What's all the Excitement About?

Particle physicists study matter at the very smallest scale, exploring the structure and interactions of the most fundamental particles that make up everything in the universe, from stars to starfish. Particle physics has made incredible progress since the discovery of the electron one hundred years ago. "The Standard Model,' a well-tested theory confirmed most recently by the discovery of the top quark, describes the particles and the way they interact. However, many fascinating questions and puzzles remain.

To study the most fundamental building blocks of matter, physicists need tools that can probe the tiniest particles in the universe and test the forces that affect them. Physicists use particle accelerators, many kilometers in length or circumference, to accelerate particles such as protons and electrons and create particle collisions. The collisions produce new particles that physicists study by surrounding the collision points with detectors, devices as big as multi-story buildings.

Fundamental Building Blocks:
The Quarks

Beginning 50 years ago, physicists using accelerators to study collisions of protons and neutrons with atomic nuclei discovered a bewildering array of more than 100 new particles. In the early 1960's, they hit upon the idea that almost all the particles could be explained by a few types of yet smaller objects called quarks. The world we see about us consists almost entirely of quarks, mostly the up and down quarks. We have now discovered four other quarks called charm, strange, top, and beauty.

The Leptons
J.J. Thomson discovered the electron 70 years before the discovery of quarks. Only recently did we learn that there is another family of fundamental particles to which the electron belongs. These particles, called leptons, have interactions quite different from those of the quarks. Two leptons, the muon and the tau, differ from the electron only in that they are more massive. The other three leptons are very elusive particles called neutrinos, which have no electric charge and very little, if any, mass.

Forces and Interactions:
Forces hold quarks and leptons, the building blocks of matter, together to form more complex systems such as nuclei, atoms, starfish, and stars. All forces we see in nature result from the interactions of the particles. Interactions come in four types: gravitational, electromagnetic, strong, and weak. "Carrier particles" mediate each type of interaction.

The Standard Model
Over the last 100 years, many experiments have confirmed the ideas of quarks, leptons and the interactions among them. There are all part of the Standard Model of fundamental particles and interactions. The Standard Model accounts for all observed particles and their processes through the strong, weak, and electromagnetic interactions of the quarks and leptons. Thus, it explains the forces that hold both atoms and nuclei together.

What Questions Remain?
The Standard Model leaves many important questions unanswered: Why do three types of quarks and leptons of each charge exist? Can we understand the source and pattern of their masses? Will we discover more types of particles and forces at yet higher-energy accelerators? Are quarks and leptons really fundamental, or do they, too, have substructure? Why does the universe consist almost entirely of matter with scarcely any anti-matter? What particles form the dark matter in the universe? Questions such as these will lead future particle physicists to do experiments at new accelerators, seeking clues to their answers.