Particle physics

Quarks, leptons, the Standard Model, and the fundamental forces

What is particle physics?

Particle physics studies the most basic building blocks of matter and energy — particles even smaller and more fundamental than the protons and neutrons covered in the Nuclear Physics article — and the forces that govern how they interact.

Protons and neutrons are not the smallest particles

For a long time, protons and neutrons were thought to be fundamental (not made of anything smaller). We now know they are each made of three even smaller particles called quarks, bound together by the strong force mentioned in the Nuclear Physics article. Quarks have never been observed on their own outside of these bound combinations — they are always found bundled together.

The basic building blocks

All ordinary matter is built from two kinds of fundamental particles: quarks (which combine to form protons, neutrons, and similar particles) and leptons (a family that includes the electron). Alongside these are force-carrying particles, which are responsible for transmitting the fundamental forces between other particles — for example, the photon carries the electromagnetic force covered in the Electromagnetism article.

The four fundamental forces

Physicists have identified four fundamental forces that govern all known interactions in the universe: gravity (covered in the Relativity article), electromagnetism (covered in the Electromagnetism article), the strong force (which binds quarks together and holds nuclei together, covered in the Nuclear Physics article), and the weak force (responsible for certain types of radioactive decay). Every interaction between particles, at the most basic level, is understood as being carried by one of these four forces.

Why particle physics matters

Particle physics answers some of the most basic questions about what the universe is fundamentally made of and why it behaves the way it does. Its discoveries — like the electron, the photon, and the Higgs boson — have also led directly to technologies including medical imaging (PET scans), semiconductor electronics, and the World Wide Web, which was originally created at CERN, a major particle physics research laboratory, to help physicists share data.