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1
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Over the last two decades our understanding of the fundamental
nature of matter has undergone a revolution. The ideas we learned as
students have been superseded by concepts at once simpler and more
elegant. The more than 100 different kinds of particles identified in the
early 1960s are now known to be made up of only a few different kinds
of more elementary (simpler) particles called quarks. Seemingly unre-
lated particles such as quarks and electrons are found to be related.
And two apparently different forces have been shown experimentally
and theoretically to be simply different manifestations of the same
more fundamental electroweak force.
This revolution in elementary-particle physics is the result of the
continuing interplay between new theories and new experimental
results. Almost all of these experiments have used accelerators, the
machines that produce the high-energy particles that are needed to
study matter on the smallest scale. These machines have included both
traditional accelerators, those in which a beam of high-energy particles
strikes a stationary target, and also the newer colliding-beam ma-
chines, accelerators in which two beams of high-energy particles collide
head on.
We are now in a position to look for the answers to yet more basic
questions: What determines the properties of the elementary particles?
How many different kinds are there? How are they related to each
other? Are the other forces in nature also simply different aspects of a
11
OCR for page 12
12 ELEMENTARY-PARTICLE PHYSICS
single, truly fundamental force? The new colliders can provide the
immense energies needed to answer some of these questions, while
others can be attacked by more traditional methods. Thus the next
several decades offer us the opportunity to continue the remarkable
progress of the recent past. This report is about that progress and also
about the opportunities for future progress.
ELEMENTARY-PARTICLE PHYSICS
Elementary-particle physics is the study of the basic nature of
matter, of force, of energy, of time, and of space. We seek to discover
the simplest constituents of matter, which we call the elementary
particles, and we seek to understand the basic forces that operate
between them. Above all, we seek the unifying principles and physical
laws that will give us a rational and predictive picture of the elementary
particles and the basic forces that constitute our world.
Elementary particles are very small, much smaller than atoms; hence
this is the physics of the very small. The size of the objects studied in
this field compared with those studied in other areas of physics is
sketched in Figure 1.1. Elementary particles are too minute to see or
study directly. We examine them and make new types of particles by
colliding particles together at high energies. High energies are needed
because the elementary particles are very small and very hard; it takes
a great deal of energy to penetrate them or to break them up. Colliding
two particles together at high energy and studying the results of that
collision is the heart of elementary-particle physics experiments. Thus
this field of research is also called high-energy physics.
In the last decade, close connections have been established between
particle physics and astrophysics. Hence elementary-particle physics
is also connected with very-large-scale phenomena.
WHAT WE KNOW
During the past two decades, particle-physics research has cleared
away much of the underbrush that had concealed from us the world of
elementary particles. As shown in Figure 1.2, we now know that there
are three basic families of elementary particles: the quarks, the leptons,
and the force-carrying particles. Some of these particles can only be
produced in the laboratory or occur very rarely in nature. But others
make up the matter of our everyday world. Thus the atoms that make
up all matter consist of electrons moving in orbits around the atomic
nucleus. The electron is one of the leptons, and the nucleus consists of
protons and neutrons that in turn are made up of quarks.
OCR for page 13
INTRODUCTION I 3
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FIGURE 1.1 The different subfields of physics study parts of nature that are very
different in size. Elementary-particle physics studies the smallest objects in nature.
objects that are smaller than 10-'3 centimeter.
Figure 1.2 also shows the four known basic forces. Two of these
have been known for hundreds of years: the forces of electromagne-
tism and of gravitation. The other two forces were discovered in the
twentieth century. One is the strong or nuclear force that holds the
atomic nucleus together, and the other is the weak force that operates
in many forms of radioactivity. One of the goals of the physicist is to
find out if these four forces can be derived from an even more basic,
single, unified force. Significant progress has been made in this
direction in the last two decades, as we now know that the electro-
magnetic and weak forces are two manifestations of a single underlying
force.
Thus our present knowledge of elementary-particle physics can now
be organized in a simple and elegant way. Chapters 2 and 3 describe
this present knowledge in some detail.
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14 ELEMENTARY-PARTICLE PHYSICS
{a)
QUARKS
LEPTONS
BEAUTY OR
BOTTOM QUARK
CHARM QUARK
STRANGE QUARK
UP QUARK
·-- DOWN QUARK
TAU
—- TAU NEUTRINO
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1 1 1
1 900 1 940 1 980
YEAR DISCOVERED
(b) KNOWN BASIC
FORCES
GRAVITATIONAL
FORCE
WEAK FORCE
STRONG OR NUCLEAR
FORCE
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FORCE
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YEAR WHEN THE PARTICLE CARRYING THE FORCE WAS DISCOVERED
FIGURE 1.2 (a) History of the discovery of the leptons and quarks. The dashed line
means that there was strong indirect evidence for the existence of the particle but that the
particle itself had not been directly identified. For example, there was strong indirect
evidence for the existence of neutrinos before 1940, but the electron neutrino was not
identified directly until the 1950s. After this report was completed. initial evidence was
reported for the existence of the sixth or top quark. (b) Each of four basic forces is
believed to be carried by different elementary particles. This figure identifies the particles
that carry the weak, strong, and electromagnetic forces and shows when these particles
were discovered. The gravitational force should also be carried by a particle. called the
graviton, but at present there is no indirect or direct evidence for its existence.
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INTRODUCTION 1 5
WHAT WE WANT TO KNOW
Now that we have discovered three basic types of elementary
particles, we are finally able to attack some of the central questions that
have for so long intrigued and baffled the physicist. As an example,
what determines the most basic property of a particle its mass? The
masses of the different kinds of particles vary enormously: the heaviest
known quark has more than ten times the mass of the lighter quarks,
while the tau lepton has about 3500 times the mass of the electron.
Another important question concerns whether there are more kinds
of elementary particles waiting to be discovered. Or, alternatively, can
we find a unifying principle that connects the known particles and tells
us that there are no more? As already mentioned, we also seek a
unifying principle for the basic forces. In Chapter 4 the questions that
intrigue and even haunt us are discussed.
THE TOOLS OF ELEMENTARY-PARTICLE PHYSICS
Elementary-particle 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 a flash of theoretical insight leads to new
experiments; at other times an experiment unexpectedly produces
surprising new data and upsets currently accepted theories. Thus
experiment and theory are two kinds of tools of elementary-particle
physics. In Chapters 5 and 6, we emphasize the experimental tools. We
do this because the size and complexity of these tools, particularly of
the accelerators, is a special quality of this field. But more fundamen-
tally, physics is an experimental science, and in the end it is only ex-
periments that can tell us if our ideas are right or wrong. Almost all
experiments in this field are carried out by using an accelerator to
produce high-energy particles, allowing those particles to collide, and
then using an apparatus called a detector to find out what has come out
of the collision. In the traditional arrangement, a beam of high-energy
particles produced by an accelerator strikes a stationary target. Much
of the experimental progress in the last two decades has come from
such fixed-target experiments. Examples are the demonstration that
protons and neutrons are made of quarks, one of the discoveries of the
c quark, the discovery of the b quark, and the discovery of the still
mysterious CP violation effect.
increasingly, however, particle colliders have come to play a dom-
inant role in contributing new knowledge to the field. In such acceler-
OCR for page 16
16 ELEMENTARY-PARTICLE PHYSICS
ators, two beams of particles collide head on, and this produces much
higher energy in the collision than is available in fixed-target acceler-
ators of the same size. Colliding-beam accelerator technology has been
paced by the development of electron-positron colliders and proton-
proton colliders. Experiments at electron-positron colliders have given
us the co-discovery of the c quark, the discovery of charmed particles,
the completely unexpected discovery of the tau lepton' the discovery
of jet structure in particle production, and much of the evidence for the
idea that the strong or nuclear force is carried by the gluon particle.
Experiments at a proton-proton collider studied the details of the
. . . .
interactions of quarks and gluons. Recently a proton-antiproton
collider has begun to contribute substantially to particle physics. The
most notable contribution is the discovery in 1983 of the W and Z
particles that carry the weak force.
Our understanding of the physics of accelerators, together with
inventions in accelerator technology, has now reached the point that
we can substantially increase the energy reached by colliding-beam
accelerators. Higher-energy electron-positron colliders and antiproton-
proton colliders are now under construction in the United States,
Europe, Japan' and the Soviet Union. The antiproton-proton collider
being constructed in the United States will be the highest-energy
collider in the world when completed in about 1985. The electron-
positron collider under construction in the United States uses a new
collider technology, called a linear collider, rather than the conven-
tional circular collider. Construction has begun in Europe on an
electron-proton collider' something that has never been done before.
More information on accelerators is given in Chapter 5.
In this Introduction we have not discussed the kinds of elementary-
particle research that do not use high-energy physics accelerators.
While not by any means the majority of experiments, such experiments
are important in our field. Some use lower-energy accelerators in-
tended for nuclear-physics studies, and some use cosmic rays. Some
are conducted in an ordinary laboratory setting, while others are
carried out on mountain tops or deep underground. The highlights of
this work are given in Chapters 6 and 7.
THE FUTURE TOOLS OF ELEMENTARY-PARTICLE PHYSICS
Experimental investigation of some of the fundamental questions in
elementary-particle physics requires energies higher than those pro-
vided by any accelerators now in operation or under construction
anywhere in the world. For this reason the U.S. elementary-particle
OCR for page 17
INTROD1JCTION 1 7
physics community is now preparing a proposal for a very-high-energy,
superconducting proton-proton collider, the Superconducting Super
Collider (SSC). It would be based on the accelerator principles and
technology that were developed in connection with the construction of
the Tevatron and on other extensive work on superconducting magnets
in the United States. This proposed collider would have an effective
energy range about 60 times higher than that of any collider now in
operation. Not only is the SSC needed to answer some of the questions
that we face in elementary-particle physics, but in addition such a large
increase in energy will open up new regions of elementary-particle
physics to be explored.
Since accelerators are at the heart of elementary-particle experimen-
tation, there is extensive research and development work on new types
of accelerators and higher-energy accelerators. An important part of
this work concerns extending the electron-positron linear collider to
yet higher energies. It seems quite likely that technology can be
developed to build a very-high-energy electron-positron collider. Since
the physics that can be done at such a collider is mostly complementary
to that which can be done at a proton-proton collider, the elementary-
particle physicist would hope to see both types of collider in operation
eventually.
Representative terms from entire chapter:
basic forces
