What is the Standard Model of particle physics?
▶ Transcript
In this chapter, we want to take stock of what we have discussed so far. I mean, we have covered quite some ground already by focusing on the central concepts of particle physics. But this also means that we have introduced a lot of terms and ideas, and throughout this course you might have encountered some new particles, charges, or fundamental interactions. Without a broader view from the outside, this can get confusing quite quickly. So, in this chapter we will finally bring all these terms and concepts together by having a proper look at the Standard Model of particle physics. Alright, let us dive right into it.
If you google the Standard Model of particle physics, you will most likely find a lot of tables of particle symbols nicely grouped together. Indeed, the Standard Model of particle physics is usually showcased as a collection of all known elementary particles. But this is only a superficial view of this fantastic scientific model. Of course, particles are absolutely crucial for particle physics. But even more important are the rules that govern the interactions between particles. You can compare it to a football match. Obviously, you need players for a match, but without precise rules of what is allowed and what not, a football match would not really make sense. The same is true for the Standard Model of particle physics. This brilliant scientific model describes the interplay between elementary particles, their charges, and fundamental interactions. This is the first important message of this chapter. The Standard Model of particle physics describes the fundamental interactions between elementary particles based on their associated charges.
Now, let us have a look at all elementary particles and anti-particles that we have introduced during this course already. We talked about the up quark and the down quark, the electron, the muon, the positron, and we mentioned the muon neutrino and even the anti-electron neutrino. Wow, that is quite a collection we have here. But to be honest, we are not even close to a full inventory of all particles that we currently consider to be elementary particles. But no worries, we will get there. And rest assured, there is no need to memorise all of the different categories and names. This is definitely not necessary and probably not useful. Instead, we will continue focusing on the central concepts of the Standard Model of particle physics while building up the complete collection of all elementary particles.
Ok, what do we mean when we describe something as an elementary particle? As mentioned in previous chapters, we consider an elementary particle to be indivisible and it has certain properties, namely a mass and one, two, or even three charges. We will use these properties to sort this bunch of particles into something more manageable. Let us start with the up quark and the down quark. A particle physicist might say that they hit the jackpot. Indeed, every quark has three charges – an electric charge, a weak charge, and a strong charge. From a particle perspective, this is the most you can get, as the Standard Model of particle physics does not include any other charges. This means that because of their electric charge, quarks can interact via the electromagnetic interaction, because of their weak charge, they can also interact via the weak interaction, and because of their strong charge, they can interact via the strong interaction. So, we will place the up quark and the down quark over here.
Next, let us have a look at the electron, the muon, and the anti-electron, the positron. When we introduced them in their respective chapters of this course, we briefly mentioned their charges. You might remember that each one of them has two charges – an electric charge and a weak charge. This means that they can interact via the electromagnetic interaction and via the weak interaction, but they cannot interact via the strong interaction. Therefore, we will place the three of them underneath the quarks. Now, we are only missing the muon neutrino and the anti-electron neutrino. Well, by now you probably already spotted why I kept them until the end. Exactly, each one of them has only one charge, namely a weak charge. This means that neutrinos and anti-neutrinos can only interact via the weak interaction. They cannot interact via the electromagnetic interaction or via the strong interaction. Let us place them all the way down there.
Good, now we have sorted this bunch of particles according to their associated charges. From quarks with three charges, to the electron, the muon and the positron with two charges each, to the muon neutrino and the anti-electron neutrino with only one charge each. Are you still with me? Good, because we can go one step further. Remember, we describe elementary particles not only with their associated charges but also with their mass. Yes, the masses of particles are incredibly small, and yes, we do not even know what a particle is :) But nonetheless, we can measure the tiny masses of particles and we can use them as a second sorting mechanism. This is the second important message of this chapter. Within the Standard Model of particle physics, elementary particles can be grouped together according to their charges and their mass.
For example, when we introduced the muon earlier, we already mentioned that it is simply a more massive version of the electron. Both have the same charges, namely an electric charge of -1 and a weak charge of -1/2, but the muon has a significantly larger mass than the electron. So, let us move the muon to the right.
Alright, what can we do with the anti-electron, the positron? Here, the situation is opposite. It has the same mass as the electron, but all its charges are opposite. So, it has an electric charge of +1 and a weak charge of +1/2. Ok, for now, we will just move it in between the electron and the muon. Good, so now we only have to sort the neutrinos accordingly. Well, looking at their names this should not be too difficult. We can move the muon neutrino to the right and add it underneath the muon, and we can take the anti-electron neutrino and place it underneath the anti-electron.
And by now you probably have an idea where this is going. Because as I mentioned earlier, these seven elementary particles are not all of the elementary particles that we currently describe with the Standard Model of particle physics. There are still quite a few missing that we have not mentioned up until now in this course. So, let us do this now really quickly. As I said, it is not necessary to memorise all of them, but we definitely want to give you the full overview and explain the logic behind it.
To some extent, you will probably be able to fill in the table yourself. Yes, even without any prior knowledge of particle physics. The reason for that is that the Standard Model of particle physics relies on symmetries. And they show. Let us have a look at the first column of our table. We see the up quark and the down quark up here and the electron underneath. To complete this column, and to make it symmetrical with the columns to the right, we would need to add a neutrino, namely the electron neutrino. And guess what, this is exactly what we can do because this neutrino exists in the Standard Model of particle physics. It has the same mass as the anti-electron neutrino but their weak charges are opposite. So, we can place it right here in its spot. Good, and now we have completed the first column of our table. This was almost too easy to be true, right? Well, of course this is not how particle physicists "discover" particles. We do have to observe them experimentally to prove their existence. But sorting already discovered elementary particles according to their charges and their mass helped experimental physicists to look for missing elementary particles that would complete columns and rows in this table.
For now, I suggest that we do not go through the whole history of how and when each individual elementary particle was found. Instead, we can go through the table and expand it as we see fit. So, let us look at the second column. Well, this is an easy one. When we discussed anti-matter in one of the previous chapters, we already mentioned that for every elementary particle there exists an anti-particle with the same mass but with opposite charges. And we already have the anti-particles of the electron and of the electron neutrino in the table. So, all we need to do is add the anti-up quark and the anti-down quark to complete this column. Boom, done. We are making progress.
Let us move on to the next column. This one indicates a step further in terms of mass. So, the elementary particles sorted in this category have higher masses than the elementary particles in the first two columns. We already added the muon and the muon neutrino in this column. This means we are missing two quarks on the top. These two we have not yet mentioned, so I will just add them. They are called the charm quark and the strange quark. In the same way as the muon is a more massive version of the electron, these two quarks are more massive versions of the up quark and the down quark. And they have the most amazing names in particle physics. I mean, can you imagine? The charm quark? The strange quark? This is yet another example that particle physicists have fantastic humour! Because at first, only the strange quark was discovered, and the particle physicists found that, well, strange. But a couple of years later, the charm quark was discovered, which completed the column, and the particle physicists found that, well, charming :)
Alright, moving on. By now you already know the drill. For every column of elementary particles, we also have a column with their anti-particles. So, let us add another column and add the anti-charm quark, the anti-strange quark, the anti-muon, and the anti-muon neutrino. Wow, look at that. Four columns already completed. We are almost done. There are only two more columns missing.
Let us quickly complete the table and finish this chapter. Up here, we have the so-called top quark and bottom quark, as well as their anti-particles, the anti-top quark and the anti-bottom quark. Compared to the other quarks and anti-quarks, these are really massive particles – whatever a particle is. Below, we have the so-called tauon and its anti-particle, the anti-tauon. These are even more massive versions of the muon and the anti-muon, and of course as of the electron and the anti-electron. But aside from this, all their respective charges are identical. And last but not least, to complete the overview of all known elementary particles, we can add the tauon neutrino and the anti-tauon neutrino.
Nice, we made it! This is the full inventory of all elementary particles that are described by the Standard Model of particle physics. We have six quarks and six anti-quarks, which all have three charges each. We have the electron, the muon, the tauon, and their respective anti-particles. Each one of them has two charges – an electric charge and a weak charge. And then we have three neutrinos and three anti-neutrinos, which all have only one charge each – a weak charge. Do you have to memorise this overview? No, of course not! Is it helpful to understand the underlying structure of the Standard Model of particle physics? Probably! Does it show that particle physics sometimes relies on creativity and inventing funny names? Definitely!
But one important aspect to consider is the fact that we can group all elementary particles and anti-particles in three generations. Yes, in particle physics, we call them generations or sometimes even families. This is just a way to illustrate the underlying symmetries of the Standard Model of particle physics. This is the third important message of this chapter. The Standard Model of particle physics sorts elementary particles and anti-particles into three generations.
For our every-day life, however, only the elementary particles of the first generation are relevant. As you can see, in the first generation we have the up quark and the down quark, as well as the electron. Remember what we discussed in one of the earlier chapters of this course: the up quark and the down quark are always bound together by gluons to form composite particle systems, such as the proton or the neutron. And together with the electron, these composite particle systems can form atoms and molecules. So, essentially, all matter of the Universe can be traced back to only three of the four particles of the first generation – the up quark, the down quark, and the electron. These are really the fundamental building blocks of our Universe.
But what about the elementary particles that are categorised in the second and third generations? Why are they not part of our daily lives? Well, the simple answer is, they are too massive. You see, the second-generation particles have greater masses than the corresponding particles of the first generation. And the third-generation particles have even greater masses than all corresponding particles of the first and second generation. You already know from one of the previous chapters that the average lifetime of an elementary particle is linked to its mass. So, as a rule of thumb we can say that the higher the mass of an elementary particle, the shorter is its average lifetime. This makes it extremely challenging for the elementary particles of the second and third generation to exist, let alone to combine into composite particle systems or even larger structures. Do not get me wrong. All of these elementary particles exist, and we can experimentally observe them. Just not in our daily lives. We usually observe them in high-energy environments, for example as part of cosmic radiation or right here at CERN in our experiments. This is the fourth important message of this chapter. Due to their high masses, elementary particles categorised in the second and third generation of the Standard Model of particle physics, can only exist in high-energy environments.
Alright, this now concludes the overview of all elementary particles. But how do the interaction particles fit into the Standard Model of particle physics? And what is the role of the famous Higgs boson in all of this? Well, these are excellent questions! We will discuss all of them in the next chapter. And we will also show you how to figure out, which particle fits your personality best. Yes, I am not kidding. So, see you soon in the next chapter.