Until the beginning of the 20th century, it was thought that the world was made up of indivisible building blocks, known as atoms. 

However, it was soon realised that atoms are themselves composed of smaller elements, known as electrons, neutrons and protons. With the help of particle accelerators, physicists discovered that the particles in the atomic nucleus – the hadrons (neutrons and protons) – consist of even smaller particles, quarks. Quarks are bound together by energy, which in turn determines the mass of a particle. New particles and their interactions were discovered by studying cosmic radiation and designing more powerful particle accelerators.