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9
Conclusions and
Recommendations
THE REVOLUTION OF THE PAST TWO DECADES
During the past two decades our understanding of the fundamental
nature of matter has undergone a revolution. We have found three
families of elementary particles: the family of leptons' the family of
quarks, and the family of force-carrying particles. The lepton family
and the quark family each consist of three generations, with strong
similarities between the generations.
The four fundamental forces electromagnetic, gravitational,
strong, and weak were known earlier, but during the last two decades
the electromagnetic and weak forces have been unified theoretically
and experimentally. In addition, the particles carrying the strong force,
called gluons, and the particles carrying the weak force, the W and Z'
have each been discovered.
During the same period, the vast family of particles called the
hadrons has been shown not to be elementary in themselves, but rather
to be made up of quarks. In particular, the quark nature of the proton
and neutron, the most common hadrons. has been studied and under-
stood in great detail. We have also acquired a good understanding of
how quarks behave inside hadrons and of how quarks interact when
hadrons are involved in high-energy collisions.
194
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CONCE USI ONS A ND RECOMMENDA TI ONS 1 95
HOUr THE REVOLUTION WAS MADE
This revolution in elementary-particle physics came about because
of three interacting components of particle research:
Progress in Accelerators. Particle colliders and higher energy fixed-
target accelerators have come into operation during the last two
decades. These new facilities provided the high-energy particles that
were needed for most of the experimental work involved in this revolu-
tion.
.
Ned Experiments and Ne`~' Experimental Techniques. In physics,
new discoveries are made experimentally. and all new ideas and
theories must be tested and validated experimentally. Some of the
revolution in particle physics occurred because theory predicted the
existence of new phenomena or new particles. That is how the W was
discovered. Conversely, sometimes experiments led the way. The tan
lepton was discovered through an experimental search, and thus the
concept of the third generation was introduced. Almost all of these
experiments used new experimental techniques such as particle-detecting
wire chambers and integrated circuitry.
Theoretical Progress. The third necessary component of elementary-
particle research is progress in elementary-particle theory. Sometimes
that progress is in the form of an elegant mathematical theory, such as
the theory that unifies the weak and electromagnetic forces. Sometimes
it is in the form of a broad insight; for example, the realization that
most,of the properties of hadrons can be directly explained by models
of the behavior of quarks inside the hadrons.
WHAT WE WANT TO KNOW
With this revolution accomplished, we are now led to deeper
questions about the basic nature of matter and energy. These questions
could not be asked in a sensible way until we had identified the three
families of elementary particles and learned how the four basic forces
behave. Some of these questions express our dissatisfaction with
present theories, which require several dozen unexplained numerical
constants. What sets the values of these constants? Are they intercon-
nected or independent? Among these constants are the masses of the
various elementary particles and the strengths of the various basic
forces. The masses of the particles are completely unexplained,
because we do not yet understand the origin of mass.
Other questions that we need to answer include the following: How
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196 ELEMENTARY-PARTICLE PHYSICS
.
many generations of quarks or leptons are there? Are the quarks and
leptons related to each other? If so, how? Can the strong force be
unified with the already unified electromagnetic and weak forces?
Then there are the questions that are related to our overview of
elementary-particle physics. Are the quarks and leptons really elemen-
tary? Are there yet other types of forces and elementary particles? Can
gravitation be treated quantum mechanically as are the other forces,
and can it be unified with them? More generally, does quantum
mechanics apply in all parts of elementary-particle physics? Do we
understand the basic nature of space and time?
These questions indicate the hopes and opportunities for continued
progress in elementary-particle physics during the next several dec-
ades. Although the United States has until quite recently been the
leading contributor to elementary-particle physics research, gradually
other regions, particularly Western Europe and Japan, have substan-
tially increased their contributions to this research. This is as it should
be, since science is a worldwide endeavor. Elementary-particle physics
is a basic science. It interacts with many other areas of physics and
astronomy, and it develops, stimulates. and uses many new technolo-
gies. In the belief that the United States should maintain a forefront
role in particle-physics research, we conclude this report with a set of
recommendations for the elementary-particle physics program in the
United States. No priority is indicated by the order in which we present
these recommendations. This program has as its centerpiece the plan.
based on ongoing accelerator development and design work, for the
construction in the United States of a very-high-energy, superconduct-
ing proton-proton collider, the Superconducting Super Collider (SSC).
In arriving at this program the elementary-particle physics commu-
nity has had to choose from a number of alternative physics directions
and from proposals for building other types of new accelerators and
facilities. A great deal of thought, research, and discussion has gone
into the program described here.
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
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CONCLUSIONS AND RECOMMENDA TIONS 197
at the heart of this field. The diversity in size, in scientific interests, and
in styles of experimentation of these research groups are essential for
maintaining the creativity in the field. Therefore itch recommend float
the strength apace din Fruity of these groups be pre.ser~'ed.
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 (AGS) at the Brookhaven National Laboratory and the
1000-GeV superconducting accelerator. the Tevatron, at the Fermi
National Accelerator Laboratory. Cornell University operates the elec-
tron-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 primarily devoted 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 allow 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, using a new linear collider princi-
ple, is now being constructed at the Stanford Linear Accelerator
Center. The Tevatron at the Fermi National Accelerator Laboratory is
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 reels colliders on their present
schedule. In addition, ice recommend that their experimentalfacilities
and programs be fully developed.
The United States elementary-particle physics community is now
carrying out an intensive research. development, and design program
intended to lead to a proposal for a very-high-energy, superconducting
proton-proton collider, the Superconducting Super Collider (SSC). It
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198 ELEMENTARl'-PARTICLE PHYSICS
a
will be based on the accelerator principles and technology that have
been developed at several national laboratories. in particular the
extensive experience with superconducting magnet systems that has
been gained at the Fermi National Accelerator Laboratory and Brook-
haven 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 ~ 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 ~ leading role in elementary-particle physics research. Sitcom the
SSC is ~ c~trc~l to to/? fitter `'f c~le~ne''tc~r!~-particl`, pl'ysics r esearch in
th`, U''itec! St~`tes lie .stro'?,£~1! r ecom',2c,'~d its e.rpeditio'~s co~zstr'~c-
tion.
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
synchrotron 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-
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CONCL USI OILS A ND RECOMMENDA TI ONS 1 99
mental research. Theorists working in elementary-particle physics
have also played an important role in forging links with other disci-
plines, including statistical mechanics. condensed-matter physics. and
cosmology. Theoretical physicists make vital contributions to univer-
sity research programs and to the education of students who will enter
all branches of physics.
W`, ret o',~7`e,Zd tl'at the e`-isti'~g' st'o''g' supper ~ fit/ c' h/ `~`c] pI'G#'/'Ci'7?
Of tI2c~'reti~al researcl2 in the `'ni`~ersitie~, instit''te.s acid '~c~ti`~al
laboratories be c o'`ti''``ecI. 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
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 searching 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 `` ill continue to
play a vital role that is complementary to accelerator-based research,
and bile 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
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200 ELEMENTARY-PARTICLE PHYSICS
extend international cooperation in the discipline. We recommendfour
guidelines for so lit a balanc ed program. First, the continued vitality of
American elementary-particle physics requires that there be forefront
accelerator facilities in the United States. The use of accelerators
developed by other nations provides a needed diversity of experimen-
tal opportunities but it does not stimulate our nation's technological
base as does the conception. construction, and utilization of innovative
facilities at home. The Superconducting Super Collider 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 collaboration in the planning and
building of accelerator facilities.
CONCLUSION
We believe that the implementation of these recommendations will
enable the United States to maintain a competitive forefront position
in 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 magnets.
Representative terms from entire chapter:
stanford linear
