School of Physics Experimental Particle Physics

What is particle physics?

Experimental Particle Physics (EPP) is the experimental arm of high energy particle physics. Our research is in probing matter to discover the underlying structure of matter; attempting to answer the age old question What are we made of?'.

Around 460 B.C., the Greek philosopher, Democritus, develop the idea of atoms. He questioned what happened when matter was divided continually; could we go on forever or would we reach a final point? He believed the latter, that you could only cut something up a finite number of times, until you reached the `atom'. His ideas were squashed by a more famous Greek philosopher, Aristotle. It wasn't until the 1800s when the English chemist, John Dalton performed experiments with various chemicals and that showed that matter did consist of elementary lumpy particles, the atoms of Democritus.

Accepted as existing today, the atom was thought to be the smallest that matter could be reduced to but the atomic revolution at the end of the 19th and beginning of the 20th centuries proved this wrong. The power of the atomic bomb and the potential danger in radiation indicated that there was still much to be discovered inside the atom. The atom was found to have a very small but heavy centre, the nucleus, made up of protons which were positively charged and neutrons, having no charge. This centre was surrounded by negative charge, electrons.

Probing the inside of an atom is not as easy as just getting the sharpest knife in your kitchen drawer. In the same way that we break the shell of an egg to see what is inside we `break' atoms to see their contents. The methods for doing this require very strong forces. Throwing a pingpong ball at a wall will do no damage but a bullet will leave a hole. We use very fast and light particle, like protons and electrons, to break open atoms, and even smaller particles. We measure what comes out and use that to work backwards and determine what was in there to start with. Another method used is to collide two particles together. The resultant pieces tell us what the colliding particles were made of. For the particles to break into the atom, or the other particle they are colliding with, they must be travelling at speeds near the speed of light, 300 million metres a second. Particles are accelerated to these speeds by electric fields in machines called, unsurprisingly, accelerators. These accelerators are either linear or circular, but always large; the largest linear accelerator is 3.2km long and the circumference of the largest circular accelerator is 27km.

Detecting the particles that are liberated from atoms, and smaller particles, is a mamoth task too. Detectors weigh thousands of tonnes and have many different functions; there are sections to measure the energy of particles along with equipment to determine their trajectories. The knowledge of the 90 odd stable atoms was once easily summarised by the now familiar periodic table. The much smaller particles now referred to as fundamental particles is summerised in the following diagram. This shows the paritcles that make up all matter, along with the particles that `carry' the four forces, gravity, elecromagnetism, the strong, and weak forces.

particle chart

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