 By the 1960s, physicists still thought hadrons were fundamental particles. New ones kept getting discovered, and it seemed like there were a lot of them, so if this was the case, the truth was inelegant. Physicists as a rule are bothered by inelegance because, more often than not, it's a sign that we're missing some key aspects of the physics we're trying to explain. What do we do in this case? Well, we start to look for patterns that might hold the clue of some underlying and more elegant explanation of what we've observed. In 1964, in an attempt to bring some order to the particle zoo, Gellman and George Zweig at CERN independently came up with the idea that the whole mess could be explained if hadrons were not fundamental, but were instead made up of fundamental particles. Gellman called these particles quarks from James Joyce's Finnegan's Wake, three quarks for muster mark. At first, they proposed three types of quarks, an up quark, a down quark, and a strange quark. Each quark has an anti-quark, labeled with a bar over the quark symbol. The anti-quarks are anti-matter and have the opposite charges in their matter counterpart. They also have the opposite baryon number. So how does this work exactly? Well, let's look at two familiar cases, the proton and the neutron. The proton consists of two up quarks and one down quark. So the two up quarks each have a charge of two-thirds. The down quark has a charge of minus one-third. That yields a proton with a charge of plus one. The neutron consists of one up quark with a charge of plus two-thirds and two down quarks each with a charge of minus one-third. That yields the neutrally charged neutron, as we know it. The spins also work out. Both the up and down quarks have a spin of one-half. The quarks that are the same in each of these scenarios have anti-aligned spins. So those spins actually cancel out. So in each case, you're left with a fermion with a spin of one-half. In this picture, all known, and some predicted baryons, could be explained by a combination of three different quarks. This produced integer charge and baryon quantum numbers, and produced spins of either one-half or three-halves, depending on how the spins of the individual quarks are aligned. Anti-particles in this family are made up of three anti-quarks, so both baryons and anti-baryons are nicely explained within the quark model. Mesons, too, could be explained by this model as quark-anti-quark pairs. This makes sense if you think of baryon number. Mesons aren't baryons, and don't obey the baryon conservation law, so the baryon quantum number should be zero. Quarks have a baryon quantum number of one-third, and anti-quarks have a baryon quantum number of minus one-third, so the baryon number cancels out in this configuration. As time went on, and heavier, even more exotic particles were discovered, three other much heavier quarks were added to the picture. The charmed quark, the bottom quark, and the top quark. So quarks, not hadrons, are the fundamental point-like particles that interact via this strong force, and our wide variety of hadrons are simply different combinations of quarks and sometimes anti-quarks. Up to this point, that leaves us with quarks and leptons as our fundamental building blocks of matter. Next time, we'll revisit some of the reactions we've been talking about, to see if we can use all of the knowledge we've just learned to figure out whether certain reactions may or may not be possible.