 In this video, we're going to examine hadrons and see if we can find any rules their reactions follow. First, we start off with some more classifications. Hadrons can either be fermions with half-integer spin or bosons with integer spin. Hadrons that are fermions are called baryons, and hadrons that are bosons are called mesons. We know some baryons already. The proton and the neutron are good examples, and as it turns out, the proton is the lightest baryon out there. Now, following on what we learned about leptons, we might expect to find some kind of baryon number conservation law. How might we test this? Well, we could look for reactions that might violate this conservation law. For instance, we could use free neutrons as our test case. When a neutron is not part of an atomic nucleus, it decays with a half-life of about 10.3 minutes. In our experiment, we would see a reaction like this. Neutron goes to a proton plus an electron plus an anti-electron neutrino. This is basically the nuclear beta-minus decay process. You see a baryon, the neutron, on the left side, and another baryon, the proton, on the right side, plus the electron and its anti-neutrino on the right, which we're familiar with. This reaction is consistent with the baryon conservation law because you have a baryon on the left side and a baryon on the right side, so that doesn't help us. Okay, so let's get a bit more clever. We can keep in mind energy conservation in the mass energy equivalence. Remember E equals mc squared Einstein's famous equation? And think about the decay processes that might really put this possible conservation law to the test. The lightest baryon, the proton, could just be the ticket. Why? Well, we're talking about a decay process. The energy to start with is just the energy equivalence of the rest mass of the proton, which happens to be 938.3 MeV. For the proton to decay while obeying some kind of baryon conservation law, there would need to be some lighter mass baryon that could decay too in order for the energy and baryon number to be conserved. If the baryon conservation law is not valid, then, well, all sorts of things could happen. So what do we observe? Well, all experiments have thus far been consistent with the idea that the proton is stable, meaning it doesn't decay. We can't rule out the possibility entirely, of course. Measuring that something doesn't occur is akin to measuring zero, and that's a very challenging task indeed. But current measurements suggest that the mean lifetime of a proton would have to be longer than 10 to the 29 years if it is indeed unstable. To put this in perspective, the universe is estimated to be just under 14 billion years, or 1.4 times 10 to the 10 years old. So that leaves us for now with a new law. Baryon number must be conserved. Baryons are assigned a quantum number of B equals plus 1, and anti-baryons are assigned a quantum number of B equals minus 1. How about mesons? Remember, these are hadrons that are also bosons, so with an integer spin. Well, things get a bit more complicated at this stage, and the reason has to do with the nature of hadrons. As it turns out, they aren't fundamental particles.