The Role of New Facilities
In Chapter 5, the progress likely to be made in experimental particle physics in the coming decade is considered. An enormous new energy range will be available, primarily with the start of operations at the Large Hadron Collider (LHC) in 2005. The LHC will be a superb instrument of discovery that supports an exciting program well into the next century. U.S. physicists, along with physicists from many other countries, are well positioned to play a leadership role in building this facility and exploring the new physics it will make available.
The tremendous enthusiasm throughout the field for the LHC derives from its power to discover new phenomena associated with breaking electroweak symmetry. Simple, elegant, and unequivocal arguments indicate that there will be clues to the origins of electroweak symmetry breaking in the energy range accessible at the LHC. It is also possible that the first hints of the origins of electroweak symmetry breaking will be seen before the LHC turns on either at the Tevatron or the Large Electron-Positron Collider (LEP). However, the physics reach of the LHC will be much greater than that of currently operating facilities, and there is every expectation that major discoveries will be uncovered with this collider. Tables 6.1 and 6.2 summarize the current and planned high-energy facilities that will define the high-energy physics program for the next decade.
Physicists are confident that some of the central questions of the field discussed in Chapter 5 can be answered conclusively with the program of the next decade. However, as this chapter shows, even the enormous power of the LHC
is not likely to provide a complete understanding of electroweak symmetry breaking or to completely unravel the origins of mass.
Because facilities that must operate at the highest energies may take two decades to plan, design, and construct, it is essential to try to anticipate now what will be learned from the LHC and other colliders by the end of the first decade of the next century and to understand what questions post-LHC facilities may have to explore. In discussing the technologies being developed now in order to address physics issues anticipated in 2010, it is important to consider both nearterm technology (for a collider that might be built in the first decade of the next century) and long-term technology (for colliders that might not operate until 2020 or later). The LHC can be built today because the planning phase of the project, as well as much of the research and development work to solve technical problems and reduce costs, started more than a decade ago in the early 1980s. This chapter considers questions that will have to be answered even after the LHC program is mature. The accelerator technologies and possible collider facilities needed to address these questions are considered here. As described in Chapter 6, much of the necessary research and development is already in progress to develop the technologies that will define the colliders and the physics program of the future.
THE LANDSCAPE IN 2010
This section describes three scenarios for the physics that may have been discovered at the LHC at the end of its first 5 years of operation. By this time, experiments currently running or being built at LEP, the Tevatron, the Stanford Linear Accelerator Center (SLAC), and the Cornell Electron Storage Ring (CESR) (as described in the previous chapter) should be finished or near completion. Although one cannot predict the future with certainty, making projections is essential in planning for long-term evolution of the field. The scenarios chosen, at least from today's perspective, effectively bracket the possibilities of the state of our understanding in 2010.
The primary missions of the LHC and of the Toroidal LHC Apparatus (ATLAS) and Compact Muon Solenoid (CMS) detectors are to find evidence for the mechanism of electroweak symmetry breaking and to uncover and explore the origins of particle masses. The simplest model of how gauge bosons, leptons, and quarks acquire mass, and how electroweak symmetry is broken, includes a single neutral boson, the Higgs particle. This mechanism makes sense only if the mass of the Higgs particle is less than about 1,000 GeV (1 GeV = 109 electron volts). However, with this model the mystery remains as to why the mass of the Higgs boson should be so small compared with the grand unification scale discussed in Chapter 3. A possible alternative model is supersymmetry, which in its simplest version replaces a single Higgs boson with five bosons—three neutral and two charged. For supersymmetry to be the source of electroweak
symmetry breaking, several of the many Higgs states in a supersymmetric model must have masses less than 1,000 GeV, possibly much lower. The lightest neutral Higgs boson should have a mass of less than 130 GeV. The current experimental lower bound is about half of this and will be extended by experiments at LEP soon. In addition, supersymmetry predicts many other new particles that are superpartners of all the particles discovered so far. Another possible model is technicolor, which postulates a new, very strong force and a number of high-mass particles. This case can be distinguished from supersymmetry both by the types of new particles seen and by the way they decay. It is also possible that a solution not yet thought of will be revealed at the LHC.
How the LHC and its predecessors, LEP II and the Tevatron, will help physicists distinguish between these three basic options—a single Higgs. supersymmetry, or new strong interactions—is considered below.
Scenario 1: If the Higgs particle exists, the LHC experiments should see evidence for it. Suppose, as an example, that a new particle with a mass of around 110 GeV decaying into a two-photon final state is discovered. This particle would be a very strong candidate for a Higgs. However, verifying that this is a Higgs and establishing experimentally that this new particle has all the properties required of a Higgs boson, such as the correct couplings to gauge bosons and fermions, may be difficult. To verify that this new discovery is a Higgs state, one would like to study many of its possible decays. To rule out the possibility that this particle is not, for example, one of several expected in supersymmetry, one will want to rule out other Higgs states with masses less than 1 TeV (1012 eV) or so.
The LHC probably will be able to detect the Higgs in only a few final states, and the Tevatron (for lack of luminosity) and LEP II (for lack of energy) will not be able to detect the state at all if it has a mass greater than about 100 GeV. It also may be difficult at the LHC to conclusively eliminate the possible existence of other Higgs states up to a mass of 1 TeV.
Scenario 2: If supersymmetry is the source of electroweak symmetry breaking, physicists will see experimental evidence for its existence at the LHC, if not earlier at LEP II or the Tevatron. Some, if not all, Higgs states should be observable at the LHC, and the experiments are likely to uncover evidence for many of the other particles associated with supersymmetry, such as the partners to gauge bosons, quarks, and leptons. A huge amount of tremendously exciting physics would pour forth from the LHC as experimenters untangled the many new particles produced. The challenge will be to verify that the new signatures are consistent with supersymmetry and to test the details of the theory. There are a number of supersymmetric models, and distinguishing among them will be difficult. The question is whether measurements at the LHC will be sufficient to ensure that supersymmetry is tested and that the patterns of masses and decay rates of the new states are understood. Determination of the masses of all of the
new particles and the relationships between their decays will require a large number of independent measurements, not all of which would be possible at the LHC.
Scenario 3: If electroweak symmetry is broken by a previously undiscovered strong interaction such as technicolor, LHC experimenters will see indirect evidence for its existence in measurements of the scattering rate of gauge bosons. There may also be direct evidence since in some models of the new strong interaction, there are new particles that can be detected at the LHC and possibly even at the Tevatron. However, in some models, all of the new particles are of such high mass that LHC experiments cannot detect them directly as resonances, and the experiments to establish strong symmetry breaking will then be very difficult. In this scenario, experiments at the LHC will provide evidence for the existence of this new strong force but may not be able to provide much information about its structure. Great challenges will remain for both theorists and experimentalists.
In considering these scenarios, it is already clear that the tremendous energy range of the LHC will enable a great step forward in physicists' understanding of nature. In the field of high-energy physics, there is consensus that it is crucial to explore this mass range, which will point the way for new investigations to explore the TeV energy scale and address major questions of the field, such as the mass scale of Higgs boson(s), the mechanism for electroweak symmetry breaking, and the properties of supersymmetric particles. This is a primary objective for the long-term future of particle physics. Other parts of the high-energy program may well make important discoveries, but definitive tests of our understanding at the most fundamental level will come from the highest-energy colliders.
Because of the long time scale to propose and build a new high-energy facility, physicists are already deeply involved in researching the new technologies for colliders that will extend their understanding of electroweak symmetry breaking and mass generation even beyond what will be provided by the LHC. The research and development for colliders of the twenty-first century has been under way for more than 10 years in anticipation of this need, and technologies have progressed sufficiently that in some cases one can begin to move to more concrete proposals for a new collider to follow the LHC. Coupled with the technical effort are strong efforts in cost optimization to make the next generation of colliders affordable, as well as efforts to ensure that the next accelerator facility will be a truly international endeavor.
Chapter 6 describes the three types of accelerators being considered by the international high-energy physics community: an electron-positron linear col-
lider, a muon collider, and a very high-energy hadron collider. The committee believes that a lepton collider with energy expandable to 1.5 TeV and sufficient luminosity would contribute essential information, complementary to that from the LHC, toward understanding the fundamental physics of electroweak symmetry breaking. The committee has concluded that it is appropriate at this time to intensify the international effort that will lead, in timely fashion, to a detailed design study for an affordable collider facility. Such a design study with an accompanying cost estimate could be ready early in the next decade. It is also the committee's belief that an even higher-energy, higher-luminosity lepton or hadron collider might be required to fully explore the experimentally challenging question of strong electroweak symmetry breaking, and R&D on both of these technologies should progress vigorously.
The Physics Need
Based on scenarios in the previous section, some of the physics issues that future colliders might have to address can be enumerated. A collider operating in the wake of the LHC will have to be capable of discovering some new particles and ruling out others. For example, if scenario 1 is correct, it will be imperative to verify that electroweak symmetry is broken by a single Higgs by ruling out any other additional Higgs states up to a mass of 1 TeV. This will require, among other things, a new collider of very high-energy. Another desirable feature is the ability to explore more decay modes of the Higgs. If, instead, scenario 2 is the path nature has chosen, a new collider will have to clarify the details of supersymmetry that have not been explored at the LHC. In particular, some supersymmetric states are very difficult to detect at a proton collider but form a very important piece of the supersymmetry puzzle. As an example, in some models the supersymmetric partners of leptons will be undetectable at the LHC. Finally, a new collider would have to provide more details on the strong symmetry-breaking mechanism if a very high-energy strong interaction is discovered at the LHC, as in scenario 3. This will require high-energy and high luminosity.
Colliders to Address the Physics Need
Since it is not known which one (or more), if any, of the three scenarios discussed is correct, one must consider how a collider that might be operating as the LHC program reaches maturity would address all three possibilities. Here, features of the three types under development are reviewed briefly.
Various accelerator laboratories in the United States, Japan, and Europe have been carrying out R&D on construction of an electron-positron linear collider that would operate with an energy of more than 1 TeV. This is the most technically mature accelerator research program, with design concepts well ad-
vanced for facilities with energies of 1 TeV, luminosities (a measure of the number of beam particles passing through a given area per second) of 1034 cm−2s−1, and ideas for extending the energy to 1.5 TeV. In many instances, the necessary subsystem components required for a 1-TeV facility have undergone proof of-principle demonstrations. Current challenges to design efforts include cost reduction, systems integration and operability, and the development of a cost effective path to 1.5 TeV. Detailed studies have explored the physics reach of the electron-positron linear collider and how it might address the issues raised in the scenarios discussed here.
The concept of a muon collider is much more speculative than an electron-positron collider. but it has generated much excitement because potentially a muon collider could operate at much higher energies. Research on muon colliders has begun at Brookhaven, Fermilab, and Lawrence Berkeley National Laboratory. Discussions have focused on a collider with 4-TeV collision energy and 1035 cm−2 s−1 luminosity. The research and development for a muon collider is less advanced than that for an electron-positron collider. Since such technology has never been attempted, a low-energy prototype may well be needed as a demonstration before a full-scale collider can be proposed.
The challenge to the research program for very high-energy hadron colliders is once again cost reduction, and research in cost-effective magnet designs is being pursued at Fermilab and Brookhaven. Detailed designs for an accelerator do not yet exist, and detailed studies exploring the physics potential of such a collider have just begun. Discussion is focused on a collider of 60-100 TeV of energy and a luminosity of 1034 to 1035 cm−2 s−1
The three scenarios can now be revisited to see how these new colliders could address the physics needs defined earlier.
Scenario 1 Revisited: If a Higgs state of 110 GeV is discovered, the challenge to the LHC and a new collider will be to try to prove conclusively that this state is a single Higgs and not one of several such states expected from supersymmetry. Information to probe the Higgs couplings would be needed, and the possibility of other new Higgs states with masses up to about 1 TeV would have to be explored effectively.
Most events in a lepton collider come from direct annihilation of the lepton and antilepton, and these events are simpler and often easier to interpret than those produced at proton colliders. As a result, experimenters at a lepton collider may be able to isolate signals for several additional final states of the Higgs such as Higgs decays to bottom and charm quarks and Higgs decays to tau leptons. This will allow experimenters to further probe the properties of this new state and test whether this Higgs couples to quarks, leptons, and gauge bosons with the expected strengths.
In the low-background environment of a lepton collider, often the discovery of new particles is relatively easy. If the collider operates above the energy
threshold necessary to produce them, new particles can be directly detected. If the collider has sufficient luminosity, experimenters can search for indirect evidence of new particles to even higher masses. For example, detailed studies show that with an energy of 1.5 TeV in the center of mass and a luminosity of 1034 cm−2 s−1, experiments at a lepton collider would directly or indirectly find evidence for any charged supersymmetric Higgs particles, if they existed, to a mass of well over 750 GeV and other neutral Higgs states to well over 1 TeV. Therefore, if only one Higgs state was seen, supersymmetry at the electroweak symmetry breaking scale could be ruled out, and scenario 1 would be established.
Scenario 2 Revisited: The discovery of supersymmetry at the LHC, if not before, would be an important verification of some of the most exciting theoretical speculations of the last 20 years. To understand the dynamics of supersymmetry breaking and how the electroweak gauge symmetry is broken will necessitate information from both a proton collider such as the LHC and a lepton collider. Detailed studies show that it might be necessary for the lepton collider to have an energy of at least 1.5 TeV to cover the full range of the supersymmetry spectrum. The information from a hadron collider and a lepton collider is largely complementary. Experiments at proton colliders would measure the properties of the supersymmetric partners of quarks and gluons. Experiments at a lepton collider would make measurements of the supersymmetric partners of leptons and of the electroweak gauge bosons. All of these inputs will be crucial in order to understand the details of supersymmetry.
Lepton colliders also have the attractive feature that the lepton beam can be polarized, which allows experimenters to selectively suppress troublesome backgrounds. The polarization can also be used to turn on and turn off different production mechanisms for the supersymmetric particles and to single out subsets of reactions. The ultimate goal would be to combine information learned at the LHC with that learned from a lepton collider. This would give physicists the power to fully test the supersymmetric relations between the couplings of various particles and to use the determination of the parameters of superparticles to study the mechanisms of supersymmetry breaking that reach to the highest mass scales.
Scenario 3 Revisited: The discovery of a new strong force at the LHC would present exciting new challenges to experimentalists and theorists alike. It will be of the utmost importance to learn more about the very high mass particles associated with this force. A 1.5-TeV lepton collider will not be able to produce particles of the new strong interaction directly if these particles have a mass greater than 750 GeV. However, with sufficient luminosity, such a collider could find indirect evidence for their existence in the process e+e−W+W−, even if they have masses as high as 4 TeV. These measurements will be essential to learn the basic parameters of this new force and begin to understand it. A detailed understanding of the strong symmetry-breaking scenario, however, will
likely require colliders of even higher energies and higher luminosities. For example, a lepton collider with an energy of 4 TeV or a very high-energy hadron collider could probe the structure of the new strong interactions and directly produce some of the resonances if they exist. Studies demonstrating that the signals for strong electroweak symmetry breaking would be detectable above background at such colliders are now under way.
The committee concludes that a lepton collider, if it could ultimately reach 1.5 TeV in energy and a luminosity of 1034 cm−2 s−1, would contribute essential information complementary to that from the LHC toward understanding the fundamental physics of electroweak symmetry breaking. The committee also concludes that a very high-energy hadron collider or a lepton collider of energy substantially higher than 1.5 TeV might be needed to explore further the experimentally challenging regime of strong electroweak symmetry breaking.
THE NEXT STEPS
The three accelerator technologies discussed in the previous section are at very different stages of development, and the next step to take is different in each case.
For the electron-positron linear collider, the next step in the process is an intensified international effort leading in a timely fashion to a complete design and cost estimate for such an accelerator. This design report should address engineering issues such as systems integration and operability, as well as identify a cost-effective path for achieving an ultimate energy of 1.5 TeV with a luminosity of 1034 cm−2 s−1. This step could be completed early in the next century.
With the muon collider, a technological basis for the accelerator must be established. If the accelerator concept proves viable, it will be necessary to produce a detailed design, to estimate costs, and to discuss the feasibility of a demonstration accelerator at lower energy to explore the new technologies required.
Continued R&D on cost reductions is necessary for the very high-energy hadron collider, along with an intensified effort elucidating the physics potential of such an accelerator. When these are further advanced, it would then be appropriate to begin a serious design effort. This will prepare the community for the possibility that when the LHC program is mature its experiments will indicate the need for a significant increase in collision energy.
The next high-energy collider after the LHC would clearly constitute a very significant fraction of the high-energy physics program in the United States and worldwide. This will be true whether this accelerator is built in the United States or abroad. It must be fully supported by the U.S. high-energy physics community. To reach such a consensus within the field, several steps will be necessary:
The physics potential of a new collider must be compelling.
The many design challenges of the accelerator technologies must be understood.
The costs to build and operate the collider must be thoroughly understood and compatible with realistic funding scenarios.
The role that such a new facility would have in the overall elementary particle physics program must be fully evaluated.
Furthermore, the United States government must be committed to significant support for such a facility, no matter where it is built.
The committee believes that early in the next decade, even before initial operations of the LHC, it may be possible for the U.S. community, in conjunction with the high-energy physics communities in Europe and Asia, to decide to pursue a particular technology and commit to building the next major collider facility. It is possible that discoveries in the intervening years will clarify the physics of electroweak symmetry breaking to the extent that one or more of the scenarios discussed could be eliminated.
Finally, the scale of particle physics accelerators is such that this facility will require strong international backing. Countries should rightly compete for the prize of having such a premier scientific instrument, but the commitment to an international collider facility means that we have to recognize and plan for the unfortunate possibility of no forefront accelerator in this country in the early decades of the next century. The management of the field of high-energy physics, both within and outside the United States, must allow and encourage truly international cooperation for all phases in the design and construction of such a facility, independent of its location. The groundbreaking progress that has been made in structuring the participation of U.S. physicists on the LHC project is an important step toward this goal.