As the preceding chapter demonstrated, particle physics has entered a special time. The most exciting scientific questions that need to be addressed are clear. The next cohort of experiments needed to address many of those questions are about to begin or are on the scientific horizon. Expert groups of scientists, engineers, and advanced students are available and eager to move this segment of the scientific frontier forward. A goal that has occupied science for centuries—gaining a fuller and deeper understanding of the origins and nature of matter, energy, space, and time—is ready for what may be a revolutionary leap forward.

With experimental study of the Terascale about to begin, physicists are finally gaining the tools needed to address questions that have been asked for decades:

• Why do the weak interactions look so different from electromagnetism, given that the fundamental equations are so similar?

• Where do particle masses come from? Does the Standard Model describe them correctly, or do the particle masses come from some more exotic mechanism?

• Are the forces of nature unified at some high energy scale? With the elementary particles known today, unification does not quite work, but it fails in a way that suggests the missing pieces will be found at the Terascale.

• Do space and time have additional dimensions? Do they have new quantum dimensions?

• What is the dark matter of the universe? Can it be produced in the laboratory?

The next generation of experiments will answer at least some of these questions.

Particle accelerators recreate the particles and phenomena of the very early universe. When particles collide in accelerators, new particles not readily found in nature can be produced and new interactions can be observed. These new particles and interactions were prominent in the early universe but disappeared as it cooled, leaving only scattered clues about their continuing influence. Understanding the properties of these particles, however, is essential to building a full understanding of the natural world and its evolution. Accelerator experiments are the sole places