Over the centuries, man has always been curious and asked questions about the world. One question which has puzzled both philosophers and physicists alike is the question of “what is matter?” The Greeks believed that matter was made up of the classical elements, earth, air, fire and water. This idea persisted throughout the Middle Ages and even into the Renaissance, greatly influencing European thought. The idea that matter was made up of small particles, as opposed to the idea that matter could be divided into infinitely smaller quantities, first began as a philosophical concept thought up by Democritus in ancient Greece. It was not until the 19th century that John Dalton concluded that matter did behave as if it were made up of tiny particles after experimenting with chemicals and their reactions. He named them atoms from the Greek word “atomos” meaning “indivisible.” It was not until 1905 that Albert Einstein proved that atoms existed through careful analysis of Brownian motion.
Over the last century the field of particle physics has expanded enormously, with the theories and discoveries of thousands of physicists culminating in the breaking down of the universe into its fundamental building blocks. We not only know that matter is made up of atoms, we also know that the atoms themselves are not fundamental and can be broken down further into neutrons, protons and electrons. The theoretical framework that describes the fundamental particles and the forces that describe how they interact is called the Standard Model.
The Standard Model is a culmination of many theoretical and experimental discoveries. The Standard Model was first devised in 1970’s and has been continually updated and improved since then. The Standard Model has been very successful so far, it has predicted several particles such as the W and Z bosons, the gluon and the charm quarks before they were actually observed.
The Standard Model is made up of twelve fundamental particles (not including their antiparticle counterparts) that are called fermions. These twelve particles are split into two distinct groups called leptons and quarks. These groups are split into three generations; the first generation is made up of the lightest and most stable particles, while the heavier and less stable particles belong to the second and third generations. A third generation particle is so heavy and unstable that it will quickly decay into a second generation particle which will similarly decay down into a first generation particle. Due to the instability of the second and third generations of particles, most matter is made up from particles in the first generation. The matter that we encounter in day to day life, the matter that we are made off, can be broken down into atoms which can then be divided up into electrons, neutrons and protons. The protons and neutrons can be broken down further into their constituent quarks; the proton is made up of two up quarks and a down quark, while a neutron is made up of two down and an up quark.
Quarks can only be found in groups forming composite particles which are called hadrons. Hadrons can also be split into two groups, baryons which contain three quarks and mesons which contain two. Quarks were proposed independently by both Murray Gell-Mann and George Zweig in 1964. They were devised in order to explain the multitude of hadrons which had been discovered and dubbed the “particle zoo.” Zweig and Mann proposed that these hadron were not fundamental particles themselves, but were actually made up of more, even smaller, particles that were themselves indivisible. The word quark actually came from James Joyce’s novel Finnegan’s Wake, where a drunken seagull in a dream, who said “three quarks for Mister Mark,” instead of “three quarts.” This name made sense then because only three quarks were originally proposed. However, there are now six quarks in the Standard Model. Reception to Zweig and Mann’s idea was mixed, most believed that it was merely a model to help understanding and that quarks did not actually exist. However, in 1968, the Stanford Linear Accelerator Centre showed that the proton was actually made up of three much smaller particles, thereby showing that quarks actually exist.
The Standard Model also describes how these particles interact with three out of the four fundamental forces, electromagnetic force, the strong force and the weak force. These three fundamental forces are carried by the exchange of force carrier particles called bosons. The electromagnetic force is the interaction between charged particles and is carried by the interaction of photons. The very successful model which is used to describe this interaction of charged particles via the exchange of particles is called quantum electrodynamics (QED).
The interaction between quarks inside hadrons and the force that keeps them together is called the strong force and is described through the exchange of gluons. The mathematical model used to describe this interaction is called quantum chromodynamics (QCD).
The weak force, aptly named because it is weak, is the cause of radioactive decay and is carried through the exchange of W or Z bosons, the only exchange particles to have mass. At higher energies the weak force and the electromagnetic force are two sides of the same coin. This unification is described by the electroweak theory.