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OCR for page 18
What Is Elementary-Particle
Physics?
Elementary-particle physics deals with questions first recorded by
the philosophers of classical Greece. What is the basic nature of the
material world around us? What are the simplest, the most elementary,
kinds of matter? What are the basic forces that operate in our material
world?
Although these are very old questions' it was not until about four
centuries ago that scientists began to make progress in trying to answer
them. Some of the first answers came with the discovery of certain of
the basic forces in nature: the gravitational force, the electrical force,
and the magnetic force. It was not until the middle of the nineteenth
century that it was discovered that the electric and magnetic forces are
in fact two different aspects of the force that we now call electromag-
netism.
Progress in the study of the basic nature of matter itself also came
slowly. Indeed, it was not until the last decade of the nineteenth century
that the first of the particles that we now call elementary was discovered;
this was the electron. In the next six decades only a few more kinds of
truly elementary particles were discovered: the moon, the neutrinos, and
the photon. It is just in the last two decades that tremendous progress has
been made in our field that we have been able to understand the families
of elementary particles and have been able to get for the first time a full
view of the basic nature of matter.
This chapter is devoted to introducing the fundamental ideas of
18
OCR for page 19
WHAT IS ELEMENTARY-PARTIC~E PHYSICS? 19
particle physics that have been developed over the last 50 years. We
will attempt to present these ideas in a way that does not require a
previous knowledge of high-energy physics nor of mathematics. Chap-
ter 3 will explore our present picture in somewhat greater detail and
will describe in particular how these ideas have been developed and
verified over the last two decades.
WHAT IS AN ELEMENTARY PARTICLE?
We call a piece of matter an elementary particle when it has no other
kinds of particles inside of it and no subparts that can be identified. We
think of an elementary particle as occupying no room in space; indeed,
we often think of it as a point particle.
How do we know whether a particle is elementary? We know only
by experimenting with it to see if it can be broken up or by studying it
to determine if it has an internal structure or parts. This is illustrated in
Figure 2.1. We know that molecules are not elementary because they
can be broken up into atoms by chemical reactions or by heating or by
other means. Nor are atoms elementary: they can be broken up into
electrons and nuclei by bombarding the atom with other atoms or with
light rays. Nor is the nucleus elementary: by bombarding nuclei with
high-energy particles or with high-energy light rays called gamma rays,
the nucleus can also be broken up into protons and neutrons.
For about 50 years physicists considered the neutron and proton to
be elementary, but in the last two decades we have found that these
particles themselves are made up of yet simpler particles called quarks.
That is, protons and neutrons have other particles inside of them,
hence they are not elementary. However' we have no evidence as yet
that the neutron and proton can actually be broken up into these
individual quarks; this is a subtle point and is discussed later.
What about the electron, the other constituent part of the atom?
Despite all of our experiments and all of our probing of the electron, we
have not succeeded in breaking up an electron, and we cannot find any
evidence that electrons have internal parts or structure. This is why we
call the electron an elementary particle.
How Many Kinds of Elementary Particles Are There?
How many different kinds of elementary particles are there in the
universe? If some physicist succeeds in breaking up an electron next
year, what has happened to its claimed elementary nature? More gener-
ally, how will we ever know if a particle is truly elementary? Will there
OCR for page 20
20 ELEMENTARY-PARTICLE PHYSICS
PART1 CLE
Large
Molecule
SIZE AND STRUCTURE
t ~
, o to Cry
10 7cm~
~ Eoch Circle
Am_ ReDresents an Atom
Atom About 10-8 cm~ ~
two
Nucleus ./ -
.
Nucleus Several x 10- 1334
I Proton or
~ Neutron
Proton about 10 ':Ouarks |
Less than
Quark -16
10 cm
ENERGY
REQUI RED
1/10 of an eV
to a few eV
· · Electron Moving
· · Around Nucleus a few eV
a few MeV
Ito IOOGoV
( (;eV = 109 eV )
iLess than
Electron {T10~86cm
T
more thon
I DO GeV
FIGURE 2.1 Many basic objects in nature are made up of yet simpler objects. For
example, molecules are made up of atoms, and atoms are made up of electrons moving
around a nucleus. To the best of our present knowledge, the elementary particles.
electrons and quarks, are not made up of simpler particles. It requires larger energies to
investigate the size and structure of the smaller particles. At the right side of the figure
are shown the energies required to study the structure of the various objects. The smaller
the object, the greater the energy required.
ever be an end to the sequence of particles within particles within
particles . . .? In Chapter 3 we describe the present research on these
questions. In this section we present a historical perspective.
Figure 2.2 sketches the history of our progress in understanding the
number of kinds of elementary particles. The classical Greeks posited
just four basic elements: earth' air, fire, and water. In subsequent
OCR for page 21
WHATIS ELEMENTARY-PARTICLE PHYSICS? 21
100
o 10
c,
-
y
1
1 1 1 1 1 1
_
.m .~
c ~
at
~ _
LL
~ _
J
1/ 1
/
1 1 1, 1 1 1 1
000 0 1000 1500
Be
800 1 900 1 950 1 980 1 990
AD
FIGURE 2.2 Mankind has always tried to explain the world as made up of a limited
number of different kinds of basic matter. Until a thousand years ago, most people
believed that the basic types of matter were earth, air, fire, and water. About 1900 the
basic types of matter were thought to be the almost 100 different chemical elements. At
present we believe there are about a dozen types of basic matter, namely the leptons and
the quarks.
centuries philosophers and alchemists added aether (to include the
heavens), mercury, sulfur, salt, and so on. Already we see a simple
picture (albeit a wrong one) beginning to expand. In 1661 Boyle defined
the concept of a chemical element, and by 1789 Lavoisier had compiled
a list of 33 known elements. At this point, a modern particle physicist
might have questioned whether these elements were truly elementary.
But the list grew steadily, doubling before Mendeleev found a convinc-
ing way to classify them into smaller related families in 1868. By 1914
the number of elements had reached 85.
Then revolutionary new developments in physics led to a much
simpler picture of matter. Discovery of the electron, the proton, and
the tiny dense nucleus of the atom gave rise to the atomic model. Each
chemical element consisted of unique atoms, defined by a specific
number of electrons surrounding a nucleus made of protons. Thus all
matter seemed to be made of only two kinds of constituents, the proton
and the electron. A dramatic reduction indeed, from 85 elements to 2
particles.
OCR for page 22
22 EfEMENTARY-PARTICLE PHYSICS
The neutron was discovered in 1932, providing a more satisfactory
picture of the nucleus as a combination of neutrons and protons and
increasing the number of fundamental particles to three. In the same
year, the positron or antielectron was also discovered. The positron
was followed by the muon, the pion, and the first strange particles, all
found in cosmic rays. These particles were the first in a long sequence
of particles that were unnecessary in the sense that they were not
needed as constituents of ordinary matter. Indeed, these particles pre-
sented a problem: why did they exist at all, and how were they related to
each other? By the 1950s, particle accelerators began to produce
hordes of new particles, and their numbers grew in a way quite similar
to the number of chemical elements in the nineteenth century (see
Figure 2.2~.
As before, scientists (now physicists) tried to find patterns in the data
that might indicate some underlying simplicity. In 1964 it was proposed
that the rapidly growing number of strongly interacting particles (called
hadrons) could all be explained as simple combinations of smaller
constituents called quarks. There should be three such quarks, and
these together with the four known leptons (electron, muon, and their
associated neutrinos) would be the seven basic constituents of matter,
including the exotic new forms produced only in accelerators. At about
the same time, more detailed study of the properties of hadrons, mainly
the absence of certain decay processes, caused theorists to suspect the
existence of a fourth kind of quark. Such speculation increased with
the observation of a new type of force, the weak neutral force. This
so-called c or charmed quark was in fact discovered in 1974, as a
constituent of a very striking new kind of particle known as the Jib.
The next year, a new lepton called the T (taut was discovered, together
with indirect evidence for an associated neutrino vat. In 1976, more
charmed particles were discovered, and in 1977 a fifth quark, the b or
bottom quark, was discovered.
Thus the number of fundamental constituents of matter has now
grown to 11, and if the expected t or top quark is found it will be 12. Is
this the final roll call of the elementary particles, or will more be found
and the situation once again become complicated? We do not know the
answer to that question. Physics, like all the sciences, is based on
experimental knowledge. At any given time, all we can do is assemble
the full body of our experimental knowledge and try to explain it with
a rational and perhaps even elegant theory. If we can explain all of our
experimental knowledge with a theory that regards only a certain set of
particles as elementary, then that must be sufficient.
OCR for page 23
WHAT IS ELEMENTARY-PARTICLE PHYSICS:' 23
The Size of Elementary Particles
As one proceeds down through the sequence of molecule, atom,
nucleus, proton, and neutron, and finally quark, the size of the particles
gets smaller and smaller. Let us begin with atoms, whose size is of the
order of 10-8 centimeter (0.00000001 centimeter). This one-hundred
millionth of a centimeter is very small by everyday standards. Mole-
cules are larger, their size depending in a rough way on the number of
atoms in the molecule. Molecules containing hundreds of atoms, such
as organic molecules, can be examined by electron microscopy, and
thus can almost be seen in the ordinary sense of that word.
But once we go below the atomic level to nuclei, there is no way to
look at these particles with any sort of microscope. The nuclei consist
of neutrons and protons packed rather closely together. The proton and
neutron are each about 10-~3 centimeter in size, about 1/100,000 the
size of an atom. Nuclei are a few times bigger than a neutron or proton,
depending on how many of these particles they contain. But the nuclei
are still not much bigger than lot centimeter. The sizes of nuclei,
neutrons, and protons are too small to be found by looking directly at
the particles; they must be measured by indirect methods.
When we come to an elementary particle such as a quark or an
electron, we go to a yet smaller scale. By indirect means the sizes of
quarks and electrons are known to be less than 10-'6 centimeter—less
than 1/1000 the size of a neutron or proton! Indeed we have no evidence
that these particles have any size at all.
Thus the scale of elementary-particle physics is distances of 1o-~3
centimeter and smaller. Elementary-particle physics in its search for
the simplest forms of matter has become the physics of the very small.
Elementary Particles and High Energy
At first it seems puzzling that elementary-particle physics, the
physics of the very small, is also called high-energy physics. The term
high-energy refers to the energies of the particles used to produce
particle reactions. By high energy we mean that the kinetic energy
(energy of motion) of a particle is much higher than its rest mass
energy. Why do we need to carry out our particle reactions with high-
energy particles? There are two reasons for this.
First, as Einstein discovered, kinetic energy can be converted into
mass, and mass can be converted into kinetic energy. The equation for
the conversion is the famous E = met, where E is the kinetic energy
OCR for page 24
24 ELEMENTARY-PARTICLE PHYSICS
that can be converted into mass m, and c is the velocity of light. Since
we want to produce new particles, and particularly new massive
particles, in the reactions that we carry out, we need a large kinetic
energy E to make a large mass m.
The second reason for needing high-energy particles is that, as we
have already said, we cannot directly see the size of a particle nor
directly see if it has internal structure or parts. We must investigate the
particle's size and structure by bombarding it with other particles. And
the deeper we wish to penetrate into a particle, the higher must be the
energy of the bombarding particles.
The famous Heisenberg uncertainty principle also leads to the
conclusion that the investigation of small distances requires high
energies. If we wish to measure small distances precisely, then there
must be a large uncertainty in the momentum associated with that
measurement. A large uncertainty in momentum can only be accom-
modated by a large initial momentum. And large momentum means
large energy.
The principal way in which we give high energy to a particle is to
accelerate it through an electric field. Thus accelerators are simply
machines that have strong electric fields and that guide the particles
through those electric fields. (Chapter 5 discusses accelerators and the
basic principles of their operation.) This leads to a convenient unit, the
electron volt (eV), for measuring both energy and mass. An electron
volt is the energy acquired by an electron or proton passing through an
electric potential with a total voltage of 1 volt. As we shall see, the
electron volt is a rather small unit of energy or mass, so the elementary-
particle physicist uses larger units:
MeV - 1046 eV = 1 million electron volts
GeV = 10~9 eV = 1 billion electron volts
TeV = 10+'2 eV = 1 trillion electron volts
The significance of these energy units can be appreciated by looking
at some particle masses expressed in electron volts:
1. The electron mass is about 0.5 MeV.
2. The proton mass is about 1 GeV.
3. The heaviest known particle, the Z°, has a mass of about 100
GeV = 0.1 TeV.
4. New kinds of fundamental particles are predicted by some
theories to lie in the still higher mass range of 0.1-2.0 TeV.
In Figure 2.1 we have indicated the range of energies needed to study
each type of particle. For the elementary particles shown in the figure,
OCR for page 25
WHA T IS ELEMENTAR Y-PARTICLE PHYSICS? 25
the quark and the electron, the highest energies are needed. In Chapter
5 we describe how the energies of accelerators are related to experi-
mental studies of the elementary particles.
THE KNOWN BASIC FORCES AND FUNDAMENTAL
PARTICLES
The Four Basic Forces
One of the great triumphs of physics has been understanding that all
-the multitudinous phenomena of the material world operate through
just four basic forces. We have already mentioned two of these forces:
the gravitational and the electromagnetic. Two more were discovered
in this century. One is the nuclear or strong force, which holds the
nucleus together and also holds the proton and neutron together. The
last force to be discovered is called the weak force; we shall describe
its behavior below.
Table 2.1 gives some comparative properties of the four forces. The
gravitational force is important in our everyday lives and in astronom-
ical phenomena because of the immense mass of the planets and stars.
But the gravitational force exerted by one elementary particle is very
small compared with the three other forces that can be exerted by that
particle.
The electromagnetic forces between elementary particles follow the
same laws as the electromagnetic forces that are used in modern
technology, such as in motors, generators, and electronic equipment.
The elementary particles simply act as small bundles of electric charge
and small magnets.
The strongest of the four forces is the nuclear force. However, the
nuclear force is not felt directly in everyday phenomena, since it does
not extend beyond a distance of about 1o-~3 centimeter from the
elementary particle. This distance is about the same as the size of an
individual neutron or proton, and thus it determines the size of atomic
nuclei. Since atoms and molecules are at least 100,000 times larger,
they do not feel the nuclear force. But at distances less than 10-'3
centimeter the nuclear force is powerful, much more powerful than the
electromagnetic force. This is why it is also called the strong force.
Finally we return to the weak force. The distance over which this
force acts is also small less than about 10-'6 centimeter and it is
much less powerful than the strong force. Yet the weak force is not
negligible. In a certain sense it is more pervasive than the strong force.
Some elementary particles such as the electron are not affected by the
OCR for page 26
26 ELEMENTARY-PARTICLE PHYSICS
TABLE 2.1 The Four Basic Forces
Type of Force
Behavior over
distance
Gravitational Weak
Electro- Strong or
magnetic Nuclear
Limited to less Extends to Limited to less
than about very large than about 10~'3
1~'6cm distances cm
l~2
Strength relative to
strong force at a
distance of 10~'3 cm
Time for a typical
small-mass hadron
to decay via these
forces
Extends to
very large
distances
1~38
Particle that carries Not
the force discovered
1~'° s
W+, W~, Photon
and Z°;
intermediate
bosons
Mass of particle Not known About 90 GeV O
1~2os l~23s
Gluon. The gluon
has been
identified
indirectly but it
has not been, and
perhaps cannot
be, isolated.
Assumed O
strong force but are affected by the weak force. The radioactive decay
of the neutron and of nuclei, as well as the decays of many of the ele-
mentary particles, occur through the weak force.
Since the 1920s physicists have speculated about the possibility that
different forces can be unified into one general theory. That is, are the
seemingly different forces simply different manifestations of one gen-
eral force? First thoughts were about unifying the gravitational and
electromagnetic forces; that has not been done, and we do not know if
it can be done. But within the last 15 years, a significant unification of
the electromagnetic and weak forces has been made and has been
verified experimentally. In Chapter 3 the state of current research on
force unification is discussed.
The Known Families of Elementary Particles
At present, all our observations in particle physics can be explained
by the existence of the four basic forces and by the existence of three
families of elementary particles. These families are the leptons, the
quarks, and the force-carrying particles.
OCR for page 27
WHATIS ELEMENTARY-PARTICLE PHYSICS? 27
THE FORCE-CARRYING PARTICLES
We turn first to this family of elementary particles. It is a basic prin-
ciple of quantum mechanics that a force has a dual nature: it can be
transmitted through a wave or through a particle. The clearest example
is the electromagnetic force, which can be treated in some situations as
being carried by an electromagnetic wave (radio waves or light waves,
for example) and in other situations as being carried by a particle (the
photon). The question then arises whether the other forces also obey
quantum mechanics in this sense and thus can be thought of as being
carried by particles. Table 2.1 summarizes our present knowledge. The
weak force is indeed carried by particles: the W+, W-, and Z°
intermediate bosons have recently been discovered. We believe that
the strong force is also carried by particles called gluons, but here the
evidence is indirect. Unlike the photon, W+, W~, and Z°, the gluon has
not been isolated. Finally, the particle conjectured to carry the grav-
itational force has been called the graviton, but such a particle has
not yet been discovered, and there is no experimental evidence for its
existence. Because of the feebleness of the gravitational interaction
among elementary particles, its detection would be extraordinarily
difficult.
THE LEPTONS
ties:
The lepton family of elementary particles is defined by two proper-
1. Leptons are affected by the gravitational, electromagnetic, and
weak forces but not by the strong force.
2. Leptons must be either created or destroyed in particle-anti-
particle pairs; the total number of leptons (number of leptons minus
number of antileptons) is conserved in all processes to the best of our
knowledge.
Figure 2.3 shows the six known leptons. They come in pairs, each
pair consisting of one charged lepton and one neutral lepton. The
neutral lepton is called a neutrino. Each pair is called a generation, and
in each generation the mass of the neutrino is much less than the mass
of the charged lepton.
In the last few years there has been speculation, but as yet no evidence,
that the proton might very rarely decay to a lepton plus hadrons. If that
turns out to be true, the total number of leptons would not be conserved
in this process.
OCR for page 28
28 ELEMENTARY-PARTICLE PHYSICS
Generation Particle Charge Mass
elect ron (e ~ - I 0.5 I MeV
electron neutrino (ye) O less than 50 eV
2*_:muon(~)
| muon neutrino (v,,)
106 MeV=0.106 GeV
O less than 0.5 MeV
3 ~ tau (T)
tau neutri no* (via)
-1 1784 MeV = 1.784 GeV
O less than 160 MeV=0.160GeV
*indirect evidence
FIGURE ~.3 The six known leptons are arranged in pairs. The members of a pair
interact only with each other. For example. the electron and electron neutrino interact
with each other but not with the muon, the muon neutrino, the tau, or the tau neutrino.
There is indirect evidence for the tau neutrino; it has not been directly detected.
The questions that we now face are profound. Are there more genera-
tions of leptons? What sets the mass of the leptons, and the difference in
masses between generations? And of course the ultimate question: are the
leptons really elementary?
THE QUARKS
The quark family of elementary particles (Figure 2.4) is also defined
by two properties:
Quarks are affected by all four basic forces. Because they are
affected by the strong force, quarks act very differently from the
leptons in many situations. In particular, it is either impossible or very
difficult to isolate quarks, whereas leptons can easily be isolated.
2. Quarks, like leptons, cannot be singly created or destroyed to the
best of our knowledge. Therefore the number of quarks, like the number
of leptons, is conserved in every physical process.
A very peculiar property of the quarks is that they have electric
charges of 2/3 or 1/3 of the unit of electric charge carried by the electron
and the proton. All other particles, elementary or not, have either zero
or integral charges. Like the leptons, the quarks fall into pairs called
generations. Each pair has a +2/3 unit charge quark and a -1/3 unit
charge quark.
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WHA T. IS E' EMENTAR Y-PARTICLE PH YSICS? 37
l
/ Axis
FIGURE 2.9 An example of fourfold symmetry in a four-bladed windmill. Its axis of
rotation, marked by the central black dot, is perpendicular to the paper. The picture is
not changed by a 90° rotation about that axis.
words, it looks the same from all sides. Invariant, a word that occurs
frequently in physics, means unchanged. Physical theories can have
symmetries of a similar kind, but what remains invariant or unchanged
after a transformation is not a pattern or an object but the mathematical
structure of the laws of the theory itself. Physicists now agree that
symmetries play a central role in our understanding of nature.
The twin concepts of symmetry and invariance can be important in
limiting the equations and theories that are applied to a phenomenon.
Consider the force of the Earth's gravity on a person walking on the
Earth's surface, and use the good approximation that the Earth is a
sphere. Then without knowing anything about the laws of gravitational
force, we can make two statements from just the arguments that a
sphere is symmetric about its center for any rotation and that the
gravitational force must be invariant to any such rotation. First, the
size of the force must be the same, no matter where the person walks
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38 ELEMENTARY-PARTICLE PHYSICS
, ~
at//
\ \\\ /~/7 7
4/1 ~ - ~ ~/1 1- ~ \\\S
L I I _ _ ~ )~N TV 1 1 1 I 1 1 OR
I I __~ 1 1R 111 ~ I I I ~ if/
\\ ~ _ _ ~ \\\\\ 1- ~ 111///
\\ \ _ ~ a\\\ \ ii~
,, ~~
ORIGINAL SPHERE GLOBAL SYMMETRY
TRANSFORMaTION
(o) ( b)
t\~\~ ~
LOCAL SYMMETRY
TRA NSFORMATION
(C)
FIGURE 2.10 The ideas of global and local symmetry can be illustrated by a sphere
marked with lines of longitude and latitude. When the sphere is simply rotated about its
axis the shapes of the lines are not changed; that is called a global symmetry
transformation. If the surface of the sphere is distorted as one might do with a sphere
made out of rubber, such that the lines of longitude and latitude are twisted, that is a local
symmetry transformation.
_.
_~
on the Earth. Second, the force must point directly toward the Earth's
center or directly away. It cannot point in any other direction' east for
example, since that direction is not invariant to a rotation. But this is
as far as this symmetry argument can go; it cannot tell us whether the
force is up or down or its strength. To know that, we need first ex-
periment and observation then a theory with explicit equations.
Physicists use other symmetry and invariance ideas in much the
same way, to provide some general information and to limit the range
of equations and theories that can apply. This is particularly important
in particle physics where the basic objects, the elementary particles,
are relatively simple and have many kinds of symmetries.
The symmetries of physical theories are of two types, called global
and local. The distinction between them may be illustrated by consid-
ering an ideal spherical balloon [Figure 2.10(a)] marked with a system
of latitude and longitude coordinates so that the positions of all points
on the surface can be identified. A global symmetry is exhibited if the
sphere is rotated about some axis [Figure 2.10(b)~. In geographical
terms, the rotation depicted is equivalent to displacing the prime
meridian from Greenwich, England, to Alexandria, Egypt. This rota-
tion is a symmetry operation because the form of the sphere remains
unchanged. It is called a global symmetry because the locations of all
the points on the surface are changed by the same angular displacement
in longitude.
Local symmetry is a more demanding statement. It requires that the
OCR for page 39
WHA T IS EN EMENTAR Y-PARTICLE PHYSICS? 39
balloon maintain its shape even if the points on the surface are
displaced independently [Figure 2.10(c)~. A local symmetry operation
stretches the balloon and therefore introduces forces between points.
Each of the fundamental forces is now thought to arise from a similar
requirement that a law of Nature be invariant under local symmetry
transformation. Because the earliest attempts to construct interactions
from symmetries dealt with invariance under a change of scale or
gauge, the resulting theories are called gauge theories.
The symmetries we have discussed so far are known as continuous
symmetries, because they may be built up from infinitesimal motions.
Another important class of symmetries of physical laws is made up of
discrete, or discontinuous, transformations. Of these the most familiar
in everyday experience is left-nght or mirror symmetry, which is mani-
fested by many objects in our environment. Many microscopic physical
processes are invariant under time reversal; a film of the event, run
backwards, would also correspond to an allowable event. Similarly, in
many situations the replacement of all particles by their antiparticles
leads to no change in the physical outcome. As an illustration, the light
emitted by an antineon lamp would be indistinguishable from the light
emitted by a conventional neon lamp.
Symmetry Breaking
It may happen that the laws of physics embody a certain symmetry,
but some of their consequences do not manifest that symmetry. An
example will show how this may come about. Above a certain critical
temperature, the individual microscopic magnets that make up an iron
ferromagnet are oriented randomly. This reflects the invariance of the
laws of electromagnetism under rotations, which is to say that there is
no preferred direction in space. When the iron is cooled below the
critical temperature, the micromagnets tend to align themselves along
some randomly chosen direction. The randomness of this direction is
attributable to the rotational invariance of electromagnetism. Once the
micromagnets have frozen along a certain direction, the ferromagnet
does not display rotational invariance, because a specific direction has
been singled out. Thus the symmetry of the laws of electromagnetism
has been hidden.
In elementary-particle physics, the most striking case of symmetry
hiding occurs in the theory of weak and electromagnetic interactions.
There the equations of the quantum theory possess a local gauge
symmetry, but the observed particles such as electrons do not display
this symmetry in their masses.
OCR for page 40
40 ELEMENTARY-PARTICLE PHYSICS
EXPERIMENTS, ACCELERATORS, AND PARTICLE
DETECTORS
Experimental Methods in Elementary-Particle Physics
The purpose of experiments in elementary-particle physics is to
study the behavior of the forces that act on the particles and to look for
new types of particles and forces. But few of these studies and searches
can be carried out using the apparatus found in the usual physics lab-
oratory. For example, elementary particles are too small to be seen
using a visible light microscope or even an electron microscope.
Furthermore, many elementary particles have short lifetimes; they
simply do not exist for a long enough time to be studied directly. A final
example is that the search for new particles usually requires that other
particles collide together at high energies to produce the new particles.
The primary experimental method in elementary-particle physics in-
volves the collision of two particles at high energy and the subsequent
study of the particles that come out of such a collision. We are
interested in the kinds of particles that come out of the collisions, how
many there are, the energies of the particles, and their directions of
motion. In this section we give an overview of how such experiments
are done.
Experiments at Fixed-Target Accelerators
The basic concept of an elementary-particle experiment using an
accelerator is shown in Figure 2.1 1. A beam of protons is accelerated
to high energy by a proton accelerator. The beam of protons leaves the
accelerator and passes into a mass of material called a target, which is
fixed in position. The collisions occur between the protons in the beam
and the material in the target. Hence this is called a fixed-target
accelerator, and the experiment is called a fixed-target experiment.
The simplest material to use for the target is hydrogen, because the
hydrogen atom consists of a single electron moving around the single
proton that forms the nucleus of the hydrogen atom. Most of the time
the protons in the high-energy beam will pass right through the
hydrogen target without striking anything, but occasionally one of the
protons in the beam will hit either a proton or an electron in the
hydrogen. We restrict our attention here to the case when a proton in
the beam hits a proton in the hydrogen atom. Then we have a
proton-proton collision. As discussed earlier in this chapter in the sec-
tion on Collisions and Decays and sketched in Figure 2.6, one of the
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WHA T IS ELEMENTAR Y-PARTICLE PHYSICS? 41
(a)
. ~
Beam of High
Energy Protons\
l
I Proton Accelerator TV=——~:
(b)
To ~ '~ /
arget' A-/
Particle Detectors
Proton in To rget
A_
Path of High Energy
Proton ~ n Beam
4 Particles
Produced
In Collision
FIGURE 2.1 1 In a fixed-target experiment a beam of high-energy particles, for example
protons' is produced by an accelerator. The beam of particles interacts with the target
producing new particles. The particles are detected and their properties studied using an
apparatus called a particle detector. In (a) the entire experiment is sketched. In (b) the
interaction of the particle itself is shown: a proton in the beam interacts with a proton in
the target and produces four particles.
things that can happen is that two protons can simply come out of the
collision again. But sometimes many other particles—hadrons and led
tons—can come out of the proton-proton collision.
In order to determine what has happened, we need an apparatus that
can detect the particles coming out of the collision. Such an apparatus
is called a paIticle detector (see Figure 2.111. Particle detectors cannot
see particles directly, but they can determine their energies and
directions of motion and the nature of the particles. How this is done
is described below. Thus the three basic elements of experiments at
fixed-target accelerators are the accelerator, the target, and the particle
detector. We next describe each of these elements in more detail.
Fixed-Target Accelerators
The particles accelerated must be stable and have electric charge,
hence either protons or electrons are used. The acceleration process
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42 ELEMENTARY-PARTICLE PHYSICS
begins with these particles at rest, and gradually gives them more and
more energy until they are moving with speeds close to the speed of
light and have high energy. The particles are given the energy by the
force of electric fields acting on their charge. Since there is a limit to
how strong an electric field we can make, higher energies require larger
accelerators.
High-energy accelerators are large and expensive machines. Thus
few are built, and these are used as intensively as possible. For example,
in the United States there are only two high-energy proton accelerators.
The Alternating Gradient Synchrotron (AGS) at Brookhaven National
Laboratory has a maximum energy of about 30 GeV and has been in
operation since 1960. The Tevatron at Fermi National Laboratory, a
circular accelerator with a diameter of 2 kilometers, has just gone into
operation; it is the first large accelerator in the world to use supercon-
ducting magnets, and it is designed to reach an energy of 1000 GeV.
Also in the United States is the 3-kilometer-long electron acceler-
ator at the Stanford Linear Accelerator Center. (The complementary
uses of the different energy ranges and particle beams are described in
Chapter 5.) In addition, the United States has lower-energy proton and
electron accelerators that are used primarily for nuclear-physics re-
search.
Targets
We have already described how hydrogen can be used as a target for
the beam of particles coming out of an accelerator. Other materials can
also be used as targets. For example, deuterium is often used. In
deuterium (heavy hydrogen) the nucleus consists of a proton plus a
neutron; hence one can study collisions between the protons or
electrons coming out of the accelerator and the neutron in the target.
Another example is provided by neutrino experiments, which often
require a dense target such as iron.
Particle Detectors for Charged Particles
Not only charged particles, such as protons or charged plans or
electrons, but also neutral particles, such as neutrons and photons, can
come out of a collision. Charged means that the particle has positive or
negative electrical charge, as opposed to a neutron or photon, which
have no electrical charge. No particle can be seen directly, but as a
charged particle passes through any kind of material, it breaks up the
atoms and molecules in that material. The technical term is that it
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WHA T IS ELEMENTAR Y-PAR TI CLE PH YSI CS ? 43
. .
ionizes the material. And through that ionization the path of the
charged particle can be determined.
The bubble chamber provides the classic example. The liquid in a
bubble chamber is heated above its boiling point, but it is prevented
from boiling by high pressure in the chamber. If that pressure is
released for a short time and then reapplied, the liquid still does not
boil. However, if a charged particle passes through the chamber while
the pressure is released, the resulting ionization leads to the formation
of a string of bubbles along the path of a particle. This string of bubbles
can be photographed, as shown in Figure 2.12, to produce a picture of
the tracks or paths taken by the charged particles in their passage
through the chamber.
Ionization produced by a charged particle is used in other ways by
other types of particle detectors. In a drift chamber, for example, the
charged particle ionizes a gas, and the electrical effect of that ionization
is used to determine the particle path. In a scintillator, the ionization
produces visible light that is detected by a phototube. Some particle
detectors, such as Cerenkov radiation detectors, do not use ionization.
Chapter 6 describes particle detectors in detail, including a discussion
of how neutral particles are detected.
Secondary Particle Beams
The primary beam produced in an accelerator is always either
protons or electrons, because stable and charged particles must be
used for the acceleration process. Once the primary beam of protons or
electrons leaves the accelerator, it is often used to produce secondary
beams of other kinds of particles. Figure 2.13 provides an example in
which the primary proton beam from a proton accelerator is used to
produce a secondary beam of charged pions. This is done in a
production target in which the protons interact with the target material
to produce the pions. The beam of plans then passes into a bubble
chamber; in this example the chamber liquid is hydrogen. The pions
finally interact with the electrons and protons in the hydrogen, those
being the collisions that are being studied. Other examples of second-
ary particle beams are neutrino beams, muon beams, and photon
beams.
Particle Colliders
In many elementary-particle physics experiments it is important to
have very-high-energy collisions. Therefore through the years acceler-
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44 ELEMENTAR Y-PARTICLE PHYSICS
FIGURE 2.12 An example of a photograph of charged-particle tracks in a bubble
chamber. Two sprays of particles emerge from the two vertex points at which they were
created. The upper vertex is the point at which a neutral charmed meson decayed into
four charged particles: Do ~ K+7~+~r-~-. The decay distance was 9 millimeters, which
corresponds to an unusually long lifetime for this particle of 5.5 x 10-'~ second. The
photograph is from the SLAC Hybrid Facility Photon Collaboration.
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WHATIS ELEMENTARY-PARTICLE PHYSICS? 45
P ro ton
Acce le ra for
Bubble
Prod uct ion Chamber
~ L_=~==-~
P roto n Charged Pion
Be a m Ben m
FIGURE 2.13 In many accelerator experiments the primary particle beam from the
accelerator is used to produce a secondary beam, and experiments are carried out with
the secondary beam. For example, a proton accelerator can be used to produce a beam
of charged pions through the interaction of its primary beam with a production target.
The secondary beam of plans is then used for experiments.
ator builders have put higher and higher energy accelerators into
operation: our phrase is ''pushing the energy frontier." But in fixed-
target experiments the useful energy for the collision does not increase
nearly so fast as the energy of the primary beam increases. Hence in
fixed-target accelerators it becomes increasingly expensive to keep
pushing the energy frontier.
The alternative is to collide two beams of particles moving in
opposite directions, as shown in Figure 2.14. In this case the useful
energy is actually the sum of the energy of each of the two beams (if the
two beam energies are equal). Particle colliders now produce the
highest useful energy of any of our machines.
In particle colliders both beams must consist of stable, charged
particles; the choice in practice has been restricted to protons and
electrons and to their antiparticles antiprotons and positrons. The
most common form of collider uses opposing beams of electrons and
positrons. This is because the collision of an electron and a positron is
often relatively simple to understand. On the other hand, the highest-
energy collisions are at present obtained with protons colliding with
antiprotons.
In Chapter 5, the section titled Accelerators We Are Using and
Building describes the world's particle colliders; here we give a few
examples. Operating electron-positron colliders range in energy from a
few GeV to 45 GeV. The Stanford Linear Collider under construction
in the United States will yield 100 to 140 GeV in energy, and the LEP
electron-positron collider being constructed at the CERN laboratory in
Europe can eventually reach over 200 GeV. CERN is now operating a
proton-antiproton collider with a total energy of over 500 GeV, and the
Fermi National Accelerator Laboratory in the United States has a
2000-GeV proton-antiproton collider under construction. The elemen-
tary-particle physics community in the United States is now discussing
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46 ED EMENTAR Y-PARTICLE PHYSICS
( o )
Col I is ion
(b) Occurs Here\
Collision
Occurs Here
SO Target
FIXED TARGET
COLLI D I NG BEAMS
FIGURE 2.14 (a) In fixed-target experiments. a beam of high-energy particles collides
with particles at rest in a target. (b) In colliding-beam experiments, two beams of
high-energy particles collide head-on. Colliding-beam experiments allow the experi-
menter to reach much higher effective energies when studying the interactions of
particles.
the possibility of the construction of a proton-proton collider to reach
40,000 GeV.
Experiments at Particle Colliders
Since there is no fixed target in a particle collider, the particle de-
tector must look directly at the region where the opposing beams of
particles collide. Figure 2.15 shows how this is done in a circular
collider where the beams of particles move in opposite directions
~ BEaMS ~
r or cO~LlDE
~ HERE ~
~ -_ ~ .~. ~ ~
FIGURE 2.15 In the simplest form of colliding-beam facilities, two beams of particles
rotate in the same direction in circles that are tangent at just one point. The beams collide
at that point.
1
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WHA T IS ELEMENTAR Y-PARTICLE PHYSICS? 47
Path of
Neutral Decay
K Meson Poi n t
F--~--~-
Vacuum Pipe
Charged Pion
Particle Detector
Path of ``'
Charged
Pion
1
FIGURE 2.16 Sometimes the decay of a particle is of interest. The sketch shows how
the decay of a neutral K meson into two charged plans is studied. This is one of the
crucial experiments in the study of CP violation.
around two circles! In this simple example the beams collide at just one
point. In a real collider, the beams would be arranged to collide at
several different points, providing the opportunity to carry out several
experiments at once.
The Decays of Particles
Until now we have discussed the most common form of experiment
in which the collision of two particles is studied. Sometimes, however,
we study the decay of a single particle. Figure 2.16 illustrates this by an
experiment that studies the decay of a neutral K meson to two charged
pions.
Experiments in Elementary-Particle Physics Without Accelerators
A large variety of experiments in elementary-particle physics is
carried out without using accelerators. Some of the experiments use
particles from fission reactors or from cosmic rays. Others look for new
particles, such as free quarks or magnetic monopoles, in ordinary
matter. Still others study with great precision the properties of the
stable or almost stable particles, testing, for example, the equality of
the size of the electric charge of the electron and the proton. In Chapter
6, the section on Facilities and Detectors for Experiments Not Using
Accelerators takes up this subject.
Representative terms from entire chapter:
strong force
