So far, physicists have discovered 25 elementary particles, which no longer make up of smaller particles. Most of these particles are unstable, decaying into lighter particles within a fraction of a second. But how can a decaying particle be fundamental? Does this mean it's made up of something else?
Are there more fundamental elementary particles?
The Standard Model of particle physics contains 25 particles, and the matter around us is made up of almost half of them. First electrons, then protons and neutrons in the nucleus: they are made up of different combinations of upper and lower quarks, with 8 kinds of gluons gluing them tightly together.
There are other particles, such as the τ, which is very similar to an electron, except that it weighs about 400 times more than an electron. It is also very unstable, with a lifespan of only 3×10^-13 seconds, and then it decays into an electron, an τ neutrino, and an electron antineutrinos. So, is the τ made up of these three particles? When it decays, the three particles fly out again?
Based on all the observations we currently have, the τ is not made up of any particles, and physicists know this for several reasons. First of all, if the τ is made up of other particles, then you need to find a way to combine them, which will require a new fundamental force, but we don't have any evidence of this force. Second, even if you come up with a new fundamental force, it doesn't help, because the τ decays in many different ways, and it can also decay into μ, τ neutrinos, and μ antineutrinos, as well as τ neutrinos and π mesons or ρ mesons, as well as other possible decay channels. Therefore, if the τ is composed of particles that decay into it, then there should be different types of τ, but this is the opposite of the observation.
Decay is really just an interaction, and in principle all these decays can occur in different orders. Let's take the τ as an example, the τ can decay into one electron and two neutrinos, but if a high-energy electron collides with an τ neutrino, this may produce an τ and an electron neutrino. In this case, we can be more certain that τ is not made up of something more basic.
All of the processes we've talked about above are examples of τ, but the same applies to unstable particles in the Standard Model.
Why do elementary particles decay
We often hear explanations that elementary particles decay in order to reach their lowest energy states. But this doesn't make any sense, because energy is conserved during decay. In fact, the cause of the decay of these elementary particles has nothing to do with energy, but with entropy.
A heavy particle can decay into several lighter particles because it has enough energy, and the rest of the energy that does not enter the mass of the new particle becomes their kinetic energy. But for the opposite process to occur, these particles must meet at the right place at a high enough energy. It is possible, but it must be provided with energy from the outside, otherwise it will be an ordered spontaneous increase, and therefore it will be a process of entropy reduction, which is why we rarely see it happening. All in all, decay is possible, but the inverse process of decay is unlikely.
summary
Decay does not mean that the decay product necessarily exists in the original particle, decay is just a specific type of interaction. And we don't have any observations that elementary particles are made up of something more fundamental, that is, they have no substructure. The reason why elementary particles decay is not related to energy, but to entropy.