Particle physics studies things which are incredibly small. The things which make up what we describe as "matter"; and "matter" is everything. People are made from matter. Your school, your house, and your town is made up of matter. Light and radio waves are matter. The whole known universe — and almost certainly the whole unknown universe — is made up of matter.
All matter is made up of particles, and there are many types of particle. There are atoms which in themselves are incredibly small. An atom contains protons, neutrons, and electrons; which are also particles. Protons, and neutrons are made of quarks, now we’re looking at very small particles indeed; a quark is smaller than 10-19 m across (that’s just 0.000 000 000 000 000 000 1 m). Quarks are subatomic particles, which also include leptons and bosons — we've all heard of the Higgs boson.
All the rules in particle physics are set out by something called "quantum mechanics". These are laws which govern very, very, exceptionally small things; things which can be simultaneously a particle and a wave, things which can be in more than one place at once, things which can disappear at the drop of a hat. Because of these rules, when we talk about "particle physics", we might not really be talking about particles at-all.
Recently, there has been a lot of excitement over work being done at CERN, on the Large Hadron Collider. Scientists have been looking for — and found — a particle called the "Higgs boson".
The Higgs boson is the particle which gives all other particles their mass. At the moment the universe was created, no particles had any mass; so they could all move around at the speed of light, almost immediately, something called the "Higgs field" was created. The Higgs field interacts with all other particles, and causes drag; heavier particles will move slower through the field, and lighter ones will move faster. The field doesn't interact much, if at-all with a photon — the particle from which light is made — and so this can still move very quickly. A photon has no mass, and can move at almost 300,000 km per second; the speed of light.
The Higgs boson is the particle which makes up the Higg;s field, and this is for what scientists have been searching.
The Large Hadron Collider (LHC) is the largest particle collider in the world. Protons are fired around the tunnel, which is a large circle with a circumference of 27 km; it needs to be this large because when the protons are travelling so fast — near the speed of light — they need a lot of space to move. A proton is quite a heavy particle, in the scheme of things, so getting it to move at this speed isn't easy.
When the protons crash together, they explode in a tiny flash of energy and quarks, and photons, and other particles are released. Very, very rarely, a Higgs boson will appear. This is what scientists have been trying to detect.
By detecting, and proving the existence of the Higgs boson, scientists have been able to prove the theory of why particles have mass. A question which has troubled science for decades.