The Standard Model, Part 2: QCD

The other particle that many of us are familiar with from school is the proton. Protons have positive charge. Since electrons, as you will of course recall, have negative charge, they are attracted to protons. They want to bind together, and this is how atoms are formed. The electromagnetic force holds protons to electrons (the electromagnetic force is described by QED). But protons are fundamentally different than electrons. It turns out that, unlike electrons, they are composite particles. They are made up of smaller particles. These particles are called “quarks” after a line from James Joyce’s Finnegan’s Wake (“Three Quarks for Muster Mark”). Like protons, neutrons are also made of quarks. The term Hadron refers to particles that are made of quarks (there are more than just protons and neutrons, but those are by far the best known). So let’s talk about quarks.

There are 6 types of quarks that we have created and measured in a lab: the up quark, the down quark, the charm quark, the strange quark, the top quark, and the bottom quark. A proton is two up quarks and a down quark (sometimes written uud), and a neutron is an up dark and two down quarks (udd). The up quark has charge +2/3 and the down quark has charge -1/3, so we recover the well known properties that a proton has charge +1 and a neutron has charge 0.



Since quarks make up protons and neutrons (among many other particles), we know that there must be something that makes them want to stick together to form these composite particles. After all, we don't see free quarks flying around, we only see groups of quarks (quarks come in groups of two or three for group theoretical reasons). The things that bind quarks together, that make them stick to each other, are appropriately known as gluons. Gluons are analogous to photons. In QED, particles have positive and negative charge feel a force provided by photons. Therefore, one would guess that there was an analogous “charge” that quarks have which is related to gluons (just so we don’t get confused, quarks DO have ELECTRIC charge, and so they do interact with photons. But since they interact with gluons, they must have another type of charge).

It turns out that quarks and gluons are much more complicated than electrons and photons. In QED, there is only one “charge” (electric charge, of course). But with quarks and gluons, there are three types of charge. For lack of any better ideas, these three qualities that a quark can have were labeled “colors.” Most people call the three charges “red,” “green,” and “blue.” So, there are 6 types of quarks, but each quark can also be red, green, or blue. So, I can have a red up quark, or a green down quark, or a blue strange quark, or a red top quark, etc etc. And since it involves colors, the theory of quarks and gluons is called “Quantum Chromodynamics,” or QCD for short.




If you ask a physicist, they will tell you that QED is a very nice and clean theory. QCD is very, very messy. The main difference between QCD and QED is the difference between photons and gluons. Photons cause interactions between charged particles (like electrons). But they are not charged themselves. This means that photons don’t “talk” to one another. Photons only interact with charged particles, and since photons themselves aren’t charged, they don’t directly interact with photons. But gluons are different. Gluons interact with colored particles. But gluons themselves, it turns out, have color. Therefore, gluons are able to interact with other gluons. Two gluons can come together to form another gluons. One gluon can split apart into two gluons, or even three gluons. So, if you have one gluon, you have many. And if you have a quark, you have gluons. And if you have gluons, you have quarks. So, QCD is a mess. A quark flying along will emit gluons, which will make more gluons, which can make quarks, which make more gluons, etc. In the end, you end up with a bunch of particles that are all flying along. This big blob of particles that comes about from QCD is collectively called a “jet.”

So, to recap, QCD holds quarks together to form protons and neutrons. They can also become more exotic particles, like pions, kaons, and others that are named after Greek letters. QCD holds protons and neutrons together to form the nucleus of an atom. So, initially, QCD was called the “Strong Nuclear Force.” It is called strong because it indeed is strong. It is able to hold two protons together even though they should be repelled by the electric force (like charges repel each other). The study of nuclear physics is the study of QCD (usually just called the strong force in the context of nuclear physics). The reason that nuclear bombs are so powerful is that the strong force is so powerful. Nuclear explosions unleash the power of the strong force, of QCD. A gluon turns one quark into another, and releases a lot of energy in the process. QCD can indeed by quite messy.