What is the charm of a charm quark?
Well, in this question, we come across terms which may not be so familiar. For one thing, charm, and then something called a charm quark. Let’s look into what these terms mean. Let’s start here with this last word, quark. What exactly is a quark? To start talking about quarks, let’s begin by talking about atoms. We know that one way to depict an atom is to show a core made of protons and neutrons surrounded by orbiting electrons. Interestingly, the word atom comes from the Greek word atomos which means indivisible. Because, originally, atoms were thought to be so small that they were the smallest particle possible. But, of course, once we discovered that atoms are actually made of protons, neutrons, and electrons, we realized that that’s not entirely true. It turns out, though, that protons, neutrons, and electrons themselves aren’t the end of the story. There are particles even smaller than these. And that brings us to the topic of quarks.
We can define a quark as a subatomic particle that carries a fractional charge. Here’s what that means. Think of the subatomic particles we know of which do have a charge, protons and electrons. We can say that the charge of an electron is represented as one times that charge, lower case 𝑒, and that the charge on a proton is negative that. It’s negative one times the charge of an electron. These two charges are not fractional, but they’re whole numbers. And any charge we would make from a collection of electrons or protons would also have a whole number charge. So far so good. But it turns out that the whole number charges are not the only kind that can exist. These particles called quarks can have charges that are not some integer multiple of the charge of an electron.
And furthermore, not only do quarks have this property of charge, but they’re also the building blocks, we could say, of the elements of an atomic nucleus. Protons and neutrons, which are the particles that make up an atomic nucleus, are themselves made of quarks. Three quarks make up a proton and three quarks make up a neutron. So, we know a bit, then, about this last word in our sentence, what a quark is. And how it fits in with the subatomic particles we’re more familiar with. But what about a charm quark, what does that mean?
It turns out that the term quark is a fairly general term, that actually there are many subatomic particles which carry a fractional charge. And they have different types to them. Now this is where some of the naming of these types got clever. They’re not called types of quarks. But, rather, they’re called flavors, almost as though you go to your favorite ice-cream store and they ask what flavor you want. In this case, we prescribe the flavor of quark we’re talking about. There are six different flavors of quark. We don’t need to cover all six types, but suffice to say that charm quark is one of those six flavors.
Remember that we said that a quark is a subatomic particle that carries a fractional charge. Well, the charge on a charm quark is positive two-thirds times the charge of an electron. This confirms to us that a charm quark, like any quark, can be made by adding some number of electrons and protons together. If we do that, we’ll just come out with an integer value. But this clearly has a fractional value of charge. We can say, then, that a charm quark is a quark, a subatomic particle carrying a fractional charge, that in particular has a charge of positive two-thirds 𝑒, where 𝑒 is the charge on a single electron.
With that understanding in hand, we then want to figure out, what is the charm of a charm quark? Isn’t this a strange-sounding question? And yet, this word does make sense in this context. Charm, when we use that word this way, is actually a type of quantum number. Now we’re familiar somewhat with quantum numbers, such as spin or magnetic quantum number. Well, charm is another type of quantum number. And believe it or not, there is actually an equation for the charm of a given particle. That equation is often written this way, where it says the charm of a given particle is equal to the number of charm quarks in that particle minus the number of charm antiquarks.
Recall that we said that protons and neutrons are made up of three quarks. So for example, if you wanted to find the charm of a proton or of a neutron, then that would involve counting the number of charm quarks and number of charm antiquarks in that proton or neutron to calculate its charmness, or simply its charm. Sound confusing enough? Well, it’s a lot of unusual terms for sure. In our case, though, our life is made somewhat more simple because we want to know the charm of a charm quark.
In order to figure out the charm of a charm quark, we’ll want to know the number of charm quarks it has and the number of charm antiquarks. This may sound confusing, but let’s think about it for a second. How many charm quarks does a charm quark have? Well, it has one, right? It must have one because it’s a charm quark. And then, how many charm antiquarks does it have? Well, it has zero because a charm quark has no charm antiquarks. That’s its opposite. So, as cryptic as this question was, when we began to understand that terminology, we found the answer is fairly simple. The charm of a charm quark is positive one. That’s the number of charm quarks minus the number of charm antiquarks in a single charm quark.