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Executive Summary
Elementary-particle physics, the science of the ultimate constituents
of matter and the interactions among them, has undergone a remark-
able development during the past two decades. A host of new experi-
mental results made accessible by a new generation of particle accel-
erators and the accompanying rapid convergence of theoretical ideas
has brought to the subject a new coherence and has raised new
possibilities and set new goals for understanding nature. The progress
in particle physics has been more dramatic and more thoroughgoing
than could have been imagined at the time of the 1972 survey of
physics, Physics in Perspective (National Academy of Sciences, Wash-
ington, D.C., 19721. Many of the important issues identified in that
report have been addressed, and many of the opportunities foreseen
there have been realized. As a result, we are led to pose new and more
fundamental questions and to conceive new instruments that will enable
us to explore these questions.
Elementary-particle physics is the study of the basic nature of matter,
energy, space, and time. Elementary-particle physicists seek the funda-
mental constituents of matter and the forces that govern their behavior. In
common with all physicists, they seek the unifying principles and physical
laws that determine the material world around us.
The atom, the atomic nucleus, and the elementary particles of which
they are composed are too small to be seen or studied directly.
Throughout this century, physicists have devised ever more sophisti-
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2 ELEMENTARY-PARTICLE PHYSICS
cased detection devices to observe the traces of these particles and
their constituents. At the same time, they have developed increasingly
energetic beams of particles to probe deeply into the structure of
matter. Early examples are x rays to probe the electronic structure of
the atom and radioactive sources to study the atomic nucleus. Some of
the constituents of ordinary matter, notably electrons and protons, are
quite stable and easily manipulated in electric and magnetic fields.
They can therefore be accelerated to high energies and used as probes
to reach the very small distance scale of the fundamental constituents.
The colliding of high-energy particles and the analysis of collision
products is at the heart of experimental particle physics. For this
reason the field is often called hi~,>l?-energy physics.
THE REVOLUTION IN PARTICLE PHYSICS
Thirty years ago, ordinary matter was thought to consist of protons,
neutrons, and electrons. Experiments were under way to probe the
structure of these particles and to study the forces that bind them
together into nuclei and atoms. In the course of these experiments,
physicists discovered more than a hundred new particles, called
hadrons, which had many similarities to the proton and the neutron.
None of these particles seemed more elementary than any other, and
there was little understanding of the mechanisms by which they
interacted with one another.
Since that time, a radically new and simple picture has emerged as a
result of many crucial discoveries and theoretical insights. It is now
clear that the proton, the neutron, and other hadrons are not elemen-
tary. Instead, they are composite systems made up of much smaller
particles called quarks, much as an atom is a composite system made
up of electrons and a nucleus. Five kinds of quarks have been
established, and initial experimental evidence for a sixth species has
been reported.
Unlike the neutron and the proton, the electron has survived the
revolution intact as an elementary constituent of matter, structureless
and indivisible. However, we now know that there are six kinds of
electronlike particles called leptons. According to our present under-
standing, then. ordinary matter is composed of quarks and leptons.
An important difference between quarks and leptons is that a
formidable interaction, known as the strong force, binds quarks
together into hadrons but does not influence leptons. Both quarks and
leptons are acted upon by the three other fundamental forces: the
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EXECUTIVE SUMMARY 3
electromagnetic force, the weak force responsible for certain radioac-
tive decays, and the gravitational force.
Over the past two decades' great progress has been made in
understanding the nature of the strong, weak, and electromagnetic
forces. A unified theory of the weak and electromagnetic forces now
exists. Its predictions have been dramatically verified by many exper-
iments, culminating in the discovery of the W and Z particles in 1983.
These carriers of the weak force are analogous to the photon, the
carrier of the electromagnetic interaction, whose existence was estab-
lished in the 1920s. In addition, there is indirect but persuasive
evidence for particles called gluons, the carriers of the strong force.
Strong, weak, and electromagnetic interactions all are described by
similar mathematical theories called gauge theories. At this time, the
role played by the gravitational force in elementary-particle physics is
unclear. We have not been able to measure directly any effect of
gravity on the collisions of elementary particles.
With the identification of quarks and leptons as elementary particles,
and the emergence of gauge theories as descriptions of the fundamental
interactions, we possess today a coherent point of view and a single
language appropriate for the description of all subnuclear phenomena.
This development has made particle physics a much more unified
subject, and it has also helped us to perceive common interests with
other specialties. One important by-product of recent developments in
elementary-particle physics has been a recognition of the close con-
nection between this field and the study of the early evolution of the
universe from its beginning in a tremendously energetic' primordial
explosion called the big bang. Particle physics provides important
insights into the processes and conditions that prevailed in the early
universe, and deductions from the current state of the universe can in
turn give us information about particle processes at energies that are
too high to be produced in the laboratory, energies that existed only in
the first instants after the primordial explosion.
WHAT WE WANT TO KNOW
Developments in elementary-particle physics during the past decade
have brought us to a new level of understanding of physical laws. This
new level of understanding is often called the standard model of
elementary-particle physics. As usual' the attainment of a new level of
understanding refocuses attention on old problems that have refused to
go away and raises new questions that could not have been asked
before. The quark model of hadrons and the gauge theories of the
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4 ELEMENTARY-PARTICLE PHYSICS
strong' weak, and electromagnetic interactions organize our present
knowledge and provide a setting for going beyond what we now know.
Although the standard model provides a framework for describing
elementary particles and their interactions, it is incomplete and inad-
equate in many respects. We still do not understand what determines
the basic properties of quarks and leptons, such as their masses. Nor
do we understand fully how the differences between the massless elec-
tromagnetic force carrier, the photon, and the massive carriers of the
weak force, the W and Z particles, arise. Existing methods for dealing
with these questions involve the introduction of many unexplained
numerical constants into the theory a situation that many physicists find
arbitrary and thus unsatisfying. Physicists are actively seeking more
complete and fundamental answers to these questions.
Another set of questions goes beyond the existing synthesis. For
example, how many kinds of quark and lepton are there? How are the
quarks and leptons related, if they are related? How can the strong
force be unified with the already unified electromagnetic and weak
forces?
Then there are questions related to our overview of elementary-
particle physics. Are the quarks and leptons really elementary? Are
there yet other types of forces and elementary particles? Can gravita-
tion be treated quantum mechanically, as are the other forces, and can
it be unified with them? More generally, will quantum mechanics
continue to apply as we probe smaller and smaller distances? Do we
understand the basic nature of space and time?
THE TOOLS OF ELEMENTARY-PARTICLE PHYSICS
Elementary-paIticle physics progresses through a complicated inter-
action between experiment and theory. As experimental work pro-
duces new data, theory is tested by the data, and theory is used to
organize the data. Sometimes theoretical insight leads to new experi-
ments; sometimes an experiment produces surprising new data that
upset currently accepted theories. Patient accumulation of data may
lead to paradoxes that cannot be resolved without major revision of
theoretical ideas. And sometimes experimenters may seek new ent'-
ties, such as free quarks or magnetic monopoles, which do not fit
known patterns. In the end, physics is an experimental science, and it
is only experiment and observation that can tell us if we are right or
wrong.
Most experiments in our field are carried out by the use of acceler-
ators, which produce beams of high-energy particles. These beam
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EXECUTIVE SUMMARY 5
particles collide either with a stationary target (a "fixed-target"
experiment) or with another beam of particles. Accelerators in which
two beams of particles collide are called colliders. Either in fixed-target
experiments or in colliders, the results of the collisions are recorded
with devices, often complex, called particle detectors. Accelerators
and particle detectors are the main tools of elementary-particle phys-
ics. Through the years invention, research, and development have led
to major innovations and vast improvement in the technology of
accelerators and detectors. In turn, these tools are fundamental to
experimental progress in our field.
- The fixed-target experiments of the past two decades have contrib-
uted much to our knowledge. Examples of these experimental results
are the demonstration that neutrons and protons are composed of
quarks, one of the two simultaneous discoveries of the fourth (or
charmed) quark, the discovery of the fifth (or bottom) quark, and the
discovery of the violation of what were thought to be fundamental
symmetries in time and space. Fixed-target experiments have accumu-
lated a large body of data that has led to the systematic understanding
of the interactions of hadrons.
Experiments utilizing colliders have become increasingly prominent
because more of the beam energy is available to the fundamental
collision processes. The extension of colliding-beam accelerator tech-
nology. was led by the development of electron-positron and proton-
proton colliders and by other basic advances in that technology.
Experiments at electron-positron colliders have given us the shared
discovery of the charmed quark; the discovery of the unexpected new
"relative" of the electron the tau lepton; the discovery of intense jets
of hadrons; and much of the evidence for the theory that the strong
force is mediated by the gluon particle. Recently the development of
the proton-antiproton collider contributed substantially to particle
physics by making possible the discovery of the carriers of the weak
force the W and Z particles. This development confirms an expanded
future role for proton-proton and proton-antiproton colliders.
Most of the discoveries described above were made possible only
through the building of new high-energy particle accelerators. This is
most evident in the discoveries of the new massive particles, such as
the W. the Z. the heavy quarks, and the new lepton. Higher-energy
accelerators in the future will similarly open up the possibility of
discovering new fundamental particles of still higher mass.
Progress in elementary-particle physics also depends on studying
rare or unusual collisions. Therefore it is important to have very
intense beams of particles to produce the rare events within a back-
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6 ELEMENTARY-PARTICLE PHYSICS
ground of less-interesting phenomena. Thus, both intensity and energy
are critical parameters of high-energy accelerators.
THE FUTURE OF ELEMENTARY-PARTICLE PHYSICS IN THE
UNITED STATES
Elementary-particle physics is perhaps the most basic of the sci-
ences; it interacts with many other areas of physics and astronomy; it
develops, stimulates, and uses new technologies. Two decades ago the
United States was the dominant force in elementary-particle research.
Gradually other regions, particularly Western Europe and Japan, have
increased their elementary-particle physics programs until together
they equal or exceed the U.S. program in personnel, financial support,
and scientific accomplishment. This is as it should be, since science is
a worldwide endeavor. International participation leads to innovation
in accelerator and detector technology, to an interchange of ideas, and
to a more rapid pace of discovery. indeed, many of the most important
recent discoveries have been made in Europe. This report includes
recommendations for the future U.S. program in this field that are
intended to exploit the scientific opportunities before us and to permit
us to maintain a competitive role in the forefront of this science.
The program for the future of the field embodied in our recommen-
dations has emerged from an intense discussion within the community
of elementary-particle physicists. During the past 3 years physics study
groups and federal advisory panels have considered several different
initiatives for new facilities. They have also considered the balance
between support of existing facilities and construction of new facilities.
Ultimately the choice was determined by the belief that new phenom-
ena that are crucial to the understanding of fundamental problems will
be discovered in the tera-electron-volt (TeV) mass range. This region
cannot be reached either by existing accelerators or by the accelerators
now under construction. The successful conclusion of the long and
difficult development of superconducting magnet technology makes a
large new machine a feasible and timely choice. Our recommendations
form a plan that has as its keystone the construction of a very-high-
energy superconducting proton-proton collider, the Superconducting
Super Collider (SSC).
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EXECUTIVE SUMMARY 7
RECOMMENDATIONS FOR UNIVERSITY-BASED RESEARCH
GROUPS AND USE OF EXISTING FACILITIES IN THE UNITED
STATES
The community of elementary-particle physicists in the United
States consists of about 2400 scientists, including graduate students,
based in nearly 100 universities and 6 national laboratories. They work
together in groups frequently involving several institutions. It is their
experiments, their calculations, their theories, their creativity that are
at the heart of this field. The diversity in size' in scientific interests, and
in-styles of experimentation of these research groups are essential to
maintaining the creativity in the field. Therefore we recommend that
the strength and diversity of these groups be preserved.
Most elementary-particle physics experiments in the United States
are carried out at four accelerator laboratories. Two fixed-target proton
accelerators are now operating: the 30-GeV Alternating Gradient
Synchrotron at the Brookhaven National Laboratory and the
1000-GeV superconducting accelerator, the Tevatron, at the Fermi
National Accelerator Laboratory. Cornell University operates the
electron-positron collider CESR. The Stanford Linear Accelerator
Center operates a 33-GeV fixed-target electron accelerator, which also
serves as the injector for two electron-positron colliders, SPEAR and
PEP. In addition, some elementary-particle physics experiments are
carried out at medium-energy accelerators that are devoted primarily
to nuclear physics.
Experimentation at the four accelerator laboratories requires com-
plex detectors that are often major facilities in their own right. The
equipment funds for major detectors and the operating funds for the
accelerators have been insufficient to permit optimum use. Because
accelerator laboratories necessarily have large fixed costs, the produc-
tivity of the existing accelerator facilities can be increased considerably
by a modest increase in equipment and operating funds. We recom-
mend fuller support of existing facilities.
RECOMMENDATIONS FOR NEW ACCELERATOR FACILITIES
IN THE UNITED STATES
The capability of two existing accelerators in the United States is
now being extended by adding collider facilities to each of them. A
100-GeV electron-positron collider, which uses a new linear collider
principle, is now being constructed at the Stanford Linear Accelerator
Center. The Tevatron at the Fermi National Accelerator Laboratory is
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8 ELEMENTAR Y-PARTIC' E PHYSICS
being completed so that the superconducting ring can also be operated
as a 2-TeV proton-antiproton collider. We recommend continued
support for the completion of these new colliders on their present
schedule. In addition, we recommend that their experimentalfacilities
and programs be fully developed.
The U.S. elementary-particle physics community is now carrying
out an intensive research, development, and design program intended
to lead to a proposal for the very-high-energy, superconducting proton-
proton collider, the SSC. This new collider will be based on the
accelerator principles and technology that have been developed at
several national laboratories and in particular on the extensive expe-
rience with superconducting magnet systems that has been gained at
the Fermi National Accelerator Laboratory and Brookhaven National
Laboratory. The SSC energy would be about 20 times greater than that
of the Tevatron collider. This higher energy is needed in the search for
heavier particles, to find clues to the question of what generates mass,
and to test new theoretical ideas. Our current ideas predict a rich world
of new phenomena in the energy region that can be explored for the
first time by this accelerator. Furthermore, history has shown that the
unexpected discoveries made in a new energy regime often prove to be
the most exciting and fundamentally important for the future of the
field. On its completion this machine will give the United States a
leading role in elementary-particle physics research. Since the SSC is
centra/ to the future of elementary-particle physics research in the
United States, we strongly recommend its expeditious construction.
RECOMMENDATIONS FOR ACCELERATOR RESEARCH AND
DEVELOPMENT
Since accelerators are the heart of most elementary-particle experi-
mentation, physicists are continuing research and development work
on new types of accelerators. indeed, technological innovation in
accelerators has been the driving force in extending the reach of
high-energy physics. An important part of this work is concerned with
extending the electron-positron linear collider to yet higher energies.
One of the purposes of the construction of the Stanford Linear Collider
is to serve as a demonstration and first use of such a technology.
Advanced accelerator research is also exploring new concepts, based
on a variety of technologies, that may provide the basis for even more
powerful accelerators, perhaps to be built in the next century. Such
research also leads to advances in technology for accelerators used in
industry, medicine, and other areas of science such as studies based on
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EXECUTI VE SUMMAR Y 9
syr~chrotron radiation. We recommend strong support for research and
development work in accelerator physics and technology.
RECOMMENDATIONS FOR THEORETICAL RESEARCH IN
PARTICLE PHYSICS
Theoretical work in elementary-particle physics has provided the
intellectual foundations that motivate and interconnect much experi-
mental research. Elementary-particle theorists have also played an
important role in forging links with other disciplines, including statis-
tical mechanics, condensed-matter physics, and cosmology. Theoreti-
cal physicists make vital contributions to university research programs
and to the education of students who will enter all branches of physics.
We recommend that the existing strong support for a broad program
of theoretical research in the universities, institutes, and national
laboratories be continued. A new element of theoretical research is the
increasing utilization of computer resources, which has spurred the
development and implementation of new computer architectures. This
trend will require the evolution of new equipment-funding patterns for
theory.
RECOMMENDATIONS FOR NONACCELERATOR PHYSICS
EXPERIMENTS
It is appropriate that some fraction of the particle-physics national
program be devoted to experiments and facilities that do not use
accelerators. These experiments include the searches for proton decay
by using large underground detectors, the use of cosmic rays to explore
very-high-energy particle interactions, the measurements of the rate of
neutrino production by the Sun, and the use of nuclear reactors to
study subtle properties of neutrons and neutrinos. There are also
diverse experiments that search for evidence of free quarks, magnetic
monopoles, and finite neutrino mass. Still other classes of experiments
overlap the domain of atomic physics; these include exquisitely precise
tests of the quantum theory of electromagnetism, studies of the mixing
of the weak and electromagnetic forces in atomic systems, and
searches for small violations of fundamental symmetry principles
through a variety of different techniques. Many of these are small-scale
laboratory experiments. Some provide a means of probing an energy
scale inaccessible to present-day accelerators.
The value of these experiments is substantial. They will continue to
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10 ELEMENTARY-PARTICLE PHYSICS
play a vital role that is complementary to accelerator-based research,
and we recommend their continued support.
RECOMMENDATIONS FOR INTERNATIONAL COOPERATION
IN ELEMENTARY-PARTICLE PHYSICS
Our program should be designed to preserve the vigor and creativity
of elementary-particle physics in the United States and to maintain and
extend international cooperation in the discipline. We recommendfour
guidelines for such a balanced program. First, the continued vitality of
American elementary-particle physics requires that there be pre-
eminent accelerator facilities in the United States. The use of acceler-
ators developed by other nations provides a needed diversity of
experimental opportunities, but it does not stimulate our nation's
technological base as do the conception, construction, and utilization
of innovative facilities at home. The SSC will be a frontier scientific
facility, and the technological advances stimulated and pioneered by its
design and construction will serve the more general societal goals as
well. Second, the most productive form of cooperation with respect to
accelerators is to develop and build complementary facilities that allow
particle physics to be studied from different experimental directions.
Third, the established forms of international cooperation, including the
use of accelerators of one nation by physicists from another nation,
should be continued. Fourth, looking beyond the program proposed in
this report, there should be further expansion of international collab-
oration in the planning and building of accelerator facilities.
CONCLUSION
We believe that the implementation of the recommendations made
above will enable the United States to maintain a competitive position
in the forefront of elementary-particle physics research into the next
century. Central to this future is the construction of the SSC, the
very-high-energy proton-proton collider using superconducting mag-
nets.
Representative terms from entire chapter:
accelerator facilities
