The Generation Game
One of the (many) strange and elegant things about the standard model is its generational structure. The matter that makes up everything around us, as any school student will tell you, is formed of protons (p), neutrons (n) and electrons (e). Electrons, as far as we can tell, are fundamental particles; they are not themselves made up of yet smaller particles. The protons and neutrons on the other hand are composite particles each containing quarks, which again we think are fundamental. They contain two
flavours types of quark: the up quark (u) and the down quark (d). Protons are made of two ups and one down (p=uud) and neutrons of two downs and one up (n=udd). Interestingly, unlike the electron, you can never see a quark on it’s own due to the nature of the force which attracts one quark to another. They only come in threes like the proton and neutron (you can also get pairs of matter-antimatter quarks together, I’ll explain antimatter in a future post). The only stable combination of these quarks is the proton, all other combinations decay pretty quickly.
So far all well and good, we have three fundamental particles which explain all the visible matter in the universe, except for some unknown reason it doesn’t stop there. Measurements of cosmic rays (the stream of particles which cascade to earth as extra-terrestrial particles collide with particles in the upper atmosphere) and of controlled collisions in laboratories show that there are six more fundamental particles and this is where things get weird. Two of those particles are exactly like the electron only heavier; another two are exactly like the down quark only heavier; the final two are exactly like the up quark only heavier! In order of increasing mass, the electron type particles are the muon (μ) and the tau (τ); the down type particles are the strange quark (s) and the bottom quark (b); the up type particles are the charm quark (c) and the top quark (t). All of these heavier particles decay pretty quickly to combinations of lighter ones as I’ll explain later, but they do exist if only briefly. It makes sense to group the particles in generations as the table shows. You can see that there are three other particles, the neutrinos (ν), which are related to the electron type particles by the weak force, again to be explained later I promise!
But why are there three generations and why is the only difference between them their respective masses? Well that I don’t know, I wish I did!