ticles. Investigating these particles is, in effect, deciphering the genetic code for the universe: why it is the way it is and how it came to be that way. The goal of elementary-particle physics is to understand the world around us by identifying the elementary particles, understanding their properties, and learning how they interact.

Researchers proceed toward this goal along two avenues: (1) by conducting experiments and (2) by trying to determine the physical principles that account for the phenomena they observe—what theoretical physicist Richard Feynman called ''the patterns [in] the phenomena of nature [that are] not apparent to the eye, but only to the eye of analysis." The dialog between experimenters and theorists shapes the research priorities of the field: Experimental research is often guided by theoretical predictions; about as often, phenomena will turn up in experimental data that no one expected to find, and theorists endeavor to account for them.

Investigating phenomena on this almost unimaginably minute scale requires the most powerful microscopes ever built: devices known as particle accelerators. In a particle accelerator, beams of subatomic particles are boosted to nearly the speed of light and then brought into collision with either a stationary target or another beam of accelerated particles coming head-on. In these collisions, remarkably, matter is actually created. The particles that emerge from the collision point, like sparks radiating out from microscopic exploding fireworks, are not contained within the original colliding particles. They are created out of the energy of the collision according to the rules of relativistic quantum mechanics. The higher the energy of the collision, the heavier are the particles it can create. Such particles, although fundamental, are often ephemeral, existing only briefly before transforming themselves into more stable particles. High-energy accelerators thus provide elementary-particle physicists with the opportunity to study phenomena that they could otherwise not observe on Earth. Today's accelerators can collide particles with such high energies that, on a very small scale, they replicate the conditions prevailing when the universe was only a fraction of a second old and enable physicists to study the kinds of particles that long ago shaped the evolution of the universe, before the cosmos cooled off too much for these particles to continue to be produced.

If accelerators function as microscopes, then the eyes and brains that see and record the phenomena that accelerators reveal are detectors. In essence, detectors are devices that surround the collision point to capture enough information about the particles produced to deduce their properties: Are they electrically charged? Are they light or relatively massive? How long do they exist before being transformed into other kinds of particles?

Over the past hundred years, advances in experimental instrumentation and technique have revealed subatomic phenomena that scientists in earlier centuries had no idea existed. These phenomena, in turn, have led to discoveries of physical principles that are crucial for understanding how the universe is put together.



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