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.

Especially when unexpected discoveries have changed the course of particle physics, accelerator energies and ongoing experiments have been modified to take advantage of the new discoveries. We don�92;t know what will be found at the Terascale; it would be foolish to extrapolate even further to the unexpected. What we do know is that unexpected discoveries such as the tau lepton have changed the paradigm of particle physics and the course of the field. As indicated on page 26, supersymmetry is rich enough to harbor surprises that could require re-optimizing the experimental triggers for the LHC experiments, guided by an ILC running concurrently. Given the expected long running time of the LHC and its upgrades, such concurrent operation would occur in any case if construction of the ILC starts after initial LHC physics results are known.

d) How would the combination of the LHC and a linear collider answer questions

that could not be addressed by either machine alone?

The report is keyed, scenario by scenario, to the theme of how the LHC and ILC would work together to discover the physics of the Terascale. Collisions of the constituents of the protons in the LHC beams have varying fractions of the total energy, extending up to many Teravolts. As a result, the LHC has the greater energy reach -- far greater than the ILC for new particles with strong interactions. The ILC, with electrons and positrons as the beam particles, produces particles solely through electromagnetic and weak interactions. The clean events, polarization, and precise knowledge of the energy and momentum for each collision allow it to zoom in on a particular energy, make precision measurements, and pin down new particles such as the superpartners of the leptons that are difficult to extract from the more complicated events at the LHC.

The case of supersymmetry is described on pages 24-25 of the report. The LHC could find superpartners with masses extending up to several TeV. The ILC would be able to find lighter superpartners, including some not found at the LHC, confirm the symmetry of supersymmetry and identify the particular supersymmetric model, plus feed this information back to obtain more accurate superparticle spectra within the LHC analyses.

e) What physics would a linear collider address that would be impossible to probe at

the LHC?

The ILC, through unique high precision measurements, will teach us about the strength of interactions and the relationships between particles that will be key to discovering how the Terascale works. Is the particle found really the Higgs and just the Higgs? When they are put into calculations that start from the Big Bang, do the mass and interactions of a candidate dark matter particle yield the dark matter density observed in the universe today? Does nature have the symmetry of supersymmetry?

Finally, with Terascale physics understood, precision measurements at the ILC would be sensitive to quantum fluctuations, allowing it to act as telescope to ultra-high energies where no accelerator will ever operate. There we may get our best experimental information on the ultimate unification of all the forces, including gravity.

f) How would the physics discoveries from experiments at a linear collider be useful

to other branches of science?

The science of the ILC is most directly related to astrophysics and cosmology. First, I have noted the possibility of gaining insight into the forces that governed the first instants of the universe, perhaps even back to the beginnings envisaged by Einstein�92;s dream of unified forces. Second, astrophysical measurements of the dark matter density in the universe are expected to reach an accuracy of a few percent in the next decade. We expect to produce particles that make up dark matter at the LHC and ILC. With measurements at the ILC of their interactions and of their mass also to a few percent, the ingredients will be present for making a definitive test of whether the dark matter candidates produced at accelerators allow us to understand the dark matter in the universe, which survives as a relic of the Big Bang.

For several decades particle theorists and experimentalists have anticipated exploring the Terascale. The Terascale is now in view. The US LHC construction projects for the detectors and the accelerator are very nearly finished. The US software, computing, and operations activities are ramping up, as faculty, postdoctoral fellows, and graduate students from my university and others work on installing the detectors in the underground halls and preparing for commissioning. They follow the progress of the machine installation on the LHC Dashboard ( and their professional lives are directed accordingly. For them, the opening of the Terascale at the LHC is now.

The ILC has taken giant steps with the technology decision and formation of the Global Design Effort (GDE) in the last year and is proceeding through the last phases of R&D and into design. That process must ramp up if we are to be ready for a timely decision on initiating construction of the ILC, once we see the discoveries of the LHC. It is the LHC and ILC together that will allow us to answer many of the compelling questions of particle physics by uncovering what happens at the Terascale and allowing us to view what lies far beyond.


Fred Gilman
Chair, HEPAP