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.

The Standard Model, part 1: QED

The generally accepted picture of particle physics is known as the Standard Model. This collection of theories is the most successful physical model ever known and is able to make predictions that agree with experiment to unprecedented accuracies. For example, a quantity known as the anomalous magnetic moment of the electron can be calculated using Quantum Electrodynamics, which is one part of the Standard Model, and it matches with experimental measurements up to 14 decimal places. More or less, this means that one can zoom in by a factor of a trillion and the theoretical answer is still correct.

The standard model is an example of a Quantum Field Theory, meaning that it describes several "fields" and lists the ways that these fields can interact. Each field describes a different type of fundamental particle, so really the Standard Model is a list of the fundamental particles and the ways that they interact with each other. One doesn't need to really know anything about field theory in order to understand the particle content of the standard model, so let's dive into that.


Most of us are familiar with the concept of atoms and understand that they are made up of protons, neutrons, and electrons. However, if we zoom in closer, we learn that of these well known particles, only the electron is "fundamental," meaning that is isn't composed of smaller particles (as far as we know, of course). We should know a few things about the electron from high school. It has a property called "charge." The charge of the electron is -1, but that's just an arbitrary defintion of some units. The important point is that electrons are attracted to things with positive charge and repelled from things with negative charge (opposites attractive, likes repell, we see this among people as well as elementary particles). But what is charge really? Is there a more fundamental way to describe it beyond the ad hoc description above? Indeed there is. One a fundamental level, one can think of charge as the ability to interact with light. This seems odd, so let's qualify it a bit. Electrons interact with other charged particles via the exchange of light. Really, they interact with the smallest possible bunches of light known as "photons."

It turns out that all forces come about as the exchange of a particle. For every force, there is one or more particle that is exchanged. When an electron comes near another electron, they exchange a photon which carries the message that they should repell each other, and so they do. So, charge is the ability to communicate with photons, and therefore anything that is charged can talk to anything else that is charged by sending out or receiving a photon messenger.

The light that we see with our eyes is really just photon messengers talking to our rods and cones. There is an explosion in the sun. Two protons fuse together, and this process creates many photon messengers that go forth into the universe in all directions. Some of these messengers, after 8 minutes of traveling, hit the chemicals in our rods and cones and say, "There was a big explosion in the sun. Part of that explosion made red come this way in the form of me, the red photon." That's how we are able to know what happened 93 million miles away without feeling it with our hands. The photons told us!!

Let's go a bit deeper. In addition to electrons, there are electrons' evil cousins: the positron. The positron is just an electron but it has positive charge. It weights exactly the same. Since they have the opposite charge, they attract each other: they exchange a photon and tell each other to come closer. How romantic. The positron is known as anti-matter because it has the opposite charge as the electron, which is matter. Of course, the distinction is arbitrary: if we were made of positrons, we would call the electron the anti-matter. When electrons and positrons touch, they can destroy each other and release a lot of photons in the process. It's a cataclysmic event and they need to send out lots of messengers to tell the world. We can produce positrons in particle colliders, and we readily do, but they are quickly destroyed since the world is full of electrons to cancel them out.

The theory of electrons, positrons, and photons is known as Quantum Electrodynamics, or QED for short. It is a major component of the Standard Model. But really, it's quite simple. QED can be summarized by the fact that an electron and a positron interact via the exchange of a photon, two electrons interact via the exchange of the photon, or two positrons interact via the exchange of a photon. It's even simpler than that sentence if one understands Feynman diagrams (there's only one vertex between an electron, positron, and photon, and all possible interactions are built out of this one vertex like legos).







As you can see in the following diagram, each interaction vertex (where two lines meet) contains an electron and a positron (the two straight lines) and a photon (the wiggly line). Any such diagram that can be drawn that is made up of straight and wiggly lines where each wiggly line meets with two straight lines is a valid "Feynamn Diagram" in QED. If you aren't familiar with these sort of diagrams, don't worry, maybe I'll describe them in more detail in another blog.







Richard Feynman, the man see above playing the bongos, was a central figure in the creation of QED in the 1970's.

The other parts of the standard model are Quantum Chromodynamics (QCD) and Weak physics (which will merge with QED to form ElectroWeak physics). So, stay tuned.