In the field of particle physics, one of the most intriguing questions today is why there are exactly six types of quarks , distributed in three generations. Let me explain to you why. Everything we see in the universe, such as an apple, car, star, or nebula, are made up of three tiny things. These particles are called up and down quarks and electrons. They belong to what is known as the first group. Atoms have protons and neutrons in their nuclei, which are made up of up and down quarks combined in threes. Electrons orbit around these nuclei.
In addition to these particles we also have neutrinos , which would complete the first generation of leptons (which contains electrons and neutrinos). Well, even though matter is “naturally” made up of only these particles, since the middle of the 20th century we began to discover other particles with properties identical to these four, but with much greater masses . In fact, the properties seemed to be repeated in groups. That is, there were 4 particles that looked like the ones named, with a higher mass and then another four that were even more massive than the previous ones.
All in all, it seems that quarks and leptons exist in three different sets, three packs, known as generations . In case you’re wondering: yes, these generations are called that by analogy with the generations of a family. In fact, we could say that the quark or lepton family has three generations. The first generation forms the stable matter of the universe. Second and third generation particles are unstable and decay rapidly into first generation particles. Despite their instability and rarity in everyday life, these particles can be created in particle accelerators to study them in a more controlled environment, although this requires high energies. The muon is the particle identical to the electron of the second generation and the tauon is the corresponding particle of the third generation. These particles have masses of hundreds and thousands of times, respectively, the mass of the electron. That means creating them in a particle accelerator will be proportionally more unlikely.
However, a fundamental question arises: why do these generations of matter exist and why are there three of them ? This is an unanswered question in current theoretical physics. The existence of these additional generations of particles suggests the possibility that there are even more quarks and leptons yet to be discovered, or even raises the hypothesis that quarks and leptons are not fundamental , but are composed of even more elementary particles and that the generations of particles arise from some property of those hypothetical particles.
For decades, the existence of a fourth generation of particles has been actively sought. However, quite important limitations have been found that suggest that a fourth generation does not exist . For example, if a fourth-generation neutrino existed, it would have to have a mass greater than 45 GeV (giga-electronvolts), which seems unlikely considering the tiny masses of known neutrinos, which are billions of times smaller. Additionally, there are cosmological limits on the number of neutrino generations, suggested by the production of helium during Big Bang nucleosynthesis , which indicate that the number of neutrino generations is around 3 or 4. That is, if many more types existed of neutrino, much less helium would have been created during the Big Bang and we would not observe the amount measured with our telescopes.
Despite this, the search for particles of this possible fourth generation continues in the most cutting-edge particle accelerators, but so far no evidence has been observed that points in that direction. In addition, lower limits have been established for the masses of the quarks and leptons that should form this new generation, based on the LHC experiments . This is because any particle below those limits should have appeared in the years that this accelerator has been operational. If this evidence is found, it would probably show us part of the way towards what is known as physics beyond the Standard Model .
String theory and unified models explain the existence of multiple generations but we don’t know why there are exactly three. Particle physics has a mystery about quarks and leptons. There are three generations of them, and there could be more. The question challenges our knowledge of small universe parts. It also leads to new research areas.
References:
- CMS Collaboration (8 May 2019). ” Boosting searches for fourth-generation quarks .” CERN Courier.
- Griffiths, David J. (2008). Introduction to Elementary Particles (Second, Revised ed.). Wiley-VCH. ISBN 978-3-527-40601-2
- Halzen, Francis; Martin, Alan (1984). Quarks & Leptons: An Introductory Course in Modern Particle Physics . John Wiley & Sons. ISBN 9780471887416.
