In addition to dark matter and dark energy unknown to science, the Standard Model of particle physics also faces difficulties in explaining why fermions add up to three almost identical sets.
For a theory that still lacks quite large components, the Standard Model of Particles and Interactions has been quite successful. It takes into account everything we encounter on a daily basis: protons, neutrons, electrons and photons, as well as exotics such as the Higgs boson and true quarks. However, the theory is incomplete, as it cannot explain phenomena such as dark matter and dark energy.
The success of the Standard Model is due to the fact that it provides a useful guide to the particles of matter known to us. Generations can be called one of these important patterns. It seems that each particle of matter can be of three different versions, which differ only in mass.
Scientists are wondering if this pattern has a more detailed explanation, or if it's easier to believe that some hidden truth will come to replace it.
The Standard Model is a menu containing all known fundamental particles that can no longer be subdivided into their component parts. It is divided into fermions (particles of matter) and bosons (particles that carry interactions).
The particles of matter include six quarks and six leptons. The quarks are as follows: top, bottom, charmed, strange, true, and adorable. They usually do not exist in isolation, but group together to form heavier particles such as protons and neutrons. Leptons include electrons and their cousins, muons and tau, as well as three types of neutrinos (electron neutrino, muonic neutrino, and tau neutrino).
All of the above particles are divided into three "generations" that literally copy each other. The top, charmed and true quarks have the same electric charge, as well as the same weak and strong interactions: they primarily differ in the masses that the Higgs field endows them with. The same goes for the down, strange and pretty quarks, as well as the electron, muon, and tau.
As mentioned above, such differences may mean something, but physicists have not yet figured out what. Most generations vary greatly in weight. For example, a tau lepton is about 3,600 times more massive than an electron, and a true quark is almost 100,000 times heavier than an up quark. This difference manifests itself in stability: the heavier generations break up into lighter ones until they reach the mildest states, which remain stable forever (as far as is known).
Generations play an important role in experimentation. For example, the Higgs boson is an unstable particle that decays into many other particles, including tau leptons. It turns out that due to the fact that tau is the heaviest of particles, the Higgs boson "prefers" to turn into tau more often than into muons and electrons. The best way to study the interactions of the Higgs field with leptons is by observing the decay of the Higgs boson into two tau.
This type of observation is at the very heart of Standard Model physics: bump two or more particles against each other and see what particles appear, then look in the residuals for patterns - and, if you're lucky, you'll see something that doesn't fit your picture.
And while things like dark matter and dark energy clearly don't fit into modern models, there are some problems with the Standard Model itself. For example, according to it, neutrinos should be massless, but experiments have shown that neutrinos still have mass, even if it is incredibly small. And, unlike quarks and electrically charged leptons, the difference in masses between generations of neutrinos is insignificant, which explains their fluctuations from one type to another.
Having no mass, neutrinos are indistinguishable from each other, with mass - they are different. The difference between their generations puzzles both theorists and experimenters. As Richard Ruiz of the University of Pittsburgh noted: "There is a pattern that is staring at us, but we cannot figure out exactly how it should be understood."
Even if there is only one Higgs boson - the one in the Standard Model - there is a lot to learn by observing its interactions and decay. For example, examining how often the Higgs boson is converted to tau compared to other particles, you can check the validity of the Standard Model, as well as get clues about the existence of other generations.
Of course, there are hardly any more generations, since the fourth generation quark should be much heavier than even a true quark. But the anomalies in the Higgs breakup tell a lot.
Again, today none of the scientists understands why there are exactly three generations of particles of matter. Nonetheless, the structure of the Standard Model is itself a clue to what may lie outside of it, including what is known as supersymmetry. If fermions have supersymmetric partners, they must also be three generations long. How their masses are distributed can help in understanding the mass distribution of fermions in the Standard Model, as well as why they fit into these particular patterns.
Regardless of how many generations of particles there are in the Universe, the very fact of their presence remains a mystery. On the one hand, “generations” are nothing more than a convenient organization of matter particles in the Standard Model. However, it is entirely possible that this organization could survive in a deeper theory (for example, a theory where quarks are composed of even smaller hypothetical particles - preons), which could explain why quarks and leptons seem to form these patterns.
After all, even though the Standard Model is not yet a definitive description of nature, it has done its job quite well so far. The more the scientific community gets closer to the edges of the map drawn by this theory, the closer scientists get to a true and accurate description of all particles and their interactions.