Interim Transmittal Letter dated July 27 2005

July 27, 2005

Harold T. Shapiro, Chair

Sally Dawson, Vice Chair

Elementary Particle Physics 2010 Committee

The National Academies

500 Fifth Street, NW

Washington, D.C. 20001

Dear Harold and Sally,

Thank you again for your letter of March 15, 2005 to me as Chair of the High Energy Physics Advisory Panel (HEPAP) and for the opportunity to answer your questions about the International Linear Collider (ILC) along the broad themes of 1) the physics case, 2) the research and development (R&D) plan, and 3) international planning. My letter to you at the end of May transmitted the answers to the questions on 2) the research and development plan. Barry Barish, the head of the Global Design Effort for the ILC, will be discussing point 3), international planning, with your Committee. This letter serves to transmit answers to 1) the physics case.

Enclosed with this letter is the HEPAP report Discovering the Quantum Universe: The Role of Particle Colliders. Drawn up by a special HEPAP subpanel, the report sets forth the science of the LHC and ILC and how that science can be addressed. It includes answers to the questions specifically asked by the Committee. With references to where fuller discussion can be found in the new HEPAP report, let me take the questions up one by one:

a) How does a linear collider address the compelling questions of particle physics? Is a linear collider clearly the right machine to address these physics objectives?

Particle physics asks what the universe is made of and how it is put together on the most fundamental scale. At the beginning of the 21st century, that definition can be articulated by the nine compelling questions set forth in the 2004 document, Quantum Universe . These science questions lead off the enclosed report.

Both theory and experiment suggest that many of the answers to the nine questions will be found in the energy domain of a trillion electron Volts (TeV), what we term the Terascale. The LHC will provide the first broad look at the Terascale. Using likely scenarios of discovery at the Terascale, the report shows that the more that is discovered at the LHC, the greater will be the discovery potential of the ILC. This connection is summarized in the Table on page 6 and described in detail in Chapter III of the report.

b) What physics does a 500 GeV linear collider address? What are the arguments

for going to an energy scale of 1 TeV? How would results from the LHC change

these arguments?

Precision measurements of the electromagnetic and weak forces indicate that a single Higgs particle is to be found at energies well below 500 GeV. The first of the detailed scenarios in the report indicates how a 500 GeV ILC would make decisive measurements of its properties, determining with high precision whether it is solely associated with the mechanism that gives mass to the elementary particles or if it is part of a more complex Higgs sector, containing admixtures of other Higgs-like particles present in theories with extra dimensions and in supersymmetry.

Other likely scenarios of discovery are presented in the report. Each has an associated energy scale. Some, like the first scenario on page 30 of the report for discovery of supersymmetric dark matter particles, could be addressed at a 500 GeV ILC; others, like the second dark matter scenario on page 32, explicitly require a higher energy machine. Varying the energy to explore the number and shape of extra dimensions (page 25 of the report) entails eventually raising the energy toward one TeV. Once the LHC makes its initial sweep of the Terascale, its discoveries will be the true guide to choosing the energy of the ILC.

c) What are the physics arguments for operating a linear collider in the same time frame as the LHC?

The decision to build the ILC would follow from its potential for discoveries based on what is found at the LHC, driven by the desire to understand the physics of the Terascale. The synergy of the two colliders is an added benefit. Historically, results have flowed back and forth between proton and electron accelerators in advancing particle physics; examples of the interplay of electron and proton machines are discussed in the synergy sidebar on page 20. The answer to question d) below deals with the synergy between physics discoveries at the LHC and ILC whether operating concurrently or not.

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