## Particle Physics

### Elementary Particles

An elementary particle (or fundamental particle) is a particle that doesn’t have any known parts. In other words, an elementary particle does not consist of smaller objects, so far as we know. Atoms were once considered elementary particles; but thanks to the work physicists like J.J. Thomson and Ernest Rutherford, we now understand that atoms consist of electrons, protons, and neutrons. (See chapter 4 for discussion of those discoveries.) But what are electrons, protons, and neutrons made of? Are they elementary particles, or do they consist of even smaller parts?

Particle physics is the branch of physics that studies elementary particles. The most successful theory of particle physics—the Standard Model, as it is called—says that electrons are elementary particles. Electrons do not consist of smaller parts, so far as we know. Protons and neutrons, on the other hand, aren’t elementary particles. They consist of smaller particles called quarks.

Protons and neutrons consist of two types of quarks, called “up quarks” and “down quarks.” Up quarks have a positive charge of ⅔ e, where the unit “e”—called the elementary charge—is the charge of a single proton (about 1.6 × 10−19 coulombs). Down quarks have a negative charge of -⅓ e. A proton is comprised of 2 up quarks and 1 down quark, so it has a positive charge of exactly 1 e (⅔ + ⅔ − ⅓  =  1). A neutron consists of 1 up quark and 2 down quarks, so it has zero total charge (⅔ − ⅓ − ⅓  =  0).

Electrons, up quarks, and down quarks constitute atoms, which in turn comprise physical objects like trees, rocks, planets, stars, and just about everything else we find at the macroscopic scale. Those three elementary particles—electrons, up quarks, and down quarks—are the fundamental building blocks of all the material objects we see. The stuff we can see, however, is only the tip of the iceberg. According to the Standard Model, there are four other kinds of quarks besides up and down quarks. Similarly, electrons belong to a group of six elementary particles called leptons. In all, there are six kinds of quarks and six kinds of leptons. These twelve types of elementary particles are all considered particles of matter, even though only three of the twelve are found inside atoms.

Moreover, there is other physical stuff in the universe besides matter. There is also something called “antimatter,” which will be discussed later in this chapter. In addition to matter and antimatter, there are other elementary particles that play a role in transmitting forces and energy. We’ll learn more about each of these types of particles in what follows. But first, it will be worthwhile to consider how physicists detect and study elementary particles.