A Very Brief Introduction to the Standard Model of Particle Physics

Credits: I prepared this note jointly with Mohit Jindal.

"Physics" is the study of Nature and natural phenomena. We see matter all around us. It comes in myriad incarnations. One of the most important discoveries of physics has been to establish, beyond doubt, that all matter—all objects around us (and indeed, our own bodies)—are made up of simpler, tiny constituents, replicated in enormous quantities.

The tiniest known matter particles are called “elementary particles”. Of course, new understanding might reveal that some of the particles that we currently think of as “elementary” are composite particles, made of something else. We think of “elementary” particles as being indivisible, incapable of being broken into smaller pieces. In this article, I will summarize the facts about structure of matter, as we know it today.

Bosons and Fermions

All particles in the universe can be divided into two classes, Bosons and Fermions. They are distinguished by a number that is called their “spin”. Spin is a very fundamental property of all matter particles, just as important as their mass or charge. If the spin of the particle is 0 or 1 or 2 or 3, etc., it is a boson. If the spin of the particle is 1/2 or 3/2 or 5/2 or 7/2, etc., it is a fermion. No other spin is possible.

The Matter Particles

There are two classes of elementary particles that make up matter: leptons and quarks. All of them are fermions, having spin 1/2.

There are six leptons, divided into three families

electron (e)	muon (μ)	tau (τ)
electron neutrino	muon neutrino	tau neutrino

The particles in the top row all have electric charge -1. All the neutrinos are uncharged.

There are six quarks, again divided into three families

up (u)	charm (c)	top (t)
down (d)	strange (s)	bottom (b)

The particles in the top row all have electric charge 2/3, the ones in the bottom row have charge -1/3.

All leptons and quarks have a non-zero mass, although the mass of neutrinos is very, very small.

The Force Carriers

There are four basic types of forces that we know of. Each of them is transmitted by certain particles. The Standard Model just deals with three of them. These forces, along with their force carriers are listed in the following table. The fourth force, the force of gravity, is not included.

Force	Electromagnetic	Weak	Strong
Carrier Particle	photon	W+, W-, Z	gluons

All the force carrying particles listed in the above table have spin 1, and are thus bosons. The photon and the gluons (there are 8 of them) are exactly massless. The W's and Z particles have mass. It is precisely because of the fact the W's and Z particles have a mass, that we need a Higgs particle in the theory. Higgs particle has spin 0, and allows the W's and Z to have a mass.

So all in all, there are six leptons, six quarks, photon, gluons, W's, Z and their anti-particles. These are the elementary particles. Everything else in this world is non-elementary, made up of a combination of these particles! Experiments done in various labs across the world have found all the particles listed above, but not the Higgs particle. That is yet to be found, and finding the Higgs is one of the main objectives of the LHC.

The electromagnetic force is experienced by all the particles that carry electric charge. All the leptons and quarks experience the weak force. The strong force, however, is felt only by the quarks. Note that the three neutrinos have a zero charge and thus can experience only the weak force.