Experimental Particle Physics (EPP) is the experimental arm of high energy particle physics. Our research is in probing matter through large-scale experiments.

Around 460 B.C., the Greek philosopher, Democritus, developed 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 that the English chemist, John Dalton, performed experiments with various chemicals and 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. The atomic revolution at the end of the 19th and beginning of the 20th centuries proved this wrong. The atom was found to have a very small but heavy centre - the nucleus - made up of protons which are positively charged and neutrons which have no charge. This centre was surrounded by negative charged electrons.

Experimental physicists collaborate with theoretical physicists who develop theories on how the underlying forces work and together seek to fit data with theory. Or the data may be such that theoretical physicists must come up with new theories. We gain new data through the use of powerful scientific instruments. Probing the inside of an atom requires very strong forces. We use very fast and light particle, like protons and electrons, to break open atoms, and even smaller particles.

Another method used is to collide two particles together. 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 mammoth task. 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. 


A pink glow illuminates the inside of this model of the LHC beam pipe, which is used to train engineers and technicians. Image credit: Guillaume Jeanneret/CERN