come available, but it also calls for a transformation in how particle physicists interact with one another at the national and international levels; this turning point in particle physics is extremely compelling.

PRIORITIES

In considering the actions recommended in this chapter, it is important to understand what the committee means by “priority.” The elements of the scientific agenda that it recommends have been prioritized based on its analysis of the importance of the underlying scientific opportunities combined with its assessment of the technical readiness and feasibility of experimental facilities located in the United States and abroad.

It also is important to keep in mind the strategic principles outlined in Chapter 4. In particular, it is important to recall the strategic necessity of mounting, regardless of budgetary constraints, a comprehensive program that reflects a diversity of scientific opportunities and approaches to the scientific challenges facing particle physics. Under no circumstances, therefore, should the committee’s top two or three priorities be permitted to exhaust the entire available budget. Indeed, in the most pessimistic budget scenario, where maintaining a position of leadership is unrealistic (Scenario B), the resources invested in the priorities outlined below would need to be adjusted, but the need for pursuing a diversified research portfolio would be unchanged.

The capacity of the Large Hadron Collider (LHC) and the International Linear Collider (ILC) to explore the Terascale directly offers the promise of deep insights into such matters as the Higgs boson, supersymmetry, dark matter candidates, and hidden spatial and quantum dimensions. At the same time, explorations of unification and particle astrophysics, including both space-based and underground observations, promise to shed light on dark energy, dark matter, and inflationary models of the universe. Moreover, new and planned precision studies of lepton and quark properties and their interactions may reveal the role of neutrinos in the universe, explain how matter came to dominate antimatter, or uncover entirely new phenomena. The committee discusses each of these scientific opportunities and the associated action items in priority order, assuming, for the moment, the constant-effort budget (Scenario A).

DIRECT EXPLORATION OF THE TERASCALE

The most compelling current scientific opportunity in elementary particle physics is aggressive exploration of the Terascale, and this is the committee’s highest priority for the U.S. program. A two-part strategy using two world-leading



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