neutrinos have a tiny but nonzero mass. Thus, despite the extraordinary success of the Standard Model, it seems likely that a much deeper understanding of nature will be achieved as physicists continue to study the fundamental constituents of the universe.

Elementary particle physicists use a wide variety of natural phenomena to investigate the properties and interactions of particles. They gather data from cosmic rays and solar neutrinos, astronomical observations, precision measurements of single particles, and monitoring of large masses of everyday matter. In addition, crucial advances historically have come from particle accelerators and the complex detectors used to study particle collisions in controlled environments. Today the most powerful accelerator in the world is the Tevatron at the Fermi National Accelerator Laboratory (Fermilab) in Batavia, Illinois, which is scheduled to be shut down by the end of the decade. A more powerful accelerator, the Large Hadron Collider (LHC) at the European Center for Nuclear Research (CERN) in Geneva, Switzerland, is scheduled to begin colliding protons in 2007. Both theoretical and experimental evidence suggests that revolutionary new physics will emerge at the energies accessible with the LHC.

Beyond the LHC, physicists around the world are designing a new accelerator known as the International Linear Collider (ILC), which would use two linear accelerators to collide beams of electrons and positrons. Together, the LHC and an ILC will enable physicists to explore the unification of the fundamental forces, probe the origins of mass, uncover the dynamic nature of the “vacuum” of space, deepen the understanding of stellar and nuclear processes, and investigate the nature of dark matter. These tasks cannot be accomplished with the LHC alone.

For more than half a century, the United States has been a leader in particle physics. But over the next few years, as the flagship U.S. particle physics facilities are surpassed on the energy frontier by new facilities overseas or are converted to other uses, the intellectual center of gravity of the field will move abroad. At the same time, the conclusion of these important experiments creates an opportunity for the United States to consider major new initiatives.

Today, the U.S. program in elementary particle physics is at a crossroads. For the U.S. program to remain relevant in the global context, it must take advantage of exciting new opportunities. Doing so will require decisive actions and strong commitments; it also will require a willingness to assume some risks. Thus, to ensure continued U.S. leadership in this important scientific area, a new strategic framework is needed that can guide the difficult decisions that have to be made.