In conclusion, the Panel endorses the energy and physics research goals of the SSC as currently proposed, and its timely completion by the end of the decade. We urge the Department to take the steps necessary to bring this about.

Enclosure

Yours sincerely,

Fraum

Francis E. Low

Chairman

High Energy Physics
Advisory Panel

Transmitted here with is the report of the Subpanel on SSC Physics which met
at the SSC Laboratory (SSCL) in Texas on January 6-8, 1990. In preparing our
report, we not only heard from Dr. Roy Schwitters. Director of the SSCL, and
other SSCL staff, but also from Dr. Carlo Rubbia, Director General of CERN.

As requested, this report focuses on the physics research to be done at the SSC
and provides advice on the range of useful machine parameters in order to
complete the design phase of the facility.

The Subpanel concludes that the SSC Laboratory, in its current proposal, has
chosen wisely among the alternatives and that the project should move
forward expeditiously. We feel strongly about the need for a flexible and
reliable facility at 20 TeV beam energy for decades to come, and, therefore, we
believe it would be very unwise to redesign the machine with a reduced
circumference.

REPORT OF THE

HEPAP SUBPANEL ON SSC PHYSICS

In 1983, the high energy physics community set as its highest priority the construction of proton-proton collider having a beam energy of 20 trillion electron volts (TeV) and a luminosity of 1033 cm 2 sec1 (see Appendix A). Approved for construction last year, th Superconducting Super Collider (SSC) will be a unique instrument for exploring new research frontiers. No comparable facility is being planned anywhere else in the world. Because of its size and uniqueness, the SSC will be expected to operate for many decades and, therefore, should be built with high reliability and with flexibility to support a broad research program. It is important that it be completed before the end of this decade to ensure continuity and productivity in the U.S. high energy physics program.

The SSC is now undergoing site-specific final design in Texas. We have been constituted as a Subpanel to the High Energy Physics Advisory Panel (HEPAP) and asked to provide "advice on the range of useful machine parameters in order to complete the design phase o the facility" in view of practical financial constraints.2 This report is our response, and, a requested, "focuses on the physics research to be done at the SSC."

The very spirit of physics is to explore the unknown. This makes it impossible for us to predict precisely what we will discover in the future. Based on our present knowledge, however, we are confident that the SSC will explore a region in which major new discoveries will be made. The SSC specifications were established with this goal in mind and experience gained since 1983 has strengthened our conviction of the importance of constructing a proton-proton collider with the beam energy of 20 TeV and luminosity of 1033 cm-2 sec-1, as originally proposed.

The SSC Laboratory (SSCL) management has developed a detailed site-specific design which resulted in an increase in the estimated total project cost. We were told that this increase is substantial (in the neighborhood of 20 to 30 percent above the $5.9 billion derived from the 1986 proposal of the Central Design Group). The cost increase arises primarily from two types of changes. The first type involves recalculation of labor and materials costs and requirements for magnet R&D and instrumentation to assure reliable accelerator performance. The second type involves technical changes (higher injection energy and a larger magnet aperture). Based on recent experimental and theoretical findings, the SSC Laboratory judges these technical changes to be required for reliable operation. We have reviewed these technical changes and the reasons for making them, and we conclude that implementing them will ensure confidence in reliable and timely operation of the SSC.

The Subpanel concluded that the only way to keep the cost of the SSC at $5.9 billion would be to seriously reduce its capabilities. We considered whether there are cuts that would allow substantial savings and that could be made in the initial design but restored later. We found none (see Appendix A). One possibility for substantially lowering the

1 Luminosity is a term used to specify the intensity of the beams in a collider. The higher the luminosity, the greater the rate of collisions.

2 Letter of December 21, 1989, from Dr. James F. Decker, Acting Director of the Office of Energy Research, to Dr. Francis Low, Chairman, HEPAP (see Appendix B). Subpanel membership is listed in Appendix C.

SSC cost would be a major reduction of the collider circumference from 54 miles. This would reduce tunneling costs and, correspondingly, decrease the number of magnets needed and the energy that could be attained. As far as we could judge, a really substantial cost saving that would return the estimate to something like the original $5.9 billion would require a reduction in circumference and beam energy by approximately 25 percent.

We have examined the physics potential that might be lost by lowering the machine energy by such an amount. It is always difficult to be precise in advance about the energy that is needed in a new machine. Certainly, no one knows enough to say that 20 TeV would be adequate, but 19 TeV would not. There are various scientific targets for which the reach of the SSC would be proportional to its energy. For instance, physicists have speculated about a variety of new particles, the discovery of which would mark a watershed in our understanding of physics, but for which we have no plausible predictions of mass.

There is, however, one class of phenomena, related to the breakdown of the symmetry between the weak and electromagnetic interactions, for which we can be more definite. The question of electroweak symmetry breaking is of truly fundamental importance. It represents the single greatest uncertainty in our present understanding of fundamental particles and forces, and it bears on such issues as the origin of mass. The best efforts of theorists indicate that 20 TeV is about the minimum energy needed to have confidence that these phenomena, in whatever form they may take, will be seen at the SSC. There is broad agreement that this confidence would be lost at 15 TeV. The SSC may well make other discoveries of equal or even greater importance, but it seems to us essential that the SSC have sufficient energy so that the problem of electroweak symmetry breaking can be solved (see Appendix A).

Apart from theoretical considerations, another guide to the energy needed in a new accelerator is that it should represent a significant increase in capabilities over other accelerators. Certainly the SSC, at either 15 TeV or 20 TeV, would represent a great step beyond any accelerator now operating or under construction. However, the existing 17-mile-circumference tunnel for the Large Electron-Positron (LEP) accelerator at the European Laboratory for Particle Physics (CERN) may allow construction of an 8-TeV hadron collider (called the LHC). Such a project is being seriously considered. For an efficient and balanced international program in particle physics, the size of any new accelerator tunnel built in the United States (54 miles proposed for the SSC) should be very different from the existing LEP tunnel, and so should the corresponding accelerator energies.

We have also considered the extent to which higher luminosity can compensate for lower beam energy. Although specific experiments may be able to use luminosities above the 1033 cm-2 sec-1 design value, present detector technology does not allow use of higher luminosities for a broad range of experiments. The potential for upgrade of the machine to higher luminosity should be retained for future evolution of the experimental program (see Appendix A).

Finally, we are concerned that the redesign of the SSC to fit a smaller circumference, with the consequent necessity for new environmental impact studies, will incur a substantial delay in project startup and raise numerous problems.