 We learned about two types of particles in the last video, leptons and hadrons. Remember, the strong and weak forces act on hadrons, only the weak force acts on leptons. In this video, we'll learn more about these classes of particles, and begin to learn about some of their key properties. We'll start with leptons. These are fundamental point-like particles, which basically means we can't break them up into smaller constituents. There are six leptons in total, which happen to fall into two categories, charged and uncharged. The charged leptons are the electron, the muon, and the tauon. All of these have a charge of minus one. The uncharged leptons are all neutrinos. The electron neutrino, the muon neutrino, and the tauon neutrino. Up until recently, we thought these particles were massless. As it turns out, they're not quite massless, but this is the story for another time. At this stage, you might be wondering why the names of these uncharged leptons are so uncreative. Well, there's a reason for this name association. If you recall a nuclear beta decay, you might have some idea why. In a nuclear beta decay process, you have something like this, where carbon-14 transforms into nitrogen-14 by basically changing a neutron into a proton. That reaction also comes with the formation of an electron and an anti-electron neutrino. Or the other alternative, fluorine-18 becomes oxygen-18, so it changes a proton into a neutron and also produces a positron and an electron neutrino in the process. The electron anti-neutrino and the electron are both reaction products in the first beta decay process, while the electron neutrino and the positron are reaction products in the second beta decay process. Basically, whenever we produce an electron or positron, we'll also produce either an electron anti-neutrino or an electron neutrino in the process. The same kind of pattern is observed for the other charged leptons. Their production only occurs with the production of an associated anti-neutrino. And as a corollary, the production of charged anti-leptons is associated with the production of an associated neutrino. We know the neutrinos and anti-neutrinos are different because of how they react with matter. Each type of neutrino or anti-neutrino leads to distinct reaction outcomes, which we'll discuss in a bit. Because of this, we say each lepton and its associated neutrino is a family. Unlike electrons, the muon first discovered in 1936 by Carl Anderson, who also happened to have discovered the positron, if you'll recall, and his colleague Seth Nettermeyer, is unstable. So the muon decays in this way, and its antiparticle decays in the opposite way. So the muon decays by producing a muon neutrino, an electron, and an anti-electron neutrino. It has a pretty short lifetime of about 2.2 microseconds. The anti-muon decays by emitting the anti-muon neutrino, a positron, and an electron neutrino. The tauon first discovered in the mid-1970s by Martin Perle and his colleagues at Stanford Linear Accelerator Center is the heaviest of the leptons, about 3,500 times the mass of an electron, and is also unstable, with a mean lifetime of 2.96 times 10 to the minus 13 seconds. It actually has a lot of different decaying modes, which we're not going to summarize here, but they all follow the same kind of pattern we've just seen for the case of the electron and the muon. So the pattern to these decay or reaction processes can be summarized like this. For every lepton that's produced, an associated anti-neutrino is produced. For every anti-lepton that's produced, an associated neutrino is produced. For every lepton that decays, an associated neutrino is produced. And for every anti-lepton that decays, an associated anti-neutrino is produced. So when I say associated neutrino or anti-neutrino, I mean the neutrino or anti-neutrino that belongs to the same family as that charged lepton or anti-lepton. Okay, so what does this pattern potentially mean? Well, let's take muon decay, for example, and make up a quantum number associated with each lepton family. For each family, the lepton in that family gets a plus one and the anti-lepton gets a minus one. So if you do that for muon decay, what you'll see is on the left side, you have a plus one for the muon family. On the right side, you also have a plus one for the muon family. You also have a plus one for the electron and a minus one for the anti-electron neutrino. So in this reaction, the lepton number for each lepton family is conserved. As it turns out, all the reactions that involve leptons follow this rule. So this leads to our first new conservation law. In any reaction, the lepton number for each lepton family must be conserved. This conservation law is in addition to the conservation laws we've already learned, either in this module and previous ones. So that's conservation of charge, conservation of linear and angular momentum, and conservation of energy. In physics, a conservation law is something we found to be true over and over and over again in many different experiments. If we were to find a reaction that seemed to violate one of these conservation laws, we would, of course, reexamine it.