Understanding matter

More on Why do physicists study particles

The world of tiny particles seems at first sight so far detached to be almost irrelevant to everyday existence. But did you know that if electrons had no mass, then solids couldn't exist? Or that if electrons were much heavier than they are, the Sun might have burned out long ago, perhaps before life on Earth had even had time to evolve? By studying the fundamental particles, how they behave and interact with each other, physicists are able to piece together the workings of the Universe on the largest scales. And by doing that, they give us a better understanding of our place in the Universe and allow us to wonder at nature's marvellous intricacy.

Particle collisions at CERN can take us back to the beginning of time, some 15 billion years ago. They can take us to the dense cores of neutron stars, where matter is so dense that a piece the size of a pinhead would weigh as much as 1000 Jumbo jets. They allow us to observe the cataclysmic explosions of stars, supernovae, from the comfort of our laboratories. And they transport us into the mysterious world of antimatter.

We still don't know everything about particles, but we have a very good idea of how they behave. A lot of painstaking research over the last 100 years has resulted in a very sophisticated collection of theories of particles. Physicists call these theories the Standard Model.

The Standard Model divides particles up into two kinds: particles of matter and force carrying particles. Particles of matter are the building blocks making up large objects like stars, planets and human beings. Force carrying particles are what hold everything together, or sometimes cause things to fall apart.

Just four kinds of building blocks are needed to account for all of ordinary matter. These are particles called up-quarks, down-quarks, electrons, and electron-neutrinos.

Up-quarks and down-quarks are embedded deep inside protons and neutrons in the atomic nucleus. Electrons orbit the nucleus to form atoms which stick together making complex objects. Electron-neutrinos complete the family, but interact so weakly with other matter that we hardly see them.

You might think that would be the end of the story, but it's not. Nature has other ideas in mind and it turns out that another two families of four particles exist. These are similar in every way to the up-quarks, down-quarks, electrons and electron-neutrinos except that they are heavier. This means that whenever particles made of them are produced, they quickly decay to the lightest possible particles, the familiar up and down quarks, electrons and electron-neutrinos of everyday matter.

The families of matter particles. Click for a bigger version.

The particles of the heavier families are called strange and charm quarks, muons and muon-neutrinos for one family, and top and bottom quarks, taus and tau-neutrinos for the heaviest family. They occur naturally in exotic places like the hot centres of stars, and they are also made at CERN. Notice that many of them are named after characters of the Greek alphabet.

Learn more about What holds matter together?
or return to
Introduction to Particle Physics

Particle Physics Education CD-ROM 1999 CERN