Hadrons

The stuff we tend to call matter is made primarily of protons, neutrons, and electrons. While we can describe electrons' behavior just based on the weak and electromagnetic interactions, we need to bring in another force to describe the behavior of protons and neutrons. It's called the strong nuclear force, and it is peculiar to particles called hadrons. There are two general classes of hadrons: mesons and baryons (meaning middle-weight and heavy, respectively, I believe), which differ in their quark content. Baryons, which include protons and neutrons, consist of three quarks. Mesons consist of a quark-antiquark pair.

The two hadrons we deal with most regularly are made of just up and down quarks. Unlike leptons and conglomerate hadrons, quarks can have fractional electric charge, which is what allows three quarks to combine for charges of 0, 1, or 2 times the electron's charge. Up quarks have +2/3 charge, while down quarks have -1/3 charge. Thus the combination $uud$ yields the positive charge of a proton, while $udd$ gives the neutral neutron. But as we've seen, there's more to life than just up and down quarks. After a while dealing with those, physicists started observing other particles that behaved strangely. They were given an attribute called strangeness, and it turns out that it's a property of a third kind of quark called the strange quark. A few years and many experiments later, physicists have concluded that like leptons, there are three generations of quarks. Each generation has a +2/3 and a -1/3 charge quark, and the weak force can couple these triples, so an up quark can be converted to a down quark and a $W^+$ boson if there's enough energy involved. It can also, with much lower probability, be converted into a strange or a bottom quark. The probabilities of such transformations are given by the Cabbibo-Kobayashi-Maskawa (CKM) matrix, which is sort of like the neutrino oscillation matrix in the lepton sector. The difference is that neutrinos mix with very high probability, while quark generations mix only rarely.

Quarks also have a funky property called color, which couples to the strong force in the same way that gravity acts on mass and electromagnetism acts on charge. The three colors are called red, green, and blue, but they have nothing to do with the colors that we see; it's just a (somewhat unfortunate) naming convention. It turns out that it's impossible to see an isolated colored object; instead, we only see colorless conglomerations. As a result, we can never detect an isolated quark. The smallest possible colorless combinations of quarks are those of three quarks (or anti-quarks), one of each color, or of a quark and an anti-quark with opposite colors. Those correspond to baryons and mesons, respectively.

The strong force is also unique in that its force carrier, the gluon, has color, and therefore feels the strong force. My understanding is that this leads to runaway strength and generally complicates the lives of theorists.