1  Particle physics in context

Although particle physics is often thought of as a ‘new’ concept, it has in fact been around for much, much longer. Like many of the great fundamental theories, the concept of an atomic like structure was first proposed by the ancient Greeks, by the philosopher Democritus (460–370BC). He believed that the matter we see around us was formed from a variety of different atoms. While this is known to be incorrect (and if you didn’t know this, you will soon!) in two ways (there are only 92 stable atoms, not a wide variety and they are divisible into smaller components), it was a great place to start from.

Fast forward a few thousand years, and by the 1930s, it was recognised that atomic nuclei are composed of protons and neutrons, and that in atoms the nuclei are surrounded by clouds of electrons. The existence of one more type of particle was surmised - the electron neutrino - which is produced in some radioactive decays when atoms change from one type to another. Finally, the photon was recognised as the quantum of energy involved in electromagnetic interactions.

Eighty years ago the account of subatomic structure would have ended with these particles, but now a deeper layer of structure is known. It is believed that protons and neutrons are composed of structureless particles known as quarks, while electrons and electron neutrinos are merely examples of a broader class of particles known as leptons. Furthermore, there are other fundamental interactions, besides electromagnetism, each of which has its own set of quanta. Leptons and quarks are discussed in the following sections, and the quanta of other fundamental interactions are discussed at the end of this course.

Before embarking on this journey through particle physics, consider the following puzzle: all of the atoms that make up the Universe (apart from hydrogen atoms) contain neutrons as well as protons in their nuclei. Yet free neutrons (that is, those not inside nuclei) undergo beta decay with a half-life of about 10 minutes. In the early Universe, soon after the Big Bang and before atoms formed, there are believed to have existed equal numbers of protons and neutrons. So why didn’t the neutrons all decay at that time, leaving a Universe made only of hydrogen? You will return to answer this puzzle at the end of this course.