The cancellation of the SSC in no way reduces the interest and importance of the scientific objectives which it was supposed to address. The study of electroweak symmetry breaking, which represented one of its major justifications, remains one of the most pressing problems of elementary particle physics. The only facility, presently on the horizon, which can hope to address this issue in a direct way is the Large Hadron Collider (LHC) being proposed by CERN. In its justification of the SSC, the American HEP community argued that the higher energy (a factor of 3) of the SSC gave it far more power to address the important issues than the LHC. This has of course not changed. The lower energy of the LHC is to be somewhat compensated by a factor of 10 higher luminosity; but this poses a tremendous challenge for the detectors, in terms of both performance and survivability. Although the attainment of the physics goals which were used to justify the SSC will be more challenging with the LHC, and perhaps not all the goals will be achieved, there simply is no alternative to the LHC as the most direct route toward the solution of some of HEP's most fundamental problems.
There is an important additional issue involved in considering U.S. involvement in the LHC program. If the SSC demise demonstrated anything, it showed that billion dollar scale scientific projects must be handled as international endeavors. If built only by the CERN member states, the LHC is already an international project, but not an interregional or global one. The LHC project offers the earliest opportunity to initiate the process of global collaboration in high energy physics. It is probably somewhat beyond the means of just the CERN community, and there is interest in it from other non-member states such as Canada and Japan. If we seize on this opportunity, interregional collaboration on the next large project, perhaps a linear collider or a larger hadron collider, will have a precedent, and should be far easier to achieve. U.S. support now for a project in Europe should make it easier to obtain European support for a future project in the U.S.
U.S. participation in the LHC program faces some significant obstacles. The CERN management has made it clear that U.S. participation in the LHC experiments must also imply U.S. support for the machine construction, and probably some U.S. support for eventual LHC operating costs. The detector and collider R&D and design work have progressed quite far, and the opportunities for U.S. input and contributions are more limited than if the project were just starting. Fortunately these problems are mitigated by the experience and expertise achieved by both the detector and the accelerator communities in the development of the SSC program. A more important obstacle is the tightness of funding for R&D work in FY1994 and FY1995. If the U.S. community, accelerator and detector, is to have some impact before all designs are completely frozen, it will need at least modest R&D funding in the near future.
In the following sections, we provide some further details on physics opportunities, and on potential U.S. participation in the ATLAS and CMS detectors, and in the LHC machine. There will also undoubtedly be U.S. interest in participation in a B physics program at the LHC, and probably even in participation in a heavy-ion program. However, since we consider the electroweak symmetry breaking area as the one which carries the justification for the LHC and for significant U.S. participation in it, we have focused this report entirely on that area.
This document is not a carefully crafted plan put together by an existing organization, and it will undoubtedly suffer from less than optimal coherence and some duplication. It is hoped nevertheless that it will convey the real interest of a substantial part of our community in maintaining a U.S. role in the pursuit of the energy frontier.
Several pieces of the standard model are missing or incomplete. The top quark remains to be discovered, and the mechanism responsible for CP violation has not been established. Planned or ongoing experiments in the U.S. and abroad will yield important information on these issues. The top quark should be discovered at the Tevatron collider; the source of CP violation can be found, and its strength determined at the B factories and through the KTEV experiment at Fermilab; ever more stringent tests of the gauge boson couplings will be carried out at LEP and SLC. We expect that the standard model will describe these phenomena, since it has already been tested in related areas, and that these experiments will yield a better determination of the model's parameters. It is imperative for the U.S. high energy physics program to continue its ongoing work on these missing pieces in our understanding of the standard model.
Despite its the great success, the standard model has the fundamental failing of being unable to explain and predict the pattern of masses. The well-tested part of the description of electroweak interactions in the standard model is built on a symmetry which requires that all masses vanish. This electroweak symmetry must be broken to produce the W, Z, quark, and lepton masses. We have no evidence for or against the mechanism used in the standard model to do this. Any real progress in particle physics is dependent upon experiments that can yield direct information on the mechanism responsible for this symmetry breaking. The energy scale at which it occurs is known to be the Fermi scale (~250 GeV), and the parameters of both the SSC and LHC were selected to reach this scale.
The simplest mechanism for electroweak symmetry breaking is that used in the minimal Higgs model. It predicts the existence of a single Higgs boson of unknown mass but fixed couplings. The lower bound on the mass of the Higgs boson, of about 63 GeV, is provided by LEP. Further work at LEPII is expected to increase this limit to about 80 GeV. Other existing facilities are inadequate to the task of finding the Higgs boson.
This minimal Higgs model is generally regarded as unsatisfactory: it fails to give any insight into quark and lepton masses and mixings, and it cannot explain why the W mass is so different from the Planck mass. The most popular class of alternatives centers around supersymmetry, which allows the W and Planck masses to be very different and which seems to be required for any unified theory involving gravity. These supersymmetric theories require the existence of new particles with masses in the TeV range or less. Searches for some of these particles have been carried out at LEP and the Tevatron. These facilities do not have sufficient energy to be able to search the full range of masses expected if supersymmetry is the correct solution.
The other class of alternatives centers on the weak scale being generated dynamically in a manner similar to the generation of the scale of hadron masses in QCD rather than from elementary scalar fields. Since these models involve new strong dynamics, they are difficult to study. They predict new interactions between W and Z bosons when they scatter at energies of order 1 TeV.
Progress on these issues cannot be made without experimental information on the nature of electroweak symmetry breaking. In order to get this information, experiments must be performed that are able to probe final states of W and Z pairs with invariant mass up to 1 TeV. The SSC's energy and luminosity were selected to ensure that it was adequate to this task. The LHC is a lower energy machine which seeks to compensate by running at higher intensity. Experiments at the higher luminosity are more challenging and the signal to noise ratio is somewhat worse. We cannot be quite so confident of success as we were with SSC, but the LHC is our best opportunity for success.
The search for the minimal Higgs boson can be carried out at the LHC over all of the interesting mass range, from that reached by LEPII up to the mass beyond which the theory is inconsistent. The search will involve the final states (in order of increasing Higgs mass) H --> (gamma)(gamma), H --> Z(l+)(l-) --> (l+)(l-)(l+)(l-), H --> ZZ --> (l+)(l-)(l+)(l-), and H --> ZZ --> (l+)(l-)(nu)(nu-bar). Extensive simulations have been carried out for both SSC and LHC, and we can be confident that the whole of the mass range is accessible at LHC provided that the luminosity of 10^34 cm^-2 s^-1 can be exploited. The SSC energy was chosen so that the search could be conducted at 10^33 cm^-2 s^-1 where the detection of isolated leptons and photons is easier. The experimentally most challenging region is at very low Higgs mass. In this region one either studies final states with relatively good signal to background and very low event rates (for example H (--> (gamma)(gamma)) + W (--> (l)(nu)), or final states with more events, but requiring excellent energy resolution to pull the signal out of the background (specifically H --> (gamma)(gamma). In either case event pile up at LHC, caused by the need to run at higher luminosity, makes the detection of isolated low transverse energy photons and leptons more challenging. At the upper end of the mass range where the Higgs becomes a broad resonance, the search is limited mainly by event rate.
The production cross-section for top quarks at LHC (SSC) is of order 1.3 (8.5) nb assuming a mass of 150 GeV. The top quark will likely be discovered at the Tevatron in the near future. However, we would like to determine its mass to an accuracy comparable to that of other quarks, so that models of quark masses can be tested, and so that the resulting error on predictions of quantities measured precisely at LEP and SLC is smaller than the experimental uncertainty. The precision of the mass measurement is limited by systematic effects even at relatively low luminosity (~10^32 cm^-2 s^-1). Detailed simulations for SSC and LHC indicate that an error on the mass of about 2 GeV should be achievable.
Strongly interacting supersymmetric particles (squarks and gluinos) have a large production rate in a hadron collider. Their signatures include events with jets and missing transverse energy or events with several isolated leptons and missing transverse energy. Gluinos of mass up to 1 (1.5) TeV should be detected at LHC running at a luminosity of 10^33 (10^34) cm^-2 s^-1. (The SSC at 10^33 cm^-2 s^-1 would have reached 2 TeV.) This is probably sufficient to detect supersymmetry if it is relevant to the breaking of electroweak symmetry.
Models in which the dynamics of electroweak symmetry breaking involve some strong interactions can be more difficult to test. Many models of this type have resonances, some of which may have strong couplings, that can be produced and detected fairly easily. Some of these resonances that do not have strong couplings can decay into channels where the backgrounds are negligible (such as Z(gamma)). The worst case scenario is represented by the "strongly coupled standard model" where the only manifestation is in the strong coupling among gauge bosons, and one must detect an excess of events over the standard model prediction in final states of two gauge bosons with large invariant mass. Of all the possible options this is the hardest to detect and the one for which the additional energy of the SSC was the most valuable. Detailed studies indicate that detection is possible only if the full luminosity of the LHC can be exploited, and even then several years of data taking may be required.
Other types of new particles not directly related to the weak symmetry breaking may also be found at hadron colliders. For example, a new gauge boson with couplings similar to the Z could be found at the Tevatron (with 1 fb^-1 of data) up to a mass of order 800 GeV, while LHC (with 100 fb^-1 of data) can reach 4 TeV and the SSC (with 10 fb^-1 of data) could have reached 5 TeV.
The production cross-section for bottom quarks at LHC is very large and only slightly lower than that at the SSC; at a luminosity of 10^32 cm^-2 s^-1. About 10^11 bottom quarks will be produced in a year of running. Some studies of CP violation in the B system will have been carried out at e+e- machines before the LHC operates, but this large sample, including Bs states, should allow more detailed studies, including the search for rare decays such as B --> (mu)(mu), to be carried out. While some interesting final states (such as B --> (pi)(pi)) are difficult to exploit in a general purpose detector that does not have particle ID, other decay modes such as Bs --> (psi)(phi) are straightforward.
In conclusion, other planned experiments at the B factory and in the Kaon system will provide vital information on the the couplings in the standard model. But, absent the SSC, the only facility that will be able to probe in the foreseeable future the fundamental question of particle physics, the origin of electroweak interaction symmetry breaking, is the LHC. We must participate in finding the answers to the origin of electroweak symmetry breaking if we are to remain at the forefront of particle physics.
Most of the institutions exploring the possibility of joining one of the large LHC detectors have been actively involved in the design and R&D efforts for the SSC detectors. Much of this experience is directly applicable to the ATLAS and CMS projects, since many of the technologies proposed for these detectors are similar, if not identical, to those that were being pursued for the GEM and SDC detectors. These detector programs were well advanced when the SSC was terminated, and their results and experience can be profitably applied to the LHC detectors. This will require sufficient funding to continue detector R&D and design in the U.S., adapted to the ATLAS or CMS requirements, and the establishment of close collaboration with non-U.S. groups already working on ATLAS or CMS detector components.
In FY1994, many of the groups formerly involved in SDC and GEM, as well as a few institutions that were not members of these collaborations, will explore CMS and ATLAS, and will likely make their decisions to join either CMS or ATLAS by about the middle of the fiscal year. The recent decision by the DOE to fund closeout activities for SDC and GEM will provide the opportunity for some groups to complete SSC R&D that may be of future use at the LHC.
It should be stressed that, although the funding for FY1995 can be modest, it does have to be significantly larger than zero if U.S. groups are to participate in the ongoing design and R&D efforts, and to develop meaningful long-term responsibilities within the collaborations. Either a series of proposals requesting R&D and design funding or a unified R&D proposal from each of the two experiments will need to be generated by the U.S. institutions seeking FY1995 funds for LHC detector work. These funding requests will likely be made by late summer of 1994. During 1995--1996, the collaborations will probably establish many of the long-term construction responsibilities for major detector components, as the designs for these components mature. One can thus anticipate the need for long-term financial commitments by perhaps as early as FY1996, and certainly by FY1997. The bulk of the construction funding should be available in FY1997--FY2000 to meet the LHC completion date of 2002, at which time the detector installation is to be complete.
Approximately 40 physicists from 17 U.S. institutions participated in the recent ATLAS Collaboration meeting of January 31-February 4 held at CERN. Several of these groups have already expressed interest in joining ATLAS and are participating in ATLAS subsystem working groups. Other institutions are still considering joining ATLAS in the near future, and thus it is difficult to present at this time a definitive summary of the U.S. involvement in the ATLAS project. Most groups expect to make decisions in the next few months, and, by April, a clearer picture should emerge.
Even with these uncertainties, it is possible to provide a preliminary description of those areas in the ATLAS project which are of potential interest to U.S. physicists and for which additional intellectual and financial resources are required. A brief description of these areas is provided below.
Muon System: Universities in the Boston area (Boston University, Brandeis, Harvard, MIT and Tufts) and the University of Washington are potentially interested in the design of the muon system, and the fabrication of chambers and parts of the alignment system. Northern Illinois University was significantly involved in the calculations of neutron and soft photon backgrounds for the SDC, and presented these results at the recent ATLAS meeting. BNL has developed a proposal for the use of Cathode Strip Chambers for the most forward rapidity interval.
Calorimetry: Arizona, BNL, Columbia, Pittsburgh, Rochester and Washington have joined the Liquid Argon Calorimeter Unit. They are involved with the mechanics and overall optimization of the EM calorimeter, the front-end electronics, the cryogenics and especially the feedthroughs, the endcap hadron calorimeter and the liquid argon option for the forward calorimeter. These groups plan a test at CERN in 1994, within the RD3 collaboration, of a small EM prototype that would validate many aspects of the design developed within the SSC/GEM group that could be applicable to ATLAS. These include a preshower detector integrated into the first longitudinal EM depth segment, the use of krypton as the active medium for better energy resolution, and the use of the massless gap technique to compensate for energy loss in the material upstream of the active part of the EM calorimeter. There will also be a small liquid argon hadronic module placed behind the EM unit to test the GEM concept of employing an electrostatic transformer for the readout. ANL, Chicago, and Illinois have expressed some interest in the iron/scintillator tile hadron calorimeter. This effort might include the structural design, the scintillator preparation and test, the fabrication of modules in the U.S. and test beam studies.
Inner Tracking Detector: LBL has expressed interest in contributing to the design and construction of the inner tracking detector. This might include the design, fabrication and testing of silicon strip detector modules and the associated radiation-hardened electronics, which were primary LBL responsibilities for the SDC detector. In addition, LBL plans to explore the possibility of contributing to the pixel detectors that are to be used at small radius in the current ATLAS design, thus taking advantage of many years of R&D in this area. The University of California at Santa Cruz, which led the SDC silicon tracker effort, has also expressed interest in working on the silicon detector and its electronics but has not yet made a decision to join ATLAS. LBL is also interested in microstrip gas chambers and their associated electronics readout, which could be very similar to the electronics developed by LBL and Santa Cruz for the SDC silicon tracker. Indiana, Pennsylvania, and Duke are exploring the possibilities for contributing to the straw tube transition radiation tracker (TRT). They might address the mechanical design, the fabrication, the simulation, radiation damage and lifetime issues, and signal processing. These groups were formerly involved in the straw tube tracker and its electronics for the SDC detector. Wisconsin has also shown interest in participating in the inner tracking detector.
Data Acquisition and Trigger: BNL, Columbia and Pittsburgh worked on the first level trigger for GEM, and will try to apply this to ATLAS. Pennsylvania has expressed interest in the TRT trigger, and Washington is exploring involvement in the level 1 muon trigger. LBL has some experience and interest in aspects of the data acquisition system, particularly high speed switches, and may also seek involvement in the DAQ and Level 2 trigger system for the inner tracking detector. The University of California at Irvine is also interested in potential contributions to the trigger system.
The muons are identified in four separate stations, each consisting of several planes of drift chambers, inserted in the barrel part of the return yoke. The endcap stations will utilize the cathode strip chamber technology since it is capable of operation at higher rates. Each station will also contain triggering planes made of resistive plate chambers. In the barrel region the momentum is measured three times: inside the inner tracking volume, just after the coil, and in the flux return. These almost independent measurements make the muon identification system very robust. Furthermore, the two measurements outside the coil are guaranteed at any luminosity. The inner tracking system aims to reconstruct all high Pt muons and isolated electrons produced in the central rapidity region with a momentum precision 0.1 x Pt (Pt in TeV), and to recognize all tracks with Pt > 2 GeV. Silicon detectors and microstrip gas chambers provide the required granularity and precision.
The electromagnetic calorimeter must be able to measure photon directions with sufficient precision so as not to degrade the di-photon mass resolution. Powerful isolation cuts and two-shower separation capability are required to eliminate background from jets. The CMS baseline is a lead-scintillator sampling calorimeter of the Shashlik design supplemented by a preshowering detector. A high resolution crystal calorimeter with good lateral granularity surrounding the inner tracking volume inside the coil would be the preferred design in the absence of cost constraints. A hadron calorimeter with copper absorber is installed between the EM calorimeter and the coil. Scintillator tiles equipped with wavelength-shifter fibers are used as detecting elements. The use of low-cost silicon pads as alternative detecting elements is being considered for the endcaps. Owing to the high radiation levels in the very forward direction (pseudorapidity between 2.6 and 4.7) a calorimeter consisting of iron sandwiching parallel plate chambers and employing an exchangeable active medium (gas) is the current baseline.
At the December 7 meeting at CERN between American physicists and members of the CMS management, spokesman Michel Della Negra stated that CMS would welcome the participation of as many as 150 to 200 American physicists, but he made the point, made also by the ATLAS group, that certain fundamental design parameters of the detector were fixed and not subject to further change. For CMS these include the 4 T magnet with its inner radius of 2.9 m. He enumerated several areas in which American groups could be incorporated into CMS: hadronic calorimetry, forward muon systems, inner tracking, and trigger/DAQ systems.
Subsequently, UCLA, one of the American signatories to the CMS Letter-of-Intent (LoI) hosted a meeting to give interested U.S. physicists the opportunity to discuss the possibilities of collaboration with CMS. Opportunities were enumerated using the mechanism of subsystem sessions jointly chaired by the CMS coordinators of the various detector subsystems and a U.S. convener. The meeting, held from February 2 to 4, attracted over 130 participants. Several specific areas of mutual interest, first defined in the December meeting, emerged in more detail.
Muon Chambers: The four American groups already present on the CMS LoI, namely UC Davis, UCLA, UC Riverside, and UT Dallas, in collaboration with a large contingent from Dubna, have elaborated a conceptual design involving cathode strip chambers (CSC) for muon detection in the forward region. R&D work has been carried out to prove the feasibility of using this technology in the CMS eofnvironment, and prototype chambers will be tested at RD5 during 1994. In addition a number of American groups have already been involved in CSC testing within RD5 including BNL, Boston University, Stony Brook and LSU. Questions of alignment and mechanical support had been studied in great detail by groups within the GEM and SDC collaborations, and CMS could benefit from their participation. These groups include Wisconsin, Brandeis, MIT, Fermilab, and LLNL. In addition, problems of system integration and the imposition of standards for CAD and file transfer have been successfully dealt with by these same groups, and their work would be very useful if the forward muon detectors were to become largely an American effort, an approach favored by the CMS management.
Calorimetry: In the area of hadron calorimetry, CMS has agreed that the center of gravity could well be in the U.S. Since the design and R&D for the CMS hadronic calorimetry is very similar to what had been proposed for SDC, it has been suggested that work on this subsystem could have a "U.S. barycenter," while also involving groups from CERN, FSU, China, and India. A transfer of the SDC calorimeter project management (FNAL) to CMS appears to be quite easy. In fact the SDC effort included Saclay, Pisa, Beijing, and Dubna so that the international aspects are also likely to transfer easily. Interested groups in attendance at UCLA were FNAL, Saclay, Purdue, Michigan, Wisconsin, Maryland, Rockefeller, Iowa, Florida State, VPI, UT Arlington, and Texas Tech.
As a beginning, the "hanging file" test apparatus, built to study hadronic calorimetry for the SDC, will be sent to CERN within a few months. More formally, an R&D proposal would be presented at the May DRDC meeting to test existing prototypes at CERN. A week-long meeting to develop such a proposal will be held next month.
The CMS baseline for the forward calorimeter is the Parallel-Plate-Chamber (PPC) proposal of the CIEMAT group in Madrid. Possible alternatives are high pressure gas tubes and quartz fibers. Tests of these devices could be conducted either as an addendum to the RD37 (PPC) experiment already in preparation, or as part of the P54 proposal.
For the electromagnetic calorimeter, interested U.S. groups are invited to take part in RD36 to test shashlik prototypes. Valuable work has been done in the U.S. on ways to minimize radiation damage to these devices. Joint radiation damage efforts on shashlik will begin at Beijing, and a radiation hard "green/orange" shashlik module will be made. Radiation damage studies on the crystal options are also a possibility.
Additional effort would also be welcome on photodetectors that will function in the 4 T magnetic field (APDs, siPDs, etc.) Several U.S. groups would plan to become involved (Minnesota, FNAL, Northeastern and Princeton). A digital pipeline developed by Fermilab might be applicable to the CMS calorimeter readout, and R&D efforts within the context of the FERMI project in Europe are the next step.
Trigger/DAQ: The CMS management indicated in December that the trigger effort would benefit from new U.S. collaborators. Interest has been shown by Wisconsin, FNAL, Saclay and by Michigan State in the area of calorimeter triggers. SDC trigger and DAQ project management resided in scientists from Wisconsin and Fermilab, so that the transfer to CMS should be painless. As a first step the SDC fast trigger simulator has been reconfigured to match CMS, and the trigger bandwidth requirements are being studied.
Inner Tracking: Promising developments in pixel devices were discussed at the UCLA meeting. This technique bears watching as a possible addition to the Si microstrip/MSGC tracker which is now the CMS baseline. The mechanical structure supporting the Si and MSGC detectors is still evolving in CMS, and further input would be useful. American groups could be of great help in obtaining radiation-hard components from U.S. sources. Although interest was shown at the meeting by some smaller groups, it appears unlikely at present that U.S. groups will make a major contribution to the CMS inner tracking effort.
General Software: In general the CMS software effort is considerably behind the level reached by the SSC software groups. Valuable contributions can be made in the areas of system architecture; detector specific algorithms, particularly for pattern recognition, for all subsystems; and global algorithms for track matching and gamma identification. Much work has been done in modeling neutron and gamma backgrounds by U.S. groups (e.g. NIU), and comparison with CMS efforts would be quite valuable.
Table 1. U.S. Institutions represented at the recent ATLAS or CMS Collaboration meetings or exploring the possibility of participating in ATLAS or CMS.
As a large segment of the U.S. HEP community looks to collaboration in the LHC physics program as its next step, the U.S. high energy accelerator community remains the willing and interested partner of the physics community in seeking collaboration on the LHC machine construction. Its willingness is in response to the clear input from the CERN management that it expects a significant contribution from the U.S. to the LHC construction in return for allowing major U.S. involvement in the LHC experimental program. Its interest arises from the view that it is imperative for the U.S. high energy accelerator community to maintain and further develop its investment in superconducting magnet technology for the benefit of future hadron collider efforts, and possibly for non accelerator applications. Involvement in a collaboration in the construction of the LHC can provide opportunities for such continuing efforts.
Representatives from CERN/LHC and from U.S. laboratories held a meeting on February 15, 1994 at Fermilab to discuss possible areas of collaboration. It is clear that CERN is interested in the intellectual as well as the financial contributions that such a collaboration could bring to the LHC construction. It was agreed at this meeting to schedule a workshop to exchange technical information relevant to the LHC among U.S. and CERN staff at BNL on March 24-25, 1994. Key areas of discussion at this workshop will include magnet and magnet system issues, superconducting strand and cables including new developments and AC loss effects, magnetic measurements, and vacuum problems arising from synchrotron light desorption.
Numerous possibilities for U.S. collaboration have been discussed. We have searched for tasks that are mutually beneficial---helping with the LHC and usefully advancing U.S. capability---and which fall within the existing expertise of the DOE laboratories. Recognizing the fiscal realities both here and abroad, we have tried to identify tasks needed in the near-term that can be accomplished with little additional funds.
Items that could be undertaken with very little extra funding include:
With a small amount of extra funding, this work could be expanded to include fabrication and testing of LHC-specific magnets.
We have also discussed larger items that may be fruitful for in-kind contributions. In general, both CERN and U.S. participants preferred in-kind contributions to be in complete systems, designed, developed, built, and delivered as working equipment. Such contributions might include:
The systems above may well represent more than the U.S. is willing or able to contribute, but they give a flavor of what is possible---ranging from the most difficult development tasks to engineering design and industrialization. It is likely that funding will be scant for at least two years, possibly increasing somewhat in FY1996, to allow an expansion of the magnet R&D program to the prototype stage. In FY1998, when the B-factory, and the new Main Injector will be completed as construction projects, enough funding may become available to start serious production.