The Standard Model Part 1: What Are Particles?
An introduction to hadrons, leptons, and quarks
Look around you and pick an object. It could be what you’re reading this on, a cup, a piece of furniture, food, etc. What would happen if you were to take this thing, and cut it in half, and then half again, and then half again, and so on? Could you go on forever?
The answer is, no. You would eventually reach a point where you could no longer cut in half — you would reach the elementary particles, the building blocks that make up all matter in the universe. These particles are infinitely small and cannot be divided into anything smaller. We’ll get back to your object in a moment.
As far as we know, this concept originated sometime in the fifth century B.C.E. and can be traced back to the Greek scientist and philosopher Democritus. Democritus’ explanation of an ‘atom’ is not the same that is used today in atomic theory. The term ‘atom’ is derived from the Greek word for indivisible. When Democritus theorized about an ‘atom,’ he was merely thinking about the smallest things in the universe that could not be divided.
His explanation used a stone. If the stone were to be unceasingly cut in half, he concluded there must be a point where a piece of the stone was small enough that it could no longer be divided. Democritus thought this must apply to all beings and any form of matter.
Democritus had other ideas, including that atoms exist separately from one another; that there are infinite numbers of atoms; that atoms can move
around; that atoms are able to unite to create matter; that atoms cannot merge to become new atoms; and that atoms cannot be divided. It is striking how similar his predictions are to modern atomic theory. Many argue he should be considered a scientist rather than a philosopher.
Within a few decades of Democritus’ death, Aristotle rose to acclaim. He promoted the idea that all matter was born from earth, air, fire, and water. Aristotle’s influence was enduring and anyone who held contrary beliefs was shunned. And so Democritus’ theories were mostly ignored.
So what does happen when you keep cutting something in half? How far can you go? Let’s break things down.
What are Particles?
A particle is a small bit of matter that can be assigned different properties such as mass, charge, or spin. All particles in the universe can be classified by mass and separated into two groups: leptons or hadrons. Hadrons themselves are not elementary particles, while leptons are. An elementary particle is a particle that can’t be broken into anything smaller.
All hadrons can be divided into two subgroups, mesons and baryons, and are composed of different combinations of quarks. Murray Gell-Mann and George Zweig taught us these combinations in 1964: intermix any three quarks to get a baryon. A proton is an example of a baryon: it is made of two up quarks and one down quark bound together. A neutron is also an example of a baryon. It has two down quarks and one up quark. Gell-Mann and Zweig also taught us that to get a meson, mix one quark and one antiquark. An example is a pion, made of one up quark and one anti-down quark. Pions are the lightest meson particle. Kaons are another example of a meson and are composed of a strange quark and an up or down antiquark.
Quarks are the smallest things we know of and the smallest particle making up your object. They have six varieties that are referred to as “flavors.” Above were mentioned three flavors: up, down, and strange quarks. The six types of quarks are split into three generations and are as follows: the “up quark” and the “down quark” complete the first generation, seconded by the “charm quark” and “strange quark,” and lastly, the “top quark” and “bottom quark.” The first generation is stable, while the second and third aren’t. All stable matter in the universe is made from first generation particles.
Quarks are extremely difficult to study due to the fact that they only come in pairs and trios and are never found by themselves. This is because of color confinement. If you were to try and pull a pair of quarks apart, you wouldn’t be able to separate them and get two individual particles. Instead, the energy binding the quarks together would turn into mass, due to Einstein’s famous E=mc2 equation. Thus, pulling a pair of quarks apart would create two new quarks, and hence, two new pairs.
When classifying particles by mass the second group is called leptons. Unlike hadrons, all particles that fall under the category of leptons are believed to be elementary particles. Leptons are not composed of smaller particles.
Similar to quarks, there are six leptons arranged in three generations. The “electron,” which is the lightest lepton, and the “electron neutrino” make up the first generation; the “muon” and the “muon neutrino” make up the second generation; and the “tau” and the “tau neutrino” make up the third generation.
All leptons can either hold one unit of electric charge or be neutral. Electrons, muons and taus are negatively charged, and have a distinct mass. Each of the three also have an associated neutral counterpart, the electron neutrino, the muon neutrino and the tau neutrino. These counterparts have no electric charge and no distinct mass. Moreover, all particles classified as leptons only respond to three of the four fundamental forces: the weak interaction, electromagnetism, and gravitation. Leptons are not affected by the strong interaction.
There are electrons in your object, too. How do electrons fit in? And what about protons and neutrons? See The Standard Model: Enter the Atom.