What is a particle accelerator?
▶ Transcript
Now it is time to discuss the most important research tools in particle physics – particle accelerators. We will look at different versions of accelerators, investigate their essential components, and examine how they are used in particle physics laboratories, for example, here at CERN.
So, why do we need particle accelerators? Well, this is simple. We use particle accelerators to accelerate particles. Ok, but why? What can we do with accelerated particles? This is also an easy one. We can make accelerated particles collide with each other. That is it. I told you that this would be easy. And now the last one of these easy warm-up questions: why do we want to have accelerated particles collide with each other? Well, this is where the magic happens! Or, let me rephrase, this is where awesome physics happens. You see, if we accelerate particles, they gain a lot of energy that can be transformed into other forms of energy. This is exactly what happens when high-energetic particles collide. Their energy transforms, and during the collision, new particles are created. Yes, it is Einstein all over again. E=mc2 – it just does not get better than this :)
Ok, before we dive deep into this chapter, we should probably clarify one definition beforehand to avoid misunderstandings. When we say that particle accelerators accelerate particles, this can mean elementary particles, such as the electron, but also composite particle systems, such as the proton. We use the term particle accelerator in both cases because the term "composite particle system accelerator" is just really unhandy. So, even though we know that the proton is a particle system made of quarks and gluons, during this chapter, we will use a more general particle model and describe elementary particles and particle systems as particles. But of course, the moment it is essential to distinguish between different kinds of particles, we will immediately switch back and use the more accurate terms again.
Now, let us start. Assume I have a bunch of protons right here. How can I accelerate them from here to there? Well, all I need is an electric field. So, let us switch on an electric field in such a way that the negative side of the field is on this side, and the positive side of the field is on that side. Since we already know that protons have a positive electric charge, it is fairly easy to predict what will happen. Exactly, the protons experience a force, which attracts them towards the negative side of the field and, at the same time, they experience a force, which repels them from the positive side of the field, just like this. And this is how we accelerate particles. Well, electrically charged particles. It does not work with electrically neutral particles because they are not influenced by electric fields. This is the first important message of this chapter. Particle accelerators accelerate electrically charged particles with the help of electric fields.
Linear accelerator
But now you might be asking, what if we want to continue accelerating this bunch of protons to increase the energies of the protons further? That is an excellent question! Well, then we have to change the polarity of the electric field just before the bunch reaches the negative side of the field. In doing so, the protons first experience a force attracting them towards the negative side of the field, but once its polarity changes to positive they experience a force repelling them and this pushes the bunch further. And now we can add another electric field in such a way that the protons again first experience a force attracting them towards the negative side of the field and then a force that pushes them away from the positive side of the field. But just before the bunch reaches the negative side of the field, the polarity of the second electric field changes as well, so that the former negative side of the field becomes positive and accelerates the protons further. And so on, and so on. If we continue doing this, we will get a long, straight device with alternating electric fields that accelerate particles along the way. The more electric fields we combine in such a device, the higher the energies of the protons get. And this is precisely the working principle of a linear accelerator. It is a long, straight device that uses alternating electric fields to accelerate particles from one end to the other end. So, quite straight forward :) This is the second important message of this chapter. In a linear accelerator, alternating electric fields are used to accelerate particles along a straight path.
However, linear accelerators have two major limitations. First, as I just mentioned, the maximum energy that particles can get depends on the length of the linear accelerator. The longer it is, the more electric fields can be used to increase the energy of the particles. At some point this gets unpractical. The currently longest linear accelerator is located in California in the USA. It is more than three kilometres long. Wow, that is a long device.
And the second limitation is that once the particles reach the end of a linear accelerator, you have to do something with them. You cannot store them somewhere for later. Experimental physicists do not really like this. They usually do not like to be rushed and prefer to enjoy a nice cup of coffee to think everything through before starting their experiments.
That is why, already a long time ago, a lot of smart physicists came up with a different approach. Their idea was to accelerate particles with one electric field, but then, instead of letting the particles travel along a straight path, bend them on a curved track. So, all the way around in a circle, until they return to the electric field, where they would get another push. Then they travel around the circle again and get another push, and another, and another. Every time they come around, their energy increases a little bit until they reach their final energy. So, similar to a small kid on a swing. You push it a little bit forward, then again, and again, and again, until at some point, it has enough energy and goes all the way around. But please do not try this at home with your friends! This is really just an easy example of how we can imagine this process.
Circular accelerator
Ok, so, how can we steer electrically charged particles around the corner, so that they travel along a circular track? Well, for this we need the most important concept in particle physics! It is this one, yes, the right-hand rule. Depending on which part of the world you are from, you might know this as Lorentz Force, or as magnetic force, or as one of Fleming's Rules. But the name is not important. As usual, we care more about the concept itself.
Let us assume we have a proton right here. We have accelerated it, so it is moving along a straight line in the direction of my thumb. Now, if we switch on a magnetic field that is perpendicular to its direction of movement, so in the direction of my index finger, there will automatically be a force on the particle, which is perpendicular both to its direction of movement and the direction of the magnetic field. So, the force will act in the direction of my middle finger. This is truly a fantastic gift of Nature! Because now we know how we can bend accelerated particles on a circular track. We just need a circle made out of magnets, with their fields perpendicular to the direction of movement of the particles. The particles would want to travel along a straight path, but due to the interaction with the magnetic field, they are being pushed towards the centre of the circle and travel around the corner. And this is precisely the working principle of a circular accelerator. It uses an alternating electric field to increase the energy of the particles and magnetic fields to steer the particles around the corner. So, it is a circular machine made out of electromagnets, with one section for the electric field to increase the energy of the particles with every turn. This is the third important message of this chapter. In a circular accelerator, electric fields increase the energy of particles, and magnetic fields are used to steer them along a circular path.
This here, the Lorentz Force, is so important. Without this concept, none of our circular accelerators would work. And you will see in the next chapter, that it also plays an essential role when it comes to particle detectors. That is why – and I am only going to tell you this now – that is why this is also the secret CERN greeting! Yes, I am not kidding. The next time you meet someone from CERN, you can use the secret CERN sign to say hello :) Let us have a quick look at how the concept of Lorentz Force looks like in an experiment.
Alright, now we know how the Lorentz force works in theory but let us try this out with real particles. In this glass tube, we can see the tracks electrons leave in a Neon gas when moving straight from one side to the other side. Once the electrons interact with this screen at the end of the tube, the screen lights up in a green colour. For the following experiment, we will focus on the position of this green spot on the screen, which is caused by the electrons. Let us position our thumb right in the direction of the electrons. You might notice that I take the thumb of my left hand. Indeed, you should always use your right hand when thinking about particles with a positive electric charge, such as protons. However, the Lorentz force has exactly the opposite direction for particles with a negative electric charge, such as electrons. Therefore we have to use the left hand for electrons.
Ok, I have my thumb in position. If I now add a magnetic field in the direction of my index finger, downwards, the electrons should experience a force in the direction of my middle finger. I will use an ordinary bar magnet to provide a magnetic field in this downward direction. Et voilà! The electrons do exactly what we predicted. We can see that the electron track appears to be bent because the electrons are deflected in the direction of my middle finger. So, Nature really works this way!
Wow, that was nice! Ok, so what are the advantages of circular accelerators over linear accelerators? Well, in a circular accelerator, we can keep accelerated particles going around in circles for hours, before we use them for our experiments. So, from this perspective, they are ideal for experimental particle physicists, who want to finish their coffees first.
However, there is one limitation that circular accelerators face, and this is their circumference. You see, in a circular accelerator, particles gain energy with every trip around the circle. This means the force caused by the magnetic fields that pushes the particles towards the centre of the machine needs to increase with the kinetic energy of the particles. Otherwise, we could not steer them around a circular path, and they would smash into the shielding of the accelerator. But the strength of magnetic fields is limited. At some point, the Lorentz Force reaches its maximum. This then defines the maximum energy particles can get in this machine. If we want to increase their energy further, we have to either invent stronger magnetic fields or increase the circumference of the circular accelerator. Or both. This is the fourth important message of this chapter. The maximum energy particles can reach in a circular accelerator is limited by the strength of the magnetic fields that steer the particles around the circle, and by the circumference of the circular accelerator.
Let us show you how this looks like in an experiment as well. Here, we see another device, this time with a sphere of glass. Outside, you can see two big coils of copper wire. These two coils create a magnetic field between them. If we switch everything on, we see a circular track. This is the track electrons leave when they move through the Helium gas inside of the sphere. We see a circular track because the electrons are influenced by the constant magnetic field created by the two coils.
Now, let us increase the energy of the electrons slightly and see what happens. Hm, the circle becomes a bit bigger, its circumference increases. If we now increase the strength of the magnetic field as well, the circumference gets smaller again, back to its initial size. If we increase the energy of the electrons even more, the circumference increases again. But, again, we can increase the strength of the magnetic field to compensate. Actually, if I increase the energy of the electrons at the same time as the strength of the magnetic field, you will not see any change in the circumference. This is the basic principle of most circular particle accelerators: We synchronise the increasing energy of the particles with the increasing strength of the magnetic fields to make sure that the particles always follow the same circular path. Therefore, we call these types of circular accelerator synchrotrons.
CERN accelerator complex
Now, let us have a look at CERN's accelerator complex. This is what we are probably most famous for. Our beautiful accelerators. Most likely, you know the biggest one, the LHC. But this is not the only accelerator we have here at CERN. You can see there are quite a few of them. They are all connected, and their purpose is to accelerate particles step by step to increasingly higher energies. It all starts with a linear accelerator down here. This one is brand-new. We call it LINAC4. It is almost 90 metres long. It accelerates bunches of negative hydrogen ions, which are particle systems made of a proton and two electrons, up to an energy of 160 MeV.
These bunches are then injected into the first circular accelerator of the complex, the so-called Booster. During injection, the two electrons of each hydrogen ion are stripped off, and only the protons are accelerated in the Booster up to a maximum energy of 2 GeV. Then, the bunches are transferred to the next circular accelerator, the Proton Synchrotron. Now you can see what we illustrated earlier with the limitation of the circumference. In the Proton Synchrotron, the protons can reach a maximum energy of 25 GeV. This is significantly higher than what the Booster can achieve. Since both accelerators use similar magnetic fields, this is only possible because the circumference of the Proton Synchrotron is also significantly larger. The Booster is only 157 metres long, but the Proton Synchrotron has a circumference of 628 metres.
Ok, let us continue. What is the next accelerator in this chain? Well, this one is easy. We can see a transfer line from the Proton Synchrotron to the Super Proton Synchrotron. Wow, we are really creative when it comes to naming accelerators. But the name is absolutely justified. This accelerator is really super. It is the first one in this chain, that is built entirely underground – about 40 metres below the surface. This makes sense since it has a circumference of 7 kilometres. Trust me, you do not want to build a circular building of this size on the ground. In this accelerator, the protons reach a maximum energy of 450 GeV.
And now we made it. Because the next accelerator in this chain is the last one. It is the LHC, the Large Hadron Collider, that is currently the largest and most powerful accelerator in the world. Truly a magnificent piece of science! It has a circumference of about 27 kilometres. That is why we also built it underground, about 100 metres below the surface. It crosses the border between Switzerland and France several times. This machine is really big. If we want to take our car from here to the opposite side of the LHC, it takes us about 20 minutes. It is a beautiful drive through Swiss and French villages and vineyards, but if we have a meeting on the other side of the LHC, it does take quite some time to get there. We usually set up a videoconference instead to have more time for coffee :)
LHC
So far, we only discussed how we accelerate particles, but not necessarily why. Yes, I mentioned collisions in the beginning, but let us now have a closer look at the LHC to discuss why particle accelerators are such essential research tools in particle physics. Ok, let us start with the name: Large Hadron Collider. The first word is easy. It is large. I mean, 27 kilometres, come on! This is large. The second word of its name we have not yet used during this course. It describes yet another category of particles. Specifically, it is the category that comprises baryons and mesons. So, hadrons are all composite particle systems that are either made of three quarks or three anti-quarks, the baryons, or of one quark and one anti-quark, the mesons. That is it. It is just another nametag to know. So, every baryon is also a hadron, and every meson as well. And this makes sense because, in the LHC, we usually accelerate protons. And the proton is a baryon, so it is also a hadron. Yes, this checks out.
Ok, now for the third word – collider. This is what it is all about. So far, all accelerators leading up to the LHC had only one job: to increase the kinetic energy of the protons. But now, in the LHC, we want to make use of these high-energetic particles. How do we do that? Well, as the name says, the LHC is a collider. This means, instead of only one beam of particles, we have two beams of protons, that are being accelerated in two separate beam tubes. The bunches of protons travel in opposite directions. One beam travels this way, and the other one goes the other way around. Did I already mention how fast they are going? I do not think so. They are incredibly fast! In the LHC the particles can reach a maximum energy of 7 TeV. At this high energy, the protons are travelling almost at the speed of light. Yes, their speed is about 99.9999991% the speed of light. Incredibly fast. They make about 11 000 turns in the LHC every second. One beam in one direction, and the other beam in the other direction.
And once the protons reach their final energy, we can manipulate both beams in such a way that their paths cross. We do that at four so-called interaction points. For this, we squeeze the bunches of protons tightly together. There are about 100 billion protons that make up one bunch. And none of them wants to be close to the others because they all repel each other due to their identical electric charge. That is why we use thousands of complex magnetic fields to focus the beams and squeeze the diameter of the bunches to a minimum of only a few micrometres. And then we let the bunches cross. So, 100 billion protons from this side, and 100 billion protons from that side. All nicely packed together in a super-thin bunch. What do you think happens in such a bunch crossing? How many of these particles will collide? All of them? Half of them? A few per cent? None of them? Well, the correct answer is almost none of them. About 50 protons from the one beam will collide with 50 protons from the other beam. All the other billions of protons continue as if nothing happened. But no worries, they will get their chance very soon. After an 11 thousandth of a second, they will have completed another turn around the LHC, and they can give it another try. If they are not part of the next 50 or so protons that collide, they will have another chance after the next turn. And again, and again, and again. Yes, this process keeps on happening for several hours. Every day, the LHC sees billions of bunch crossings and billions of collisions.
Ok, but why? Why do we put so much effort into making high-energetic particles collide? Well, by now you probably already know how much we rely on Albert Einstein in physics. Because, in every collision, the energies of the particles are transformed into new particles. So, we can use collisions to create new particles. The higher the energies of the particles that collide, the higher the chances that we create new particles. This is the fifth important message of this chapter. When high-energetic particles collide, their energies can transform, and new particles are created.
Ok, but why do we need so many collisions? I mean, billions of collisions every day? That sounds a little bit excessive. Well, the reason is that we do not know which particles are created in any given collision. This is an entirely random process. So, if we are looking for new particles, for example, as we did when we looked for the Higgs boson, we need to investigate a lot of collisions to build up enough statistics. If you are curious to learn how we detect and analyse particles that are created in collisions, then I have good news for you! Yes, this will be the topic of the next chapter!