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OCR for page 81
4
Elementar~r-Particle Physics
What We Want to Know
INTRODUCTION
.
We saw in Chapters 2 and 3 that developments in elementary-particle
physics during the past decade have brought us to a new level in the
understanding of fundamental physical laws. This new level of under-
standing is often called the "standard model" of elementary-particle
physics. The establishment of the standard model has brought new
maturity to elementary-particle physics, which strengthens its interac-
tion with other areas of physics such as cosmology. Although the
standard model provides a framework for describing elementary par-
ticles and their fundamental interactions, it is incomplete and inade-
quate in many respects. As usual, the attainment of a new level of
understanding refocuses attention on many old problems that have
refused to go away and raises new questions that could not have been
asked before.
One measure of the inadequacy of the standard model is the number
of basic physical parameters that are required to specify it. At one
level, one might accept the existence of certain particles and forces as
given a priori. Even then, there remain many mysterious inputs, such
as the masses of the different particles and the relative strengths of the
different forces. At a more fundamental level, one seeks explanations
for the choices of elementary-particle species and for the gamut of
different fundamental forces.
81
OCR for page 82
82 E f EMENTAR Y-PARTICLE PHYSICS
Thus one may ask how the masses of the different elementary par-
ticles are determined what is the underlying mechanism for mass
generation, and how are the individual particle masses related? Why do
elementary particles come in sets, or generations, whose individual
members have similar masses but different fundamental interactions?
Why has this generation structure been copied more than once, and
how many copies exist? What is the origin of the overall scale for
elementary-particle masses? We know that all the stable matter in the
universe is made out of the lightest first generation of elementary
particles, while the existence of higher generations might have been
essential for the synthesis in the early universe of the matter present in
it today. The amount of helium in the universe depends on the number
of species of light neutral particles. Stellar evolution and astrophysics
would be vastly different if elementary-particle masses were substan-
tially altered. Thus these basic questions about the masses and number
of elementary particles bear directly on some of the fundamental aspects
of astrophysics and cosmology.
Although the standard model certainly represents a great step
forward in the unification of the fundamental interactions, a completely
unified framework has yet to be developed. It is natural to suppose that
the different strong, weak, and electromagnetic forces known today are
simply different manifestations of one underlying force, which may
also be related to gravity. Such a grand unified theory would tell us why
we have the particular set of force-carrying vector bosons that we
know, and why their interactions have such different strengths. Grand
unified theories can also tell us why elementary particles like to
assemble in the observed generations. In particular, they explain why
the electric charges of the electron and proton are simply related, so
that conventional matter is electrically neutral. If the electric charges
of the electron and proton were not equal and opposite to an accuracy
of about 20 decimal places, the electrostatic forces between planets,
stars, and galaxies would be stronger than their gravitational forces.
Thus any explanation of this equality would be welcome to astrophys-
icists and cosmologists. They would also welcome the new and ex-
ceedingly weak forces expected in some grand unified theories that
violate previously sacred physical laws, enabling baryons like the
proton to decay. Although the basic principles of such grand unified
theories are not necessarily compromised, the simplest examples of
such theories make predictions for proton decay that appear to conflict
with experiment, and an important question for the future is whether
there are alternatives that make testable and successful predictions.
It may well be that none of the above questions has a simple answer
OCR for page 83
WHAT WE WANT TO KNOW 83
when posed at the level of the constituents of matter that currently
seem to us to be fundamental. Some physicists believe that the
particles that we currently regard as elementary are still so numerous
and diverse that they may be composites made up from a smaller and
simpler set of more fundamental constituents. Just as our predecessors
discovered that the atoms of previous generations can be subdivided
into more elementary physical objects—culminating in the recent
discovery that protons, neutrons, and other strongly interacting parti-
cles are actually made out of quarks so perhaps we too may discover
that quarks and leptons are themselves divisible.
- It is possible that free magnetic monopoles (particles containing an
unpaired north or south magnetic pole) may exist. They are predicted
by some unified theories and may remain as relics of an early stage of
the birth of the universe. If they do exist, their masses may be
enormous—perhaps 10'6 times the mass of a proton. Definitive evi-
dence for such monopoles would be extremely important for both ele-
mentary-particle physics and astrophysics. In any event, experimentalists
must be alert for surprises and unpredicted phenomena. Many of the most
exciting and most important discoveries in elementary-particle physics
have been the least expected.
It is apparent from this discussion that many fundamental questions
are left unanswered' and new ones raised, by the standard model.
There is no consensus among elementary-particle physicists as to
which of these problems are the most ripe for solution, still less what
form any such solution might take. The experimental confirmation of
some of the ideas incorporated in the standard model has forced
theorists to speculate in many new directions that are not all mutually
compatible. Ultimately it will be experiment that has to determine
which if any of the different possibilities considered by theorists is the
path followed by nature. At the moment, theorists' ideas are insuffi-
ciently constrained by experimental realities. Balance can be restored
to the science of elementary-particle physics, and a new phenomeno-
logical synthesis achieved, only if experiments are soon performed that
discriminate among the different physical alternatives. Let us now
examine some of these more closely, with a view toward refining our
intuition about the most appropriate lines for future experiments.
The Problem of Mass
The elementary-particle masses that are known range between zero
and about 100 GeV, as shown in Figure 4.1. Generally accepted gauge
symmetries mean that some particles, such as the photon' the gluans,
OCR for page 84
84 El EMENTARY-PARTICLE PHYSICS
l2
lot 1
lolo
109
loo
107
In
In
J
~ lob
CL
loo
104
10
lo2
10 1
10°
Charged
Leptons
—Tou Neutra I
Leptons
- Muon ~ Neutrino
Electron A- Muon
I Neutrino
-
-T-
I Electron
I Neutrino
1
Force-Corrying ~—1 TeV
Particles
—W and Z
*~ 1 GeV
*~ 1 MeV
~—1 keV
Photon is
Far Below
levy
FIGURE 4.1 Some examples of the range of particle masses. The scale extends from
I eV (1 electron volt) to 10" eV (1,000,000,000,000 electron volts). We are only sure of
upper limits on the masses of the neutral leptons or neutrinos. Their masses could be
zero. The upper limit Qn the photon mass is far below the bottom of the page.
OCR for page 85
WHAT WE WANT TO KNOW 85
and the graviton, are firmly believed to have zero mass. There is no
such gauge symmetry to prevent the neutrinos from having masses,
although there is as yet no confirmation that any of three known
species of neutrino does in fact have a mass. The most stringent
experimental upper limit on a neutrino mass is about 10-4 of the elec-
tron mass for the electron neutrino, and there is an experimental
suggestion that it may have a mass just below this limit. There is a
much larger mass scale of a different sort associated with gravity,
whose extremely weak coupling strength to
relativistic matter would become strong for matter at a mass or energy
of about 10~9 GeV.
conventional non-
Where Do All These Mass Scales Originate?
Gauge invariance is now part of the theoretical framework of
elementary-particle physics, but it forbids masses for all the known
particles. For them to acquire masses, gauge invariance must be
broken in some way. If desirable features of gauge theories such as
their calculability are to be maintained, gauge invariance can only be
broken spontaneously. This means that the underlying equations of the
theory must possess gauge symmetry, but their solutions need not.
This is analogous to the observation that most human beings are not
spherical, despite the fact that the laws of physics underlying their
construction are themselves rotationally invariant.
The symmetry of a gauge theory will be spontaneously broken if
some gauge noninvariant scalar quantity is nonzero in the theory
lowest energy state. Quarks, leptons, and intermediate bosons can then
acquire masses in proportion to their couplings to this nonzero scalar
quantity. Thus we have a mechanism for generating masses for all the
known elementary particles. Unfortunately, gauge theory per se pro-
vides little information about the magnitudes of the scalar's couplings
to the different quarks and leptons. Thus the wide range of their masses
can be accommodated but not explained by gauge theories. To explain
their magnitudes we would need an additional dynamical principle.
The original version of the standard model introduced a new
elementary scalar particle, called the Higgs particle, to make gauge
invariance break down spontaneously. The Higgs particle's couplings
to other particles are proportional to their masses, and are hence fixed
though unexplained. Clearly it is of vital importance to search for the
Higgs particle. Colliding e+e~ and hadron-hadron beam experiments
seem to offer the best prospects, and suitable experiments are envis-
aged at present and future colliding-beam accelerators.
OCR for page 86
86 ELEMENTARY-PARTICLE PHYSICS
We note that the ad hoc introduction of a Higgs particle raises new
questions. What should its mass be? The standard model provides no
answer, and keeping the elementary Higgs mass within acceptable
bounds (less than about 1 TeV) proves to be a difficult technical
problem.
Composite Quarks and Leptons?
The idea that quarks are the fundamental constituents of strongly
interacting nuclear matter was advanced 20 years ago. Since that time
this idea has gained universal acceptance, and we know from current
experiments that quarks and leptons are structureless, pointlike parti-
cles at least down to a scale of 10- '6 cm. However, the number of these
apparently fundamental particles has increased recently to at least 11,
not counting the separate red, green, and blue colors for each kind of
quark, and also not counting the 11 analogous antiparticles. Thus some
physicists are beginning to believe that quarks and leptons may be
composites of even more fundamental constituents.
While this hypothesis of another layer to the onion is very seductive,
some cautionary remarks are in order. The first is that there are no
compelling reasons why any compositeness of quarks and leptons must
show up on a scale of 10-'7 cm, rather than at much smaller and more
inaccessible distances. Second, to date there exists no model for
composite quarks or leptons that satisfies all the theoretical constraints
that such a model should obey. However, our ignorance of a satisfac-
tory model may simply be attributable to a lack of theoretical ingenu-
ity. The only way we shall be able to determine if there is in fact
another layer of the onion is by building accelerators that enable
experiments to probe distances smaller than those accessible today.
Unification of the Fundamental Forces?
Another persistent theme in physics is the unification of the different
particle interactions, the most recent success being the combination of
weak and electromagnetic interactions in a unified gauge theory frame-
work. However, the standard model is not completely unified and has
three independent gauge couplings. Nevertheless, the underlying gauge
principle provides hope that one might be able to find a truly unified
theory. One would expect such a theory to make definite predictions
for the strengths of all the gauge interactions in the standard model,
related to the strength of the underlying unified gauge interaction. This
potential unification was described in Chapter 3.
OCR for page 87
WHA T WE WANT TO KNOW 87
Interaction of Hadrons
So far in this chapter we have been concerned with the properties
and interactions of the elementary particles, the quarks and leptons.
Although the hadrons are themselves not elementary, we do have a
promising theory, quantum chromodynamics (QCD), for the strong
interaction of quarks and of hadrons. However, we have not generally
been able to apply QCD in a quantitative manner to the interactions of
hadrons. These interactions include the dependence of the total
interaction probabilities, or cross sections, of hadrons on one another
as functions of energy; the elastic scattering of hadrons, in particular at
large values of angle or exchanged momentum; the detailed study of
lifetimes and decay processes; and the specific production probabilities
of hadrons in collision processes as functions of energy and other pa-
rameters. One particular class of strong-interaction experiments stud-
ies the ejects of the spin (intrinsic angular momentum) of hadrons on
production and scattering processes. At present we do not know how
to use QCD to explain these interactions in detail. We may not be able
to do so because the detailed calculations are too difficult to carry out,
or because QCD may only be an approximation to the correct theory of
the strong interactions.
USING EXISTING ACCELERATORS AND ACCELERATORS
UNDER CONSTRUCTION
One of the purposes of this chapter is to set out, in the context of our
theoretical understanding, the ongoing program of experimentation at
existing accelerators, our expectations for the devices now under
construction, and the imperative for major new facilities in the 1990s.
For the machines now available we are able to pose many sharp
questions. For the machines of the future, the issues are necessarily
less specific, but of greater scope. It is, of course, most important to
continue to test the standard electroweak theory and QCD and to
explore the predictions of unified theories of the strong, weak, and
electromagnetic interactions. The degree of current experimental sup-
port for these three theories is rather different. For the electroweak
theory the task is now to refine precise quantitative tests of detailed
predictions. In the case of QCD, most comparisons of theory and
experiment are still at the qualitative level, either because a precise
theoretical analysis has not been carried out or because of the
difficulties of the required measurement. We find ourselves in the cur-
ious position of having a plausible theory that we have not been able
OCR for page 88
88 ELEMENTARY-PARTICLE PHYSICS
to exploit in full. So far as unified theories are concerned, we are only
beginning to explore their consequences experimentally. Although the
simplest model provides an elegant example of how unification might
occur, no preferred unified theory has yet been selected by experiment.
Many specific experiments at our existing accelerators will address
these issues. Each in its own way, the electron-positron storage rings
(SPEAR, DORIS, CESR, PETRA, PEP. and TRISTAN) and the
fixed-target proton accelerators (the AGS, the SPS, and the Tevatron)
will contribute to the refinement and testing of the standard model.
These low-energy tests include the following:
· The study of static properties of hadrons, such as their magnetic
moments, charge radii, and masses.
· Studies of polarization effects in hadron physics.
· Further detailed study of the quarkonium states in the ~ and Y
families, with their implications for the force between quarks.
· Investigation of scaling violations in deeply inelastic scattering of
electrons, muons, and neutrinos from nuclei.
· Study of the energy dependence of the rate of hadron production
in electron-positron annihilations.
· Exploration of how quarks and gluons materialize into hadrons.
· Probing the quark structure of the proton.
· The search for hadrons with unusual composition, such as the
quarkless glueball states suggested by QCD.
· Measurement of the rate of dimuon production in hadron colli-
sions, and allied tests of QCD.
· Study of the spectroscopy and decays of states containing c and b
quarks.
· Study of the phenomenon of CP violation.
· Searches for rare decays of K mesons to probe for effects of
particles perhaps so massive that they cannot be produced at any
existing or conceivable accelerator.
· Examination of the interplay of strong and weak interactions in
weak decays of one hadron into others.
· Observation of the interactions of neutrinos produced in decays of
shot-lived hadrons, and demonstration of the existence of the tau's
neutrino.
· Refinement of properties of the neutral and charged weak cur-
rents.
Many of the experiments listed here are new uses of existing
accelerators. In many cases the accelerators were built before the
physics of these experiments was known or ever conceived. For
OCR for page 89
WHA T WE WANT TO KNOW 89
example, when the AGS was built, there was little known about K
mesons and no conception of glueball states. It was years later that it
was realized that the AGS is tremendously useful for searching for the
rare decays of K mesons and decades later when it was realized that
one could use the AGS to search for glueball states. In general, the
elementary-particle physics community has kept old accelerators going
only when one could use them for new physics.
We consider next three higher-energy colliders now under construc-
tion. Two are electron-positron colliders: the Stanford Linear Collider
(SLC) and the LEP facility at CERN. The third collider under
construction is the 2-TeV proton-antiproton Tevatron at Fermilab.
SLC and LEP will act as Zen factories, as shown in Figure 4.2,
yielding studies of the production rate and decay modes of the neutral
intermediate boson. Precise measurements of the mass and lifetime of
the Zip may be confronted with detailed theoretical predictions. This is
an important part of the program of probing the electroweak theory in
the same way as quantum electrodynamics has been verified. The
lifetime and production rate are also measures of the number of quark
and lepton species that occur as decay products. This information
could provide, among other things, a determination of the cosmologi-
cally important number of light-neutrino species. Specific studies of the
decays of the Zt' into heavy quarks will determine the neutral-current
interactions of the heavy quarks and also make available a rich source
of heavy quarks for the study of their spectroscopy and decays. Some
aspects of the strong interactions, including the reliability of QCD
calculations and the way in which quarks and gluons materialize into
hadrons, will also be explored at the SLC and LEP. It is also
conceivable that a light Higgs boson could be observed; it will in any
event be important to search for it.
Perhaps the most important work done at the SLC and LEP will be
none of the above. Rather, it might be the discovery of another
generation of leptons or quarks, or the discovery of a new type of
elementary particle, or even the discovery of a new type of force. LEP
can eventually produce a higher energy, 200 GeV, than the SLC; hence
it will allow exploration to higher energies.
The Tevatron Collider at Fermilab, a 2-TeV proton-antiproton
storage ring, will also have a rich and significant physics program. This
machine will be a copious source of the charged intermediate bosons
W+ and W~, whose decays into quarks and leptons define the structure
of the weak charged-current interaction. The mass and lifetime of the
W are critical parameters of the electroweak theory, like those of the
Z°. Although the Tevatron will not produce as many Oh's as the SLC or
OCR for page 90
90 ELEMENTARY-PARTICLE PHYSICS
1 0000
in
o
J
I 1 000
At
Z {~)
LL
Z J
O ~
=~ 100
cn or
O d:
ILL
1
Z
o I
~ t; 1 0
~ 0
LL
·. 1
0.1
=
- zo Produced
- JO ~
_ ~
-
-
0 1 00 200 300 400 500 600
TOTAL ENERGY (GeV)
FIGURE 4.2 The rate at which electrons and positrons annihilate to produce other
particles is shown as a function of the total energy. In general the rate rapidly decreases
as the energy increases. But at about 100 GeV, where the Z° particle is produced, the rate
has a sharp and useful peak. The electron-positron colliders that will operate at this
energy, the SLC and LEP. are called Z° factories.
LEP, there should be some systematic advantages to studying both
charged and neutral intermediate bosons in the same detector, under
similar production conditions. The difference between W. and Z°
masses is a particularly acute probe of the correctness of the elec-
troweak theory. Should there be another intermediate boson in addi-
tion to those expected in the standard model, Tevatron experiments
would be sensitive to it up to a mass of about 500 GeV. A favorite
possibility in theoretical speculations is a right-handed W`.
Extensive studies will be made of hard collisions among quarks and
gluons leading to two or more hadronic jets produced at large angles to
the incident beams. This is a superb laboratory for the study of QCD in
OCR for page 91
WHAT WE WANT TO KNOW 91
constituent collisions at energies up to about 600 GeV. Gluon-gluon
collisions are quite effective at producing pairs of heavy quarks up to
masses of 100 GeV, which would not be accessible in W or Z° decays.
The discovery of Higgs bosons would also be possible if the mass does
not exceed 100 GeV. In any case, the Tevatron represents our first
sortie into the several-hundred-GeV regime.
THE NEED FOR HIGHER-ENERGY ACCELERATORS
By early in the 1990s this vigorous experimental program will have
subjected QCD and the standard electroweak theory to ever more
stringent testing of the kind that is essential to verify that the theories
are indeed accurate descriptions of the energy regime below about 100
GeV. Although surprises may well be encountered, it is likely that our
efforts to understand why these theories work and to construct more
complete descriptions of nature will remain without any direct new
experimental guidance. In order to explain what sort of guidance we
require, it is useful to summarize some of the shortcomings and open
problems of the standard model. Even if we suppose that the ideas of
a unified theory of the strong, weak, and electromagnetic interactions
are correct, there are several areas in which accomplishments fall short
of the announced aspirations, and there are also a number of specific
problems to be faced.
· No particular insight has been gained into the pattern of quark and
lepton masses or the mixing between different quark and lepton
species.
· Although the idea that quarks and leptons should be grouped in
generations has gained support, we do not know why generations
repeat or how many there are.
· The number of apparently arbitrary parameters needed to specify
the theory is 20 or more. This is at odds with our viewpoint, fostered
by a history of repeated simplification, that the world should be
comprehensible in terms of a few simple laws. Much of the progress
represented by gauge theory synthesis is associated with the reduction
of ambiguity made possible by a guiding principle.
· CP violation in the weak interaction does not arise gracefully.
· The most serious structural problem is associated with the Higgs
sector of the theory. In the standard electroweak theory, the interac-
tions of the Higgs boson are not prescribed by the gauge symmetry as
are those of the intermediate bosons. Whereas the masses of the
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92 ELEMENTARY-PARTICLE PHYSICS
TABLE 4.1 Questions that Lead to Higher-Energy Accelerators
What is the origin of mass?
What sets the masses of the different particles?
Why are there quark and lepton generations?
Are the quarks and leptons truly elementary?
Can the strong and electroweak interactions be unified?
What is the origin of gauge symmetries?
Are there undiscovered fundamental forces'?
Are there undiscovered new types of elementary particles?
What is the origin of CP violation?
intermediate bosons are specified by the theory, the mass of the Higgs
boson is only constrained to lie within the range 7 GeV to I TeV. In a
unified theory, the problem of the ambiguity of the Higgs sector is
heightened by the requirement that there be a dozen orders of mag-
nitude between the masses of W and Zt' and those of the leptoquark
bosons that would mediate proton decay.
· Gravitation is omitted from the quantum theory, although the
unification scale for the strong, weak, and electromagnetic interactions
is only four orders of magnitude removed from the Planck mass at
which gravitational effects become strong. Can gravity be made con-
sistent with quantum theory, and can it be unified with the other
fundamental forces?
· Faced with the large number of apparently fundamental quarks
and leptons, we may ask whether these particles are truly elementary.
· Are there other types of elementary particles?
· Finally, we may ask what is the origin of the gauge symmetries
themselves, why the weak interactions are left-handed, and whether
there are new fundamental interactions to be discovered.
Given this list, summarized in Table 4. 1, it is not surprising that there
are many directions of theoretical speculation that depart from the
standard model. Many of these have important implications that cannot
yet be tested. Although theoretical speculation and synthesis is valu-
able and necessary, we cannot advance without new observations. The
experimental clues needed to answer questions like those posed above
can come from several sources, including
· Experiments at high-energy accelerators;
· Experiments at low-energy accelerators and reactors;
· Nonaccelerator experiments; and
· Deductions from astrophysical measurements.
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WHAT WE WANT TO KNOW
93
However, according to our present knowledge of elementary-particle
physics, our physical intuition, and our past experience, most of the
clues and information will come from experiments at the highest-
energy accelerators.
Since many of the questions that we wish to pose are beyond the
reach of existing accelerators and those under construction, further
progress in the field will depend on our ability to study phenomena at
higher energies or, equivalently, on shorter scales of time and distance.
What energy scale must we reach, and what sorts of new instruments
do we require?
- The mystery of symmetry breaking in the electroweak theory, which
is to say the nature of the Higgs sector of the theory, presents an
especially important and exciting challenge to experimental high-
energy physics. This is because there are rather general theoretical
reasons why the characteristic scale of the symmetry-breaking phe-
nomenon can be no more than a few TeV. While this probably lies
beyond the reach of the current generation of colliders, it is certainly
accessible to a hadron machine of multi-Ted capability.
The excitement of the search is heightened by the fact that we know
so little of what will be found. Whatever it may be, there is little doubt
that further theoretical progress depends critically on finding out. Until
we know, the idea of unified theories will rest on a questionable
foundation.
Although the Higgs phenomena might possibly occur at less than 1
TeV, building a comprehensive theory in which this occurs proves to
be a difficult problem, unless some new physics intervenes.
One solution to the Higgs mass problem involves introducing a
complete new set of elementary particles whose spins differ by one-half
unit from the known quarks, leptons, and gauge bosons. These
postulated new particles are consequences of a new supersymmetry
that relates particles of integral and half-integral spin. The conjectured
supersymmetric particles stabilize the mass of the Higgs boson at a
value below 1 TeV and are likely themselves to have masses less than
about 1 TeV. Up to the present, however, there is no experimental
evidence for these superpartners.
A second possible solution to the Higgs problem is based on the idea
called technicolor that the Higgs boson is not an elementary particle at
all but is in reality a composite object made out of elementary
constituents analogous to the quarks and leptons. Although they would
resemble the usual quarks and leptons, these new constituents would
be subject to a new type of strong interaction that would confine them
within about 10- '7 cm. Such new forces could yield new phenomena as
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94 ELEMENTARY-PARTICLE PHYSICS
.
rich and diverse as the conventional strong interactions but on an
energy scale a thousand times greater around 1 TeV.
The origin of electroweak symmetry breaking is only one of many
puzzles that define the cutting edge of our field. However, because of
its importance and accessibility, it imposes a clear minimum require-
ment on our planning for future facilities. The next high-energy
accelerator to be designed and constructed in the United States should
be comfortably able to make a few TeV of energy available for new
particle production.
Either an electron-positron collider with beams of 1 to 3 TeV or a
proton-(anti~proton collider with beams of 5 to 20 TeV would allow an
exploration of the TeV region for hard collisions. The higher beam
energy required for protons simply reflects the fact that the proton's
energy is shared among its quark and gluon constituents. The parti-
tioning of energy among the constituents has been thoroughly studied
in experiments on deeply inelastic scattering, so the rate of collisions
among constituents of various energies may be calculated with some
confidence. As examples, we show in Figure 4.3 how the relative
importance of hard gluon-gluon collisions at different energies depends
on the energy of the colliding protons. A similar plot for collisions of up
quarks and antiup quarks is shown in Figure 4.4.
The physics capabilities of the electron-positron and proton-
(anti~proton options are both attractive and somewhat complementary.
The hadron machine provides a wider variety of constituent collisions,
which allows for a greater diversity of phenomena. The simple initial
state of the electron-positron machine represents a considerable mea-
surement advantage. Also, electron-positron collisions give a larger
ratio of interesting events to uninteresting background events' and it is
easier to find these interesting events. However, the results of the
CERN proton-antiproton collider indicate that hard collisions at very
high energies are relatively easy to identify. Because the current state
of technology favors the hadron collider, it is the instrument of choice
for the first exploration of the TeV regime.
A multi-Ted hadron collider will surely reveal much more than the
mechanism for electroweak symmetry breaking. Surprises and unex-
pected insights have always been encountered in each new energy
regime, and we confidently expect the same result at TeV energies.
Conventional possibilities and existing speculations about the Higgs
sector serve the important function of calibrating the discovery reach
of a planned facility. They also help to fix the crucial parameters for a
new machine: the energy per beam and the rate at which collisions
OCR for page 95
.
105
104
103
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2
101
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c
'~ 1 0°
° 10-1
-
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10-4
10-5
lo-6
WHA T WE WANT TO KNOW 95
Total Energy
(TeV)
~ 1
. I
\10 \20\-
\ \ \
\ \
\2
1\
10°
lo-2
10 1
Total Energy of Colliding Gluons (Te\/)
101
FIGURE 4.3 In high-energy collisions of protons. some of the events actually consist
of the collision of gluons within the two protons. Very-high-energy gluon collisions are
most interesting. The numbers on each curve give the total energy of the colliding
protons; as that energy increases the rate of occurrence of the rare very-high-energy
gluon collisions also increases. This is one of the many reasons for wanting to study
very-high-energy proton-proton collisions.
occur. Because the most interesting of the anticipated new phenomena
are rare occurrences' an ideal storage ring must provide a high collision
rate as well as high energies. A total energy of 40 TeV and a collision
rate of at least 107 interactions per second would allow a thorough
exploration of the TeV regime. These parameters define a reasonable
target for the next major facility for the study of particle physics in the
United States.
Whatever the physics of the TeV energy regime turns out to be, its
exploration will provide sorely needed guidance for the attempts at a
deeper theoretical description of nature that is now necessarily highly
conjectural.
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96 ELEMENTARY-PARTICLE PHYSICS
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101
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Tote I Energy of Col I id i ng Quark ond Antiquark ~ TeV ~
FIGURE 4.4 Quark-antiquark collisions also occur in the collisions of protons. The
most interesting quark-antiquark collisions are those that occur at the highest energy.
SOME FUNDAMENTAL ISSUES
It is appropriate to close this chapter with a brief discussion of some
fundamental issues for which we do not yet know how to frame a
definite experimental program. All the ideas discussed in this report
have been formulated within the general framework of quantum field
theory. This prescribes that the principles of quantum mechanics be
applied locally to fields such as that carrying familiar electromagne-
tism. A little over a decade ago' there was no such unanimity that
quantum field theory was appropriate for describing elementary-
particle physics, and many rival approaches were being considered.
These have been abandoned since gauge theories have provided such
a successful description of the fundamental particles and their interac-
tions. This is not to say that quantum field theory is without its
problems.
For example, infinities tend to occur in diagrammatic calculations of
the kind described in Chapter 3, but these can be controlled so that
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WHA T WE WANT TO KNOW 97
computations yield finite and reliable answers. Many physicists have
found the existence of even controllable infinities unaesthetic and have
sought theories that are completely finite. A class of such theories has
recently been discovered, but their relevance to reality is unclear.
These theories embody supersymmetry, which has already been men-
tioned in connection with the Higgs problem, and may aid in the
application of quantum principles to gravity.
Unlike the quantization of the electromagnetic field, the gravitational
field has never been successfully quantized, and all attempts have
ended in a maze of uncontrollable infinities. Some of these infinities are
removed by supersymmetry, but others remain. It may well be that the
marriage of quantum mechanics and gravitation requires a few more
drastic revisions of our ideas. For example, our description of space-
time as a continuum may have to be replaced by a discrete, granular
structure at extremely short distance. Familiar symmetries such as the
equivalence of the laws of nature at all times and places and time-
honored conservation laws like the conservation of electric charge may
break down in the presence of intense gravitational fields. Perhaps the
quantum field theory itself must be rethought or abandoned. Perhaps
the usual laws of quantum mechanics should be modified, as has been
suggested by some physicists working on quantum gravity.
It does not seem likely that any of these ideas will have a great
impact on experimental physics in the near future, but the possibilities
should be kept in mind. One of the best laboratories for probing
quantum mechanics has been the K"-~° system studied at high-energy
accelerators. Thus even these fundamental problems may have some
impact on elementary-particle physics within the next two decades.
Representative terms from entire chapter:
elementary particles
