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6
.
Instr uments and Detectors for
Elementary-Particle Physics
INTRODUCTION
Elementary particles cannot be seen directly; their path and energy
as they come out of an accelerator or out of a collision must be
determined indirectly. It is also important to identify the type of
particle: electron, muon. proton, or photon, for example. Thus the
detectors, which determine the path, the energy, and the particle type.
are often complex. The development and construction of these detec-
tors, and the analysis of the data produced, are the province of the
particle experimentalist. The strong interest in rare processes (such as
W and Z production in the recent CERN experiments) and the need to
characterize events completely have led to the development of detec-
tors sensitive to almost the total solid angle. At all accelerators but
particularly at particle colliders it is essential to provide detectors
capable of making the fullest use of the particle beams. A wide range
of detectors exists including the small but often sophisticated instru-
ments designed for fixed-target work, the large detectors used tor
recording rare events such as the interactions of neutrinos or the
decays of nucleons, and the large collider detectors that provide almost
complete angular coverage and characterization of the interactions
occurring in these machines. This latter class of collider detectors 1`
among the most costly and demanding. Their technological problems
and solutions are, in large part, shared with the other classes fit
132
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INSTRUMENTS AND DETECTORS 133
detector. The following discussion. for simplicity. will largely concen-
trate on the development, construction, and needs of this collider
detector class. However, later in this chapter in the section on
Detectors in Fixed-Target Experiments we present some highlights of
the detectors used in these experiments. And in the final section of this
chapter, we discuss experiments that do not use accelerators.
As an introduction to large detectors. we present the Mark 1 detector
first used at SPEAR in 1972. It was the first electronic detector for a
particle collider with close to full angular coverage and with a magnetic
field to provide momentum and energy measurements for charged
particles. A view of the Mark 1 is shown in Figure 6. 1, and a
reconstruction of a ~ (psi) event is shown in Figure 6.2. This detector
was sophisticated for its era, allowing the discovery of the ’, the tau
lepton. and charmed particles.
A bare decade later comparably important results have begun to flow
from the detectors at the CERN proton-antiproton collider with the
discovery of the Z and W particles. Again these discoveries and the
realization of the potential of the accelerator are only made possible by
the sophistication of the detectors. The enormous advances in detector
technology over the last decade are best illustrated by contrasting the
view of a ~ event in the Mark 1 detector with the enormously more
detailed picture from the UAI detector at CERN of an event with a Z°,
shown in Figure 6.3. The actual configuration of the immense 5000-ton
UA1 detector is shown in Figure 6.4.
Fixed-target detectors usually are more selective than the large
collider detectors. The neutrino detectors are vast instrumented tar-
gets, usually incorporating magnetic analysis of produced muons and
calorimetric measurement of produced hadrons. The target of a
Fermilab neutrino detector, shown in Figure 6.5, has an instrumented
mass of 690 tons, consisting of 20-cm iron slabs interleaved with drift
chambers, followed by 420 tons of momentum-analyzing toroidal
magnets. Such large masses are required to achieve a reasonable
interaction rate from the weakly interacting neutrinos. In fixed-target
experiments, the kinematics of high-energy relativistic collisions re-
sults in most of the final-state particles from an interaction being
thrown forward into a relatively narrow cone. Consequently, fixed-
target experiments generally appear as a linear sequence of detector
elements downstream from the target, as Figure 6.5 clearly illustrates.
We return to fixed-target detectors in a later section.
The larger detectors constitute major facilities, with a lifetime of
usage of typically more than 10 years. They cost in the range of $10
million to $50 million and compete for resources with even the large
OCR for page 134
134 ELEMENTARY-PARTICLE PHYSICS
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collider was the Mark I detector shown here. (b) Cross-sectional view of the Mark 1
detector.
OCR for page 135
INSTRUMENTS AND DETECTORS 135
.
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FIGURE 6.2 A computer reconstruction of an event found by the Mark I detector in
which a +' particle decays to a ~ particle plus two pions; the ~ then decays to two
electrons. The ’' particle was in the small circle at the center. and the four lines coming
out indicate the paths of the four particles produced in the decay. The lower-energy pion
tracks are curved more strongly by the magnetic field. This particular picture became
well known because the four paths also happened to form the Greek letter ’.
parent accelerators. Unlike the early pioneering experiments of parti-
cle physics, a modern experiment may well require the simultaneous
collaboration of several hundred physicists from 20 or more institu-
tions. These major facilities require resources comparable with those
used in the construction of the parent accelerator.
DETECTOR REQUIREMENTS AND PHYSICAL PRINCIPLES
OF DETECTION
A detector system should be able to measure, as completely as
possible, all the characteristics of the produced particles within an
event. This implies that the detector should function over the largest
possible angular range, measure with the best attainable precision, be
provided with instrumentation to identify particle characteristics, and
simultaneously provide a wide range of cross checks to protect against
measurement artifacts. Additionally for use with hadron colliders,
detectors must be able to extract interesting classes of physics events
from backgrounds of events perhaps a hundred million times more
frequent. These challenges must be met while simultaneously keeping
OCR for page 136
136 ELEMENTARY-PARTICLE PHYSICS
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FIGURE 6.3 A computer reconstruction of an event produced at the CERN' proton-
antiproton collider; this event includes the production of a Z° particle. in a) the tracks of
all the particles produced in tile event are shown. In b) the two tracks of the electron and
positron produced in the event are shown by themselves; this electron-positron pair
comes from the decay of the Z°.
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INSTRUMENTS AND DETECTORS 137
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FIGURE 6.4 The UAI defector et CERN which produced the event shown in Figure
6.3. Note the enormous size of the detector compared with that of the person standing
at its lower right side.
the combined costs of construction, operation, and data-reduction
within reasonable bounds.
The basic physics underlying detector operation can be summarized
as follows:
· Charged particles lose energy by ionization processes and leave a
track or trail of ionized atoms and electrons as they pass through
gasses, liquids, or solids. A wide range of techniques serves to measure
the position and magnitude of these ionization trails. The magnitude of
this energy loss per unit length is a measure of particle velocities.
· Velocities of particles may be determined from the time interval
required to pass between two points. In general this technique differ-
entiates velocities only for relatively small particle energies.
· In the presence of a magnetic field, charged particles are deflected
into curved orbits. Measurement of this curvature permits the mo-
menta of these tracks to be determined. The particle's energy can be
calculated from its momentum if the mass is known.
· Characteristic, but weak, radiation is coherently emitted by par-
ticles passing through material Cerenkov radiation; or by particles as
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138
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OCR for page 139
INSTRUMENTS AND DETECTORS 139
they cross an interface between different materials transition radia-
tion; or as they pass through magnetic fields synchrotron radiation.
The intensity and characteristics of these radiations can serve as the
basis of a velocity measurement.
· When electrons or photons pass through matter they produce
characteristic electromagnetic cascades of secondary radiation, which
in turn leave an intense core of ionized atoms. The energy of the
original electron or photon is ultimately completely converted into
ionization by these processes. Measurement of this converted energy
in a sufficiently thick block of material determines the total incident
electromagnetic energy and constitutes a calorimetric shower-energy
measurement. A detector constructed to make use of this property is
an electromagnetic calorimeter.
· Hadrons, such as protons and mesons. interact strongly as they
pass through matter, producing secondary hadrons. This again results
in the production of intense cores of ionized atoms. The total energy of
the incident particles can be measured from the total energy deposited
in the form of ionization. Hadronic cascades can be differentiated from
the electromagnetic cascades that develop in much thinner layers of
material. The technique of measurement is known as hadron calorim-
etry. Typically a hadron calorimeter might require a thickness of 3 to
4 feet of instrumented steel with a total weight of hundreds of tons.
· Energetic muons are uniquely characterized by the property that,
although charged, they penetrate large thicknesses of material and
emerge with a relatively small change of energy at the outside of a
detector.
DETECTORS FOR COLLIDER EXPERIMENTS
Modern detectors typically make use of all or many of the above
properties to characterize detected events. The characteristics of these
detectors have many features in common and share similar design
architectures. The detectors for collider experiments are based on a
series of concentric shells or layers, one behind the other, each of
which is devoted to some particular aspect or aspects of the detection
process. The initial detector layers are used to characterize the
charged-particle component and are designed to be nondestructive,
i.e., to contain little material so that charged particles will not interact
or degrade in energy and thus will maintain their identity while
traversing the layers. The outer detector layers deliberately use large
amounts of material in order to materialize the neutral particles and to
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140 El EMENTARY-PARTICLE PHYSICS
convert the energy carried by the particles into detectable ionization;
such a device is known as a calorimeter.
.
The outer layer of the calorimeter or an additional detection layer is
frequently used to detect muons. As mentioned earlier, energetic
muons are usually the only particles that can reach this outermost
layer.
Inevitably as the layered levels of detection systems are built up, a
detector will become large and correspondingly complex and expen-
sive. A prime objective of detector development is, therefore' to keep
detection systems as compact as possible and to combine detection
roles whenever possible.
Additional demands are imposed on detector systems associated
with hadron colliders by the high ambient radiation levels at the
detector and by the fact that events of interest may be separated only
by short times from uninteresting background events.
Summarized below are the elements or layers constituting a typical
modern detector system and some of the ongoing research and
development aimed at maximizing present or future detector capabili-
ties.
. . .
Close-in Detection: Vertex Detectors
A fraction of the particles emerging from a collision point decay in
flight at distances as close as 0.001 cm. Such
decays provide charac-
tenst~c signatures as to the nature of the decaying particles. Therefore
use can be made of charged-particle detectors with high spatial
resolution that are placed as close to the interaction point as possible.
The first such vertex detector for collider work was recently con-
structed to operate with the Mark It detector at the PEP collider at the
Stanford Linear Accelerator Center. Several such detectors are now
being constructed or are operating at electron-positron and proton-
antiproton colliders.
The first generation of such vertex detectors was h~cer1 on ~nn~ren-
tional track-detection methods for charged particles, using multiwire
drift chambers or time-projection chambers (TPC). (The principles of
operation of these track detectors are described below.) The chambers
are typically only about 10 cm in radius, are fabricated with fine
subdivisions to provide separation between adjoining tracks that might
otherwise overlap, are aligned with great precision (about 0.009- to
0.005-cm tolerance), and ultimately are likely to be operated under high
pressures to provide sharp internal localization of the trail of ionization
left by the charged particles. The limits of precision for such detection
__ ~ W~ ~ v~ vim Al ~~!
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INSTRUMENTS AND DETECTORS 141
are at present about 0.01 cm; this should eventually improve by a factor
of 2 or 3.
The second-generation vertex detectors now under development are
based on modern silicon semiconductor technology. This technology
has already been used successfully on a small scale in experiments
designed to measure decays of short-lived particles. which decay close
to the parent event. In these experiments the target is constructed of
microstrips' and a detailed history of events occurring within these
active targets can be recorded. Prelirrunary tests have been made and
have established the feasibility of the proposed vertex detectors.
Full-scale detectors should come into operation during the next 5
years. Three approaches are being tried: ( 1 ) the use of packages of long
thin silicon strips with widths of about 0.009 cm (2) the use of a mosaic
of semiconductor squares as currently exist in the charge-coupled
devices (CCD) that are used as a basis for image detection in astro-
nomical and other applications, and (3) a more speculative idea that
involves drifting the ionization over a relatively long distance within
the silicon.
The handling, precision alignment, and electronic readout of such
miniaturized devices present fascinating but soluble problems. With
reasonable confidence this second generation of detectors should pro-
vide previsions an order of magnitude better than those currently
obtainable.
Charged-Particle Tracking Chambers
In a typical collider detector, beyond the vertex detector are
charged-particle tracking chambers. These chambers serve to measure
the directions and curvatures of the paths of the individual particles.
These paths are called tracks. The principle of operation of a multiwire
drift chamber is shown in Figure 6.6. The first such devices were
coarse and measured only a few tracks. Modern devices are fine
"rained in subdivision and may provide over a hundred measured
points to a track. The resulting image is almost of photographic quality
and is reminiscent, even though produced at electronic speeds, of the
superb track detection provided in bubble chambers. With some
additional effort and with certain possible compromises these detectors
can also be used to measure the ionization of the produced tracks.
An elegant variant of this detection method is to remove the fine grid
of wires and to drift the ionization with a collection electric field to the
end caps, where the arrival positions of the ionization are measured
and also the arrival times and the degree of ionization. The arrival
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142 ELEMENTARY-PARTICLE PHYSICS
+ H V
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FlGURE 6.6 Most detectors use drift chambers or devices derived from drift cham-
bers. In a drift chamber, slender wires are strung parallel to each other in a volume of
gas. When a charged particle passes through the gas it leaves a track of ionized gas
molecules and electrons. Electrons are attracted by the electrical voltage on the wires,
and when they reach the wire they send an electrical pulse down the wire. Those pulses
are collected and amplified electronically and recorded on magnetic tape. The position of
the track is given roughly by knowing the wire that gave the signal. But a more accurate
position is obtained knowing the time the electrons took to drift to the wire, hence the
name drift chamber.
times provide a measure of the depth at which the particle was
produced, because the ionization drifts under the influence of the
collection fields with a fixed velocity. This system. known as the
time-projection chamber (TPC), has been implemented and is currently
used in the TPC detector at PEP.
Charged-particle tracking detectors are often immersed in a magnetic
field in order to make it possible, from measurements of the track
curvatures, to determine the signs of the charges and the momenta of
the particles. Magnetic fields in the range of several kilogauss to
several tens of kilogauss are used. The larger the field, the more the
tracks curve, and the easier it is to measure the track momentum. To
provide the highest possible magnetic fields, it is desirable to use
superconducting coils to carry the required large currents. Although a
number of these coils have been constructed and successfully oper-
ated, the technology of fabrication is demanding, and further research
is desirable.
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Representative terms from entire chapter:
proton decay
146
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INSTR UMENTS A ND D, TECTORS 147
muon neutrinos, produced by the primary proton beam, are allowed to
travel a long distance before striking the detector. The detector is of
moderately large size but simple construction; its function is to detect
neutrinos and to distinguish between muon neutrinos and electron
neutrinos.
Large or Complex Fixed-Target Experiments
A major fixed-target facility that demonstrates many instrumentation
techniques is the Fermilab Tagged Photon Spectrometer, shown in
Figure 6.9. It is intended primarily for studying the production of
charmed particles. A photon beam produced by the primary proton
beam strikes a liquid hydrogen target. Recoiling protons are identified
by the recoil detector. and the produced particles are analyzed in the
forward spectrometer. The spectrometer has magnetic analysis to
measure charged-particle momenta; Cerenkov counters to identify
pions. kaons. and protons; electromagnetic calorimetry to measure the
energy of neutral hadrons; and finally a set of scintillation counters
behind an iron filter to detect penetrating muons. This sequence of
analysis steps is the same as that used in most collider detectors, but
the target is not surrounded by all the components of the detector as it
is in a collider detector.
Figure 6.10 shows a rather complex detector that combines modern
detector elements with the old nuclear emulsion technique. A nuclear
emulsion is a thick photographic emulsion that when developed shows
the paths of charged particles that have passed through it. This detector
was used at Fermilab to measure the lifetimes of charmed mesons. The
emulsion gave precise pictures of how the mesons decayed close to
their production point.
Bubble Chamber
The bubble chamber, invented in the 1950s, was for many years the
workhorse of elementary-particle physics experiments. A bubble
chamber uses a superheated liquid, such as liquid hydrogen, neon, or
Freon. Charged particles passing through this liquid leave tracks of tiny
bubbles, which are photographed. The bubble chamber has gradually
been replaced in most experimental applications by electronic detec-
tors. The latter are more versatile, often give more information about
the products of the collision, and usually provide that information in a
form that can be directly used in computers. Nevertheless, the large
volume and precise track information provided by bubble chambers
148
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INSTRUMENTS AND DETECTORS
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FIGURE 6.10 This fixed-target particle detector combines the old technique of the
nuclear emulsion with new techniques of chambers. Used to measure the lifetime of
charmed particles, the emulsions give a precise location for where the charmed particle
decayed.
still makes them most suitable for certain types of experiments. Chief
among these are the study of neutrino interactions and the study of
particles with short lifetimes. Recent improvements in bubble-chamber
technology include high repetition rates, precise track measurements,
and holographic photography.
DATA REDUCTION AND COMPUTERS
Experiments in high-energy physics characteristically have pro-
duced great quantities of data, whose reduction and physical interpre-
tation have made up a significant component of the experimental effort.
In recent years, apart from the sheer increase in scale of these
experiments, data reduction has evolved to become more integrated
with and intrinsic to an experiment's operation. A particular example
is Monte Carlo computer simulation, which has become an important
means of experimental calibration. New detector capabilities have
made this evolution both possible and necessary. Very precise time
resolution (billionths of a second. in some cases) has become possible
over large detection volumes, allowing selective recording of particular
event classes that make up only a tiny fraction of the total rate. In turn,
these fast detectors can supply information in a form that can be
.
150 ELEMENTARY-PARTIC!E PHYSICS
rapidly digitized and processed to provide the criteria for real-time
event selection.
Advanced systems for the reduction of high-energy data have unified
the traditionally separate functions of trigger decision making, data
logging. experimental control, and off-line event analysis. The trigger is
critical to the success of many experiments because collisions may
occur at a rate exceeding 106 per second. In order to select those
interactions that are of particular interest in an experiment. a trigger is
used. The trigger is a fast electronic decision made to record the data
from a particular event on the basis of the signals received from the
detector.
The triggering decision itself may be based on detector input that no
single computer could process fast enough. Fortunately, the repetitive
nature of these calculations can be adapted to the use of fast but
relatively primitive processors working in parallel. These are the first
steps in what traditionally would have been termed off-line analysis.
Without the results of these and other computations' the operation of
advanced detectors cannot be monitored or controlled. Even the
logging of data onto tape may use many levels of the data-acquisition
system. For example, a single trigger may contain 100~000 characters
of data (the equivalent of a 30-single-spaced-page report), which must
be assembled from widely distributed local memories into the image of
one event. One experiment in one year can accumulate data amounting
to 10 million such reports. Because of the enormous software devel-
opment required programs for real-time and off-line event analysis
increasingly must share common subroutines and other features. The
distinction between real-time and off-line analysis is further blurred by
the scale of processing power that must be dedicated -to a single
experiment, in notable cases reaching a level comparable with that of
a mayor computer.
The trend toward large-solid-angle general-purpose detectors at the
colliding-beam facilities has been noted. As the collider energy rises,
the events become more complex, and the amount of computer time
required to analyze these events becomes large. For example, a Z"
decay into hadrons has an average of about 20 charged particles and "
neutral particles, and the off-line analysis time for such events in the
detectors proposed for the SLC or LEP is of the order of 100 seconds
of central processor unit (CPU) time for moderate-size computers.
Millions of such events per year may need to be processed. As another
example, it has been estimated that a dedicated capacity equivalent to
tens of moderate-size computers will be required to process the inter-
esting events from the 2-TeV proton-antiproton collider at Fermilab. it
INSTRUMENTS AND DETECTORS 151
is expected that the computer requirements for the high-luminosity
SSC machine will greatly exceed the present level, as event rates,
complexity, and detector sophistication will all increase.
These requests for computer power are marginally met by current
commercially available computers. The supercomputers tend to be
machines optimized for vector or array calculations typically encoun-
tered in solving large sets of partial differential equations, such as in
weather prediction. These machines are not well suited to the large
input-output (10) requirements of higb-energy physics nor to the large
address-space requirements of the detector analysis codes. Some
manufacturers have addressed the 10 problems and have reasonable
CPU power but are relatively weak in modern software tools and in the
system architecture to support them. These are important issues in
high-energy physics, where the data production codes are being
constantly improved. and data analysis involves significant amounts of
programming, almost all of which is done by the physicist. Tools that
improve the efficiency of the physicist are clearly valuable. Finally, the
manufacturers of superminicomputers who have advanced the soft-
ware state of the art do not produce machines of sufficient CPU power
to analyze the data from a modern collider detector.
The present situation is sufficiently serious to motivate noncommer-
cial attempts to provide adequate computing power. Most of these
attempts are based on the relatively large fraction of CPU to 10 activity
that characterizes detector event analysis, so that many relatively
simple. laboratory-designed processors can work on events in parallel,
while being controlled by one commercial host computer with exten-
sive 10 facilities. Examples are the emulator developments at SLAC
and CERN and the multiple-microprocessor project at Fermilab.
Related projects involve even more specialized processors designed for
lattice gauge theory calculations or accelerator ray tracing. It is, of
course, possible that commercial developments will prove adequate in
the next few generations of machines, but the present situation is
murky. Computing represents a significant expense at the national
laboratories and universities in terms of actual hardware, support
personnel, and physicist involvement. Even with the rapid decline in
the unit costs of computing (crudely a reduction by a factor of 2 every
3 years)' the overall costs of computing increase.
Thus computers of both large and medium size are now necessary
parts of almost all elementary-particle physics experiments. Not only
are they needed to reduce and study the data, but they are also used to
monitor and control the experimental apparatus. The extensive use of
computers has stimulated advances in some types of computer tech-
152 E~EMENTARY-PARTICLE PHYSICS
nology and systems. This is because physicists have been willing to
work with computers that were still in an early stage of production,
interacting closely with the computer manufacturers.
.
FACILITIES AND DETECTORS FOR EXPERIMENTS NOT
USING ACCELERATORS
Most particle-physics experiments use accelerators, but some do
not. In this section we sketch some of the ways in which elementary
particles are studied without using accelerators.
Atomic, Optical, Electronic, and Cryogenic Experiments
Elementary-particle physicists are concerned with precise measure-
ments of the properties of the more stable particles, such as electrons,
positrons, muons, protons, and neutrons. For example, the electric
charge of the proton is the same magnitude as the electric charge of the
electron according to the most precise measurements that can be made.
This is one of the reasons why we believe that there is a connection
between the leptons (the electron is a lepton) and the quarks (the
proton is composed of quarks). Such precise measurements are carried
out using the methods of atomic, optical, electronic, or cryogenic
physics. These include measurements of the electron g-factor, of the
positronium Lamb shift, and of parity violation in atomic systems.
These methods are also used to search for new types of particles in
matter. Two sorts of searches have particularly intrigued particle
physicists. One is the search for free quarks as contrasted with the
quarks that are bound together inside protons and neutrons. The other
is the search for magnetic monopoles, that is, for an isolated magnetic
pole. All known particles that have magnetic properties have two
magnetic poles, one north and one south, of equal size. None of these
searches has produced generally accepted evidence for free quarks or
monopoles. But more definitive searches are under way.
Experiments Using Radioactive Material or Reactors
There are experiments using radioactive material or reactors that are
important in both elementary-particle physics and nuclear physics. An
outstanding example is the study of the radioactive beta decay of
tntium. An electron and a neutrino are produced in this decay' and the
mass of this neutrino can be measured. The mass of the neutrino is a
pressing question: is it exactly zero or, as indicated in a recent tri-
INSTRUMENTS AVID DETECTORS 153
tium-decay measurement in the Soviet Union. is the mass nonzero?
Another example involves different forms of beta decay that have been
proposed, such as the production of two electrons but no neutrino.
This would violate our present theory of the weak force. and thus it is
important to ascertain if such a decay exists.
Reactors produce electron neutrinos, which are being used to study
the stability of these particles. The most recent experiments find no
confirmed evidence for neutrino instability. lncidently, evidence for
neutrino instability is also being sought with solar neutrinos, as de-
scribed below. as well as with neutrinos from accelerators. This
illustrates how the exploration of an area in particle physics spreads
over all the experimental techniques. Reactor experiments have also
set important upper limits on the neutron's electric dipole moment.
Experiments Using Cosmic Rays
Cosmic-ray physics is concerned with three areas: the origin of
cosmic rays, the use of cosmic rays as probes of extraterrestrial
phenomena, and the use of cosmic rays to study elementary particles.
It is the third area that concerns us here.
Earlier in this century, cosmic rays were the only source of very-
high-energy particles; hence substantial discoveries in particle physics
were often made with cosmic rays. Prominent examples are the
discoveries of the positron, the muon, and some of the strange
hadrons. However, accelerator experiments have gradually displaced
cosmic-ray experiments. Cosmic-ray experiments at present can only
contribute to elementary-particle physics at extremely high energies,
but unfortunately the flux is then small. This is shown in Figure 6.1 1.
The most ambitious facility for the study of the highest-energy
cosmic rays in the United States is the University of Utah~s Fly's Eye
detector. Here a matrix of about 1000 phototubes in two clusters
separated by 3.3 km observes, with good spatial and time resolution,
the atmospheric scintillation light from cosmic-ray air showers. These
air showers come from the interactions of very-high-energy cosmic-ray
protons (energies above 108 GeV) with nuclei in the upper atmosphere.
Because the cosmic-ray flux is so low at the highest energies (less
than one per year per km' above 10'° GeV), it seems impractical to
study these events in any way except through the study of air showers.
Even an ambitious space station would not be able to support a
detector sufficient to address this energy regime. By utilizing the
atmosphere as a target, the Fly's Eye technique can explore a very
large area, of the order of 100 km or greater. If the detector were high
154 ELEMENTARY-PARTICLE PHYSICS
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FIGURE 6.11 The integral cosmic-ray flux. the number of primary cosmic-ray nuclei of
energy greater than E. plotted versus E expressed in electron volts (eV). The vertical
scale is expressed as particles per square meter per second per steradian (left) and as
particles per square kilometer per year per steradian (right). The energy scale is also
given in terms of the equivalent proton-proton center of mass energy, `/.s. The shading
represents the experimental uncertainty of the flux determinations.
in the atmosphere, the details of the first interaction would be more
accessible. Again, this approach is the only access to particle physics
at energies greater than even a 40-TeV proton-proton collider can
provide, which is equivalent to about 109 GeV (10~8 eV) for cosmic-ray
interactions.
Turning to another example, current unified theories predict that
there should be massive magnetic monopoles with a rest mass of about
10'6 times the proton mass and further that these should have been
produced in the early universe and still be present among cosmic rays.
An experiment in early 1982 reported evidence for the passage of one
such monopole through a superconducting coil of a few cm' area. A
large number of other experiments, using larger superconducting coils
INSTRUMENTS AND DETECTORS 155
and also searching for slow, lightly ionizing particles, have subse-
quently failed to see evidence for monopoles. Larger monopole detec-
tors are now being built.
Another volume in this survey describes cosmic-ray experiments in
more detail. with particular emphasis on their impact on questions in
astrophysics and cosmology.
The Solar Neutrino Experiment
A major nonaccelerator facility with ramifications in particle phys-
ics, nuclear physics, and astronomy is ~ Brookhaven National Labo-
ratory detector for solar neutrinos. This detector is seeking evidence
for inverse beta decay produced by neutrinos from the Sun. This
process. whereby a neutrino produces a transition of a chlorine nucleus
to an argon nucleus, would be a clean signature for solar neutrinos.
After some years of operation and a multitude of careful checks, the
observed rate of argon production is only about one third to one fifth of
the predicted rate. Either our nuclear physics and astrophysics under-
standing of the solar furnace in which hydrogen is burned in thermo-
nuclear reactions is incorrect or some of the neutrinos decay before
they reach the Earth (and neutrinos are unstable) or the experiment is
wrong. Although this important problem remains unsolved, no new
solar neutrino detectors are now being built. However. a new kind of
solar neutrino detector using gallium has been proposed and designed
in detail. Such a detector would be sensitive to the lower-energy
neutrinos coming from the Sun. This is an advantage, since the number
of these neutrinos predicted by theory is less dependent on a complete
understanding of the conditions in the interior of the Sun.
Searches for the Decay of the Proton
Until the last decade the proton was regarded as absolutely stable;
that is, it was assumed that the proton could not decay to any other
particle. However, the realization that quarks, and hence the proton,
are related to leptons has been growing. Therefore we have begun to
consider the possibility that the proton could decay to either an
electron or a muon (both leptons) plus other particles.
This possibility, made quantitative by grand unification theories, has
led to a first-generation family of underground proton-decay experi-
ments. As of the end of 1983. the earliest very large detector, built by
the lrvine-Michigan-Brookhaven (1MB) group, had seen no evidence of
proton decay in about 4000 tons of water. This implies that, depending
156 ELEMENTARY-PARTICLE PHYSICS
.
on the mode of decay, the proton lifetime is probably greater than 103'
to 1032 years. This detector employs about 2000 photomultipliers
mounted on the walls of an 8000-m; water tank. The signals are from
Cerenkov radiation in the water.
Several large detectors in Europe, India. Japan, and the United
States are or soon will be operating with only somewhat smaller
masses. Some of these detectors use ionization for tracking and
calorimetry' rather than the Cerenkov technique, and will therefore be
more sensitive to some decay modes than the IMP experiment.
Together with further 1MB data these detectors should either identify
proton decay or set lower limits to the proton decay lifetime of between
1032 and 1033 years for each of several expected decay modes.
SUMMARY AND FUTURE PROSPECTS
In the preceding discussion a number of trends in instrumentation
and detection systems for high-energy physics have been identified.
Jets signatures produced by quark interactions at high momentum
transfers have led to great emphasis on finely segmented detection
systems and to calorimetric detectors for the measurement of energy
flow. New generations of quarks and leptons with lifetimes in the range
of one trillionth of a second have stimulated new progress in the
development of electronic systems for high-resolution vertex detec-
tion. Emphasis on rare processes has required virtually all major new
detectors to be designed for efficient operation over nearly the full
angular acceptance. For successful detection under conditions of high
ambient rate, new levels of sophistication and integration of data-
acquisition systems have become necessary.
Continued progress in the development of instrumentation and
detection systems is the experimental high-energy physicist's greatest
challenge: these systems provide both the raison d'etre for accelerator
facilities and the means by which our theoretical understanding can be
confronted by experiment. Support for the development of high-energy
physics instrumentation should grow in breadth and intensity. Basic
research in the development of new detectors for high-energy physics
increasingly should be recognized and supported as a fundamental
source of much progress in the field. Correspondingly, the experimen-
tal stations at our front-line accelerators should be used as effectively
as available technology will reasonably permit.
