeological dating, a microscopic picture of
the nuclear fission of uranium-235 still eludes us. Indeed, to fully explain the inner
workings and multiscale complexity of protons, neutrons, and nuclei remains an
enormous undertaking. The challenge is to include all the relevant physical features
in deciphering truly complex problems rather than being forced to rely on simpler
models that do not take into account the full physics involved. Examples of the
pathways that have been mapped to overcome the daunting computational chal-
lenges include microscopic calculations of the properties of quark-gluon plasma
and how it flows, how the quarks and gluons spin in a proton, and how protons
and neutrons conspire to produce the collective phenomena and simple regularities
seen in nuclei. A new generation of exascale computers, capable of performing a
million trillion calculations per second, will allow simulations of nuclear fission,
nuclear reactors, and hot and dense evolving environments such as those found in
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Overview 21
neutrons
FIGURE 1.4 From quarks to neutron stars: Different technologies are being brought to bear on the
myriad challenges in understanding nuclear matter at different spatial resolution scales or, equivalently,
1-04.eps
at different energy scales. At the shortest distance scales, relativistic heavy ion collisions are used to
study quark-gluon plasma and how protons and neutrons and other hadrons condense from it as it
cools. Electron-scattering experiments are used to study the complex structure of those protons and
neutrons, with varying spatial resolution. Rare isotope beams are used to understand the patterns and
phenomena that emerge as protons and neutrons form larger and larger nuclei. Nuclear phenomena
occur on truly macroscopic distance scales in stars, in the nuclear reactions that drive certain classes
of cataclysmic stellar explosions and in the description of the structure, formation, and cooling of neu-
tron stars, which are basically gigantic nuclei. Building bridges of understanding between the physics
at different spatial resolution scales is one of the paramount challenges facing contemporary nuclear
science. For example, the most natural description of nuclei is in terms of neutrons and protons, and
the most natural description of neutrons and protons is in terms of quarks and gluons. However, a
rigorous connection between these two descriptive frameworks requires a description of the lightest
nuclei in terms of quarks and gluons. This is the challenge for which the coming generation of accel-
erators, detectors, and computers is being designed, and is one of the great challenges for theoretical
nuclear physics as well. SOURCE: Courtesy of Witold Nazarewicz, University of Tennessee.
inertial confinement fusion, nuclear weapons, and astrophysical phenomena and
will provide a consistent picture of the fission data needed for national security
and nuclear energy applications. Such computational capability, coupled with
conceptual and algorithmic advances, will allow the physics of simple nuclei to be
understood directly from QCD in terms of interacting quarks and gluons in a way
that will serve as a benchmark for a rigorous computational approach to the full
nuclear many-body problem. This bridge would link a century’s worth of classic
questions directly to the fundamental interactions that are now known to be basic
to the structure of all matter.
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22 Nuclear Physics
ARE THE FUNDAMENTAL INTERACTIONS THAT ARE BASIC
TO THE STRUCTURE OF MATTER FULLY UNDERSTOOD?
The first part of the answer to this question is known: The fundamental strong
interactions between quarks and gluons (the laws of QCD) are known, and these
elementary laws must be responsible first for the emergence of protons, neutrons, and
their interactions, and then of nuclei. The interactions of QCD are not the only fun-
damental interactions we know of, however. All matter interacts by the gravitational
force, and electrons are bound to nuclei (making atoms) by the electric and magnetic
forces. Finally, the weak interactions (“weak” because they act only over distances
much smaller than the size of a proton) are responsible for the radioactive decay of
the majority of unstable nuclei—for example, carbon-14, used to estimate the age of
carbon-bearing materials, and fluorine-18, used in medical imaging—and they deter-
mine the properties of the elusive neutrinos that pass through space, Earth, and our
bodies, without us ever noticing. Nuclear scientists are able to utilize handpicked nuclei
as laboratories in which to make extraordinarily precise measurements that provide
stringent tests of our theories of all the fundamental interactions except gravity, which
does, however, play a role in nuclear physics in the context of neutron stars, where
neutron stars can be thought of as giant nuclei with a mass comparable to that of the
sun. The theory that describes these fundamental interactions, namely QCD, together
with the unified theory of electromagnetism and the weak interactions, is called the
Standard Model. (It could more descriptively be called the “Theory of Visible Matter.”)
By testing the predictions of this theory for nuclear phenomena to exquisite precision,
nuclear physicists are challenging the Standard Model and seeking evidence for new
interactions that go beyond it.
Nuclear physics has played a key role in the most significant revision to our
understanding of the fundamental laws of nature that has come since the last
decadal assessment—the discovery that neutrinos oscillate, transforming from one
type, or “flavor,” to another, even perhaps to a third type, and back and then repeat-
ing. The first evidence for this discovery came from Ray Davis’s historic Nobel
prize-winning experiment designed to measure the flux of neutrinos from the sun
in conjunction with John Bahcall’s precise modeling of how the sun shines, based
on nuclear theory and nuclear data from laboratory experiments. Comparing the
measured neutrino flux with solar model expectations, Davis found that about two-
thirds of the expected neutrinos were missing, a mystery that remained unsolved
for more than 30 years. Since the last decadal survey, however, two nuclear physics
experiments, one at the Sudbury Neutrino Observatory in Canada (SNO) and the
other in Japan (KamLAND), established convincingly that the neutrinos were not
missing at all. Davis’s experiment was sensitive to only one of the three flavors of
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Overview 23
neutrinos in nature, meaning that most of the neutrinos from the sun were hid-
ing from Davis’s detectors by oscillating into another flavor. Neutrino oscillations
require that neutrinos must come with different masses, implying that at least two
of them must have masses that are not zero. This discovery constitutes the first
change in several decades in our understanding of the fundamental laws that gov-
ern the elementary constituents of all matter, namely the Standard Model (see Box
1.1). It opens new questions, the most profound of which are the determination of
the average neutrino mass and the source of thier mass and the determination of
whether neutrinos are their own antiparticles. Concerted efforts to answer these
and other questions are now being mounted by nuclear physicists in a mutually
beneficial partnership with their particle physics colleagues.
Physicists do not expect the appearance of neutrino masses to be the last word
in the quest to understand the laws of nature at the level of elementary particles
and their interactions. Our current understanding, as codified in the Standard
Model, has had an extraordinary run of success in describing many phenomena,
but it is incomplete. Nuclear and particle physicists are seeking a new Standard
Model (NSM), which will incorporate the many successes of the Standard Model
but will in addition provide an understanding of aspects of physics that are now
mysterious. Questions here include: Why do quarks and electrons have the masses
that they have? What is the nature of the dark matter and dark energy that per-
vade the universe? and Why is the universe filled with matter but little antimatter?
One approach to answering these questions, led by particle physicists, is to push
back the high-energy frontier, seeking to create whatever new particles and new
interactions that may exist in the NSM in proton-proton collisions at the Large
Hadron Collider. An alternative approach, where nuclear physicists play a lead-
ing role, is to make advances on the precision frontier, where exquisitely sensitive
measurements may reveal tiny deviations from Standard Model predictions and
point to the fundamental symmetries of the NSM. For example, the symmetries of
the Standard Model do allow a neutron to have a very tiny permanent separation
between the center of mass of the positively charged quarks and the center of mass
of the negatively charged quarks within it, but many ideas for the NSM allow for
a possibly larger charge separation, known as the neutron dipole moment. In the
coming decade, nuclear physicists are planning a campaign to detect such an effect
or at least greatly reduce the experimental limits on it. The detection of a charge
separation larger than that allowed by the Standard Model could have decisive
implications for our understanding of the NSM and would naturally accommodate
mechanisms for the generation of an excess of matter over antimatter when the
universe was a trillionth of a microsecond or less old.
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24 Nuclear Physics
Box 1.1
The Fundamental Matter Particles of the Standard Model,
Also Sometimes Called the “Theory of Visible Matter”
There are six quarks in the Standard Model: the up (u), down (d), charm (c), strange (s),
top (t), and bottom (b) quarks. Quarks are matter particles that emit and absorb massless gluons,
meaning that they experience the strong interactions. The matter particles that do not participate
in the strong interactions (called “leptons”) include the electron (e) and its two cousins, the
muon (μ) and the tauon (τ). Leptons and the quarks are charged, meaning that they emit and
absorb massless photons and thus experience electric and magnetic interactions. Neutrinos (ν)
are the only known fundamental matter particles that do not absorb or emit either photons or
gluons. All matter particles, including neutrinos, emit and absorb W and Z bosons; the conse-
quent interactions are weak because the W and Z bosons are heavy, each having about half
the mass of the heaviest known matter particle, the top quark. All matter particles also feel the
force of gravity. All the particles in Figure 1.1.1 except the neutrino have antiparticles with the
same mass and the opposite electric charge. Each of the three neutrinos may have an antiparticle
or may be its own antiparticle; ongoing nuclear physics experiments aim to determine which.
The three neutrinos have different masses and so, when labeled by their masses as in
this figure, they can be called “light,” “medium,” and “heavy.” The pattern of neutrino masses
shown is one of the possibilities suggested by the recent discovery of neutrino oscillations,
captured in the pie chart for each neutrino. The three colors reflect the flavors of the electron,
muon, and tauon charged leptons. They show that the only way to construct a neutrino that is
the exact partner of the electron (called an “electron neutrino,” it is blue in this diagram) is to
combine neutrinos with differing masses in a certain way. Nuclear reactions in the sun produce
electron neutrinos. And, all Standard Model processes in which a neutrino is made produce the
exact partner of one of the charged leptons. The fact that these are combinations of neutrinos
with differing masses is what causes the neutrino to oscillate as it flies through space. (Quarks
also show this mixing property but to a much lesser degree.) Although the discovery of neutrino
oscillations has given us good information about the differences between the three neutrino
masses, the mass of the lightest neutrino is not known precisely; it could even be zero. Ongo-
ing nuclear physics experiments seeking to measure the average neutrino mass directly, not via
oscillations, will help. But we do not yet have any fundamental understanding of the pattern
of the masses of the 12 Standard Model matter particles, in particular of why the neutrinos are
millions of times lighter than any of the other particles.
HOW CAN THE KNOWLEDGE AND TECHNOLOGICAL PROGRESS
PROVIDED BY NUCLEAR PHYSICS BEST BE USED TO BENEFIT SOCIETY?
Nuclear physics is not only a basic scientific enterprise, but it also has stunning
practical applications. These in themselves can justify the cost and effort of the research,
going beyond the basic knowledge that has been gained. Nuclear science has a many-
decade-long history of accomplishments that benefit our health, the economy, and
our safety and security: The societal benefits derived from nuclear physics are by now
ubiquitous. This track record continues, with many new accomplishments since the
last decadal assessment and many more under development. Smoke detectors in our
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Overview 25
FIGURE 1.1.1 The masses of particles. The vertical scale is the particle mass in electron-
volts, with each tick representing a 1,000-fold increase. SOURCE: Courtesy of R.G. Hamish
Robertson, University of Washington.
homes, new medical diagnostic imaging methods, therapies using ion beams and new
isotopes for cancer treatments, and new methods for assessing breaches in national
and homeland security are just some of the ways that nuclear physics makes a differ-
ence to our safety, health, and security. Technological advances driven by advances in
nuclear physics, which range from particle accelerators (most of which are now used
either for medical purposes or in the semiconductor industry) to supercomputers, make
significant contributions to our economy. A mutually beneficial synergy has developed
in which a fundamental intellectual enterprise has consistently produced technological
gains that, pursued with societal benefit in mind, have more than compensated the
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26 Nuclear Physics
public support required to pursue this science. Also beneficial to society is informing
people about the discoveries from nuclear physics and explaining to them the origin
and structure of matter and the fundamental interactions.
Positron emission tomography exemplifies the synergy between nuclear sci-
ence, technological advances, and benefits to society. This medical imaging tech-
nique has become a powerful new tool for the diagnosis of cancer. The positron
sources as well as the highly segmented crystalline detector elements come directly
from nuclear physics research. Another example of synergy: Over the last two
decades, nuclear physicists interested in the structure of the neutron have developed
spin-polarized helium-3 targets, and it now turns out that these very techniques
can be used to make spin-polarized helium-3 or xenon-129. These, in turn, can
be introduced into the air a patient breathes, allowing for a new kind of magnetic
resonance imaging (MRI) of the lungs. Without these developments, MRI could
not be used to image gases and thus would not be able to accurately visualize lung
function.
Nuclear science plays a role in treatment as well as in diagnosis. Nuclear medi-
cine is a well-established field within medical research and therapy, with techniques
that originated in nuclear science now used as a matter of course in the irradia-
tion of tumors with high-energy particles. One of the many exciting advances in
treatment being pursued today is targeted radionuclide therapy, which has been
the most highly sought-after goal of nuclear medicine physicians and scientists for
decades. Targeted therapy involves attaching a “targeting molecule” to a relatively
short-lived radioactive isotope. The isotope emits radiation (alpha particles, for
example) that reacts strongly with nuclei of atoms that comprise the tissues in the
body and so deposits most of its energy nearby. The biologically active targeting
molecules are carefully designed to bind to receptors on cancer tumor cells. When
the radioactive nuclei attached to the targeting molecules bound to the tumor
decay, they deliver a lethal dose of radiation only to the tumor tissue. By careful
construction, the targeting molecule will pass through the body quickly if it does
not bind to tumor cells, thus minimizing the exposure of healthy tissue to radiation.
The use of these techniques in human clinical trials and in actual clinical therapy
has just started. Two radiopharmaceuticals are now in use to treat non-Hodgkins
lymphoma. And, recently, researchers at the Institute for Transuranium Elements
in Karlsruhe, Germany, treated neuroendocrine tumors with the alpha-emitting
bismuth-213 nucleus attached to a biological molecule that targets these particular
tumor cells; they found in a small initial trial with human patients a reduction in
the size of some tumors with no discernible negative side effects. If this approach
can become routine, the treatment of cancer will undergo a paradigm shift. Nuclear
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Overview 27
scientists play an essential role in this interdisciplinary effort, which blends biology,
medicine, modern technology, and nuclear physics.
The years since 9/11 have seen important advances in nuclear forensics. An
attack using a nuclear or radiological explosive device would of course be cata-
strophic, but it would also raise a set of urgent and crucial questions: What was
exploded? Who did it? Do they have more? Was the device improvised or sophis-
ticated? Did they steal it, and if so from where? Is the material reactor-grade or
weapons-grade fuel? How old is it? Nuclear forensics refers to the techniques and
capabilities needed to answer these questions. It can be likened to forensics-style
exercises in nuclear astrophysics, whereby scientists analyze and evaluate the debris
left behind by a stellar-scale nuclear explosion. Both efforts—nuclear astrophysics
and nuclear forensics—are led by nuclear physicists.
The last decade has also seen major advances in the use of nuclear physics
techniques to detect heavy nuclei like uranium or plutonium in a truck or cargo
container—one such technique might detect the cosmic ray muons, elementary
particles similar to electrons, that scatter off these elements at large angles. The
techniques are based on well-understood basic nuclear physics, reminiscent of
Rutherford’s early experiments, but their application to the detection of nuclear
contraband crossing U.S. borders is new. The detector and computational chal-
lenges are related to very recent developments in basic nuclear physics.
Nuclear physics has long been a driver in the development of accelerators and
computers, both of which are prevalent in our lives and in many sectors of the
economy. Solving the design challenges associated with the building of very high
energy accelerators being used to probe the fundamental nature of the matter in
our universe will bring advances that improve the more than 30,000 accelerators
used around the world for radiotherapy, for ion implantation to precisely embed
dopants in semiconductor chips, and for other applications from developing new
materials to improving food safety and benefiting other areas of industrial and bio-
medical research. Advancing nuclear science also drives innovations in computer
architecture. For example, when IBM developed the Blue Gene line of computers
that have become successful commercial machines with an impact on climate
science, genomics, protein folding, materials science, and brain simulation, it
employed a paradigm that had been developed first for lattice QCD machines—in
fact, IBM employed people who had previously designed a computer called the
QCDOC (QCD on a chip).
These examples all show how investment in nuclear physics has benefits beyond
addressing the fundamental overarching questions earlier in this chapter. These
investments are yielding progress on some of the nation’s biggest challenges as well
as innovations that help to drive the economy.
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28 Nuclear Physics
PLANNING FOR THE FUTURE
As the complexity of the main challenges in the field has grown, so have the
cost and size of the experimental nuclear physics tools. What began 100 years ago
primarily as efforts of individuals or small groups has grown into a mix of small
and large groups working as teams, both here and abroad. The U.S. nuclear physics
community has developed a number of complementary processes for establishing
consensus and setting priorities and future directions. The Division of Nuclear
Physics in the American Physical Society, one of the most active divisions, provides
help with planning and outreach for the benefit of nuclear physics. Another effec-
tive element is the Long-Range Planning process organized by the Nuclear Science
Advisory Committee (NSAC) of the Department of Energy and the National Sci-
ence Foundation. Using this tool, the community has been establishing its priorities
and providing guidance and advice to the funding agencies. The present decadal
assessment of nuclear physics brings experts from the diverse areas of the field to
assess the achievements and provide a forward-looking vision of the new horizon.
Anticipating needs for personnel and for building new facilities as well as develop-
ing and improving infrastructure for the field are all important components of the
planning process. The charge for this study reflects the mission of decadal studies:
The new 2010 NRC decadal report will prepare an assessment and outlook for nuclear
physics research in the United States in the international context. The first phase of the
study will focus on developing a clear and compelling articulation of the scientific rationale
and objectives of nuclear physics. This phase would build on the 2007 NSAC Long-range
Plan Report, placing the near-term goals of that report in a broader national context.
The second phase will put the long-term priorities for the field (in terms of major facilities,
research infrastructure, and scientific manpower) into a global context and develop a strat-
egy that can serve as a framework for progress in U.S. nuclear physics through 2020 and
beyond. It will discuss opportunities to optimize the partnership between major facilities
and the universities in areas such as research productivity and the recruitment of young
researchers. It will address the role of international collaboration in leveraging future U.S.
investments in nuclear science. The strategy will address means to balance the various
objectives of the field in a sustainable manner over the long term.
This present report offers the committee’s assessment and outlook. Chapter 2
summarizes the main scientific areas and the science questions addressed by nuclear
physics, focusing on accomplishments since the last decadal assessment and direc-
tions for the decade to come. From the beginning the diversity of the science is
evident in the range of topics, from the behavior of quarks and gluons to the uni-
verse. In this introduction, the committee has highlighted the interconnections of
these main scientific areas with each other.
In Chapter 3 as well as elsewhere in this report, some of the ways in which
society benefits from applications of nuclear physics are emphasized, and snapshots
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Overview 29
of various important uses of the knowledge and know-how gained from nuclear
physics are provided. Again, the topics of application are astonishingly diverse.
Nuclear physics in the global context is described in Chapter 4. Resulting from
remarkably productive international cooperation, the global program in nuclear
physics combines competition, cooperation, and communication in a way that is
benefiting all the participants and accelerating scientific progress.
Chapter 5 addresses the important issues related to decision-making processes.
The critical NSAC Long Range Planning exercise and other less structured global
planning processes have become vital for keeping the nuclear physics enterprise on
a successful path to the future. Workforce issues are explored in this chapter along
with the steps being taken to ensure a productive workforce in the coming years.
Finally, the committee discusses its findings and recommendations in Chapter 6.
