These four particles and four antiparticles are enough to describe just about everything we encounter in our daily lives. But, they are not enough to build all of the other particles that physicists have observed in the laboratory (e.g., during high-energy particle collisions). Soon after particle accelerators reached the energy equivalent to the mass of a proton, two more leptons were discovered. These were named the muon and the tauon (μ and τ for short).
The muon and tauon were observed to have all of the same properties as electrons, except that they were more massive. This also makes them unstable, as they can always decay by becoming an electron, releasing energy, much as excited atoms can decay down to their ground states.
QUANTUM LEAP
All systems in nature have a tendency to seek their lowest energy state. In particle physics, this manifests itself in the tendency of particles to decay into less massive particles, if allowed by the conservation laws. If all the relevant quantities can be conserved (electric charge, momentum, angular momentum, etc.), then a particle will spontaneously transform into one (or more) other particle(s) whose total mass is less than the mass of the original particle. The mass difference turns into an amount of energy determined by E = mc2.
The muon and tauon were soon found to have their own respective neutrinos (νm and νt), distinguishable from the electron’s neutrino, which always accompanied them during reactions. So it seemed that the whole original lepton family was simply copied twice, for a total of three “generations.” Now we are up to six leptons (plus their six antimatter partners). None of these leptons feel any influence from the strong nuclear force at all.
Similar to the situation with leptons, scientists who studied quarks found that they needed more than just the up and down variety to account for all the observed hadrons. To make a long story short, we ended up with two more generations of quarks, just as in the lepton case. Each successive generation is just a copy of the previous one, the only difference being the higher generation quarks have greater mass. The pair of quarks in the second generation were named “charm” and “strange” (or c and s), and the third pair “top” and “bottom” (or t and b). Remember that these are just arbitrary labels, and there is nothing more “charming” about any single quark than any other.
ATOM TRAP
One thing that makes particle physics so confusing is that there are several different ways to classify the same group of basic particles. Subatomic particles have many intrinsic characteristics, such as mass, electric charge, spin, and more. They can be put into groups based on any of these characteristics or combinations thereof. What this means for us is that the groupings we describe here are not mutually exclusive, and any given particle can be in more than one group.
At this point, you may legitimately ask, is this the end? Over time, physicists have discovered ever more massive particles as their accelerators achieved higher energy capabilities (recall the equivalence between mass and energy). Should we expect this to continue, beyond the three generations now known?
Theoretically, more generations are possible, even if the particles that make them up are rarely or never found in the present universe. Due to the uncertainty principle, such massive particles would have extremely short lifetimes even if they could form anywhere. But in the distant past, the energy density of our universe was much greater in the seconds following the Big Bang.
If a fourth generation were allowed, the even heavier quarks and leptons would have existed for some small fraction of time, and their existence would have affected how the universe evolved. The present universe would be different in some key aspects depending on whether or not a fourth generation of elementary particles can exist. The present universe as it actually exists is not consistent with any more than three generations of quarks and leptons.
So even though we lack the ability to create even heavier fundamental particles to verify whether or not they can exist, we actually have evidence that they are not allowed, and three generations is the limit. This is just one example of how cosmology, the study of the universe on a large scale, intersects with particle physics, which studies the universe on an incredibly tiny scale.
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