 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 in 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 equivalents. 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 equivalents 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 inch or 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. We'll talk about this in the next video, but before we get to mesons, baryons, and what particles really are fundamental building blocks of matter, we have one other quantum number in conservation law to learn about. This quantum number is called strangeness, and the first hint of its existence appears in 1947 with the discovery of cations in cosmic ray experiments by G.D. Rochester and C.C. Butler. So what was strange about cations? Well, cations and other strange particles were known as v-particles, because they produced a characteristic v-shape decay track within a cloud chamber, like the one shown in this cloud chamber picture here. These v-shapes were consistent with two possibilities. Either they were neutral particles decaying into two particles with opposite charge, or they were charged particles decaying into two particles, one charged and one uncharged. Basically these v-shapes looked like they were characteristic of a decay process. The particles producing these v-tracks turned out to be easy to create in cosmic ray collisions, suggesting that the strong force had a role in their production. But what was odd was that the lifetimes of these particles appeared to be much longer than one would expect for a particle created by the strong force. In other words, another force was responsible for triggering the decay, and it appeared that something was inhibiting, or decreasing, the decay probability. This means, essentially, that the lifetimes of these particles were longer than one would expect, based on our knowledge of how they were formed. Up until this point, the force involved in creating a particle would be instrumental in triggering its decay. To see a particle that didn't fit this pattern was, well, strange. As a result, these particles were called strange particles. Now, the fact that these strange particles were always produced in pairs led to it in explanation. A new quantum number and associated conservation law must be playing a role. This is called this quantum number S, strangeness. And in 1953, Murray Gellman at Caltech proposed that the long lifetimes of these particles were because only the weak force could change the particle strangeness, while the strong force and electromagnetic forces could not. So only the weak force could trigger the decay process, in other words. He would go on to win the Nobel Prize for this idea in 1969. Strangeness is adopted as a quantum number, and based on observations, a new conservation law is born. Strangeness is a conserved quantum number for reactions involving the strong force. Protons and neutrons were assigned S equals 0. More exotic hadrons were assigned to range a positive and negative integer values of S, depending on their observed reaction and decay patterns. So to recap, reactions have to conserve charge, linear and angular momentum, energy, leptome number, baryon number, and strangeness, but only for reactions involving the strong force. So as you can see, there are a lot of requirements for each type of reaction, but mess and conservation isn't one of them. As you'll learn in the next video, that's because there's another more fundamental class of particles that we have yet to discover.