2 The Standard Model

Today, most educated people know that the world is made of atoms, and that atoms, in turn, are made of electrons, protons, and neutrons. The electrons orbit a dense nucleus made of protons and neutrons, and as the outermost layer of any atom's structure, they are responsible for all chemistry. They are held close to the nucleus by electromagnetic forces: The electrons carry a negative electric charge, and protons carry a positive charge. Opposite charges attract, and this keeps the electrons and the nucleus together.

At one point in time, electrons, protons, and neutrons were all believed to be fundamental and without any constituent parts, just as atoms themselves were once believed to be, before the discovery of the electron. Electrons are the only one of these subatomic particles still considered fundamental, and it is with this venerable particle that we begin a table of the basic constituents of matter, called `fundamental fermions'. We will see more soon.

Fundamental Fermions (first try)
Name Symbol Charge
Electron $e^-$ -1

Since the electron is charged, it participates in electromagnetic interactions. From the modern perspective of quantum field theory, electromagnetic interactions are mediated by the exchange of virtual photons, particles of light that we never see in the lab, but whose effects we witness whenever like charges are repelled or opposite charges are attracted. We depict this process with a diagram:


Here, time runs up the page. Two electrons come in, exchange a photon, and leave, slightly repelled from each other by the process.

The photon is our next example of a fundamental particle, though it is of a different character than the electron and quarks. As a mediator of forces, the photon is known as a gauge boson in modern parlance. It is massless, and interacts only with charged particles, though it carries no charge itself. So, we begin our list of gauge bosons as follows:

Gauge Bosons (first try)
Force Gauge Boson Symbol
Electromagnetism Photon $\gamma$