Down Quark
Classification: quark, fermion
Fundamental: yes
Family: first
Mass: 4.1 - 5.8 MeV
Interactions: Electromagnetic (charge -1/3), Strong, Weak, Gravity
Spin: 1/2
Lifetime: dependent on meson/baryon containing down quark
The up quark's buddy in just about everything is the down quark. He's just a little bit heavier, and he has a charge of -1/3. Like the electron and electron neutrino are paired together in all weak interactions, the up and down quarks show up together when the weak force is involved. These four particles (electron, electron neutrino, up quark, down quark) make up the first family of particles. Together, the up and down quarks can take credit for making up most of the matter of our universe. To revisit some particles we've already met:
a proton = two up quarks + a down quark
a neutron = an up quark + two down quarks
pi+ = an up quark + an anti-down quark
pi- = an anti-up quark + a down quark
pi_0 = a linear combination of up + anti-up and down + anti-down
Now you begin to see why quarks made keeping track of the particle zoo much simpler. You dream up every possible combination of two or three quarks, and there is an observable particle corresponding to that combination. Since you know the properties of the quarks, you can make really good estimations of what the properties of the observable particle should be.
But why only two or three? Why do we need to have those numbers of quarks? It has to do with how the strong force behaves. Like the electromagnetic force has a charge associated with it, the strong force has charges, too. However, the electromagnetic force has one type of charge and its opposite, while the strong force has three types of charge and their opposites. For lack of better names, these charges got labeled red, blue, and green, or the color charges. Their opposites are called anti-red, anti-blue, or anti-green.
Stuff that's charged wants to interact with other charged stuff; neutral stuff tends to be more stable. So atoms are more or less stable, being electrically neutral, while the constituent electrons and protons would zip around on their own. The charges pull on things until they are canceled out. The same applies to the color charges, but they have two methods of canceling themselves out. You can either pair up a color with its anti-color, or you can have one of each color. So the mesons with two quarks are a color plus its anti-color, while the baryons like the proton are a mix of one of each type.
This explains while the stable hadrons only have two or three quarks, and starts to explain why we never see quarks on their own. The strong force, or what quarks feel when they aren't paired up, is exactly that--strong. Very strong. Strong enough that the leftover bit of attraction that quarks in one proton feel to the quarks in the next proton or neutron over because they happen to be a bit closer is enough to overcome the electromagnetic repulsion and hold all the protons in the nucleus together. So when one quark gets knocked around inside a hadron, the pull it feels is tremendous. It's so strong and packs so much energy that it will start pulling quark-anti-quark pairs of particles into existence.
Since those new pairs must be color + anti-color (there was no charge to the energy before, and charge must be conserved), they then neutralize the color force and the original hadron breaks. The quark that got kicked takes one (or two) or the newly made quarks with it in the form of a hadron, and the rest of the new guys stay behind filling in the gap it left and producing another hadron. This process is called hadronization and the stream of particles you get is typically called a jet in my circles. Still, the point is that throughout the whole process you never managed to get one quark by itself out of a hadron.
Like I said, the quarks are clique-ish like that.
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