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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
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
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 objectsculminating 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 enormousperhaps 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,
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.
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.
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.
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
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
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
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
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
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.
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
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
. 105 104 103 u) 2 101 o c '~ 1 0° ° 10-1 - lo-2 _ 10 3 cr 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.
96 ELEMENTARY-PARTICLE PHYSICS 104 '\ = O - o In y o v ._ - y o lo 2 101 10° 10 I_ o, 10 2 - o cr 10 4 10-3 10-5 - ~1005 (Te\/) \ \\~\\ 1 o-2 10-1 10° 10' 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
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.
