What is a particle detector?
▶ Transcript
In the last chapter, we discussed the most important research tools of particle physics, particle accelerators. However, a particle accelerator alone is not of that much use because there is no way to figure out what kind of awesome physics happened in a particle collision – unless you add another super important research tool, a particle detector. Indeed, in this chapter, we will discuss how particle detectors work and how we use them to record the results of particle collisions.
It is quite easy to explain what a particle detector is. It is a device that detects particles. Depending on the type, particle detectors can also measure different properties of particles, such as their electric charge or their energy. This is the first important message of this chapter. A particle detector is a device that detects particles and measures their properties.
So, in principle, we could end this chapter right here. But you might wonder what exactly we mean by "particles" in this case and also what exactly we mean by "detect". So, let us look at this a bit more in detail.
Which particles can a particle detector detect?
Particle detectors detect elementary particles or anti-particles, such as the electron and the positron, or simple composite particle systems, such as the proton or the neutron. We use the term particle detector in both cases. You probably remember that we had a similar situation in the last chapter when discussing particle accelerators. There, it made sense to change the model of how we describe particles. But in this chapter, we will switch back to the way we described them throughout this course because the distinction between elementary particles and particle systems is really important for particle detectors.
Now, we know already that there are quite a few elementary particles and anti-particles, let alone all the different composite particle systems that we call mesons and baryons. But, you do not need to worry. Nature makes particle detection a bit cleaner. Particle detectors cannot detect all types of particles for a very simple reason: the lifetime of many particles and particle systems is too small. Particles can only be detected if they make it from the point they were created to the particle detector within their respective lifetime. For example, the Higgs boson only has an average lifetime of about 10-22 seconds. This is an incredibly short lifetime. Therefore, Higgs bosons transform into other particles before any particle detector can detect them directly. This is the second important message of this chapter. A particle detector can only detect types of particles with a sufficiently long average lifetime.
This makes our lives much easier because it reduces the types of particles we need to worry about dramatically. The only elementary particles that we can detect directly are the electron and the positron, the muon and the anti-muon, and neutrinos and anti-neutrinos. However, neutrinos are a special case since they rarely interact with other particles.
In addition to these elementary particles, there are a few composite particle systems we can detect directly, such as the proton, the neutron, or pions. Ok, but what about interaction particles? Can we detect them with particle detectors? Well, yes, but only one of them. We can detect the photon, the interaction particle of the electromagnetic interaction.
However, even if the lifetime of a certain particle, such as the Higgs boson, is too short, we can still prove its existence. We already mentioned this trick in one of the previous chapters of this course. In this case, we need to measure very precisely all the particles that are created when a Higgs boson transforms. We will come back to how exactly this works at the end of this chapter.
We already showed you one fantastic particle detector, the cloud chamber. It is a beautiful device because it is easy to set up and it visualises tracks of high-energetic particles. Today, cloud chambers are no longer used in research because they have a few disadvantages. For example, to record the observations of a cloud chamber, we would need to take a lot of photographs and go through them one by one with a ruler to analyse the tracks. This is super time consuming, but it is really how particle physics research looked like about 100 years ago.
You can imagine that particle physicists got annoyed by that relatively quickly. After all, we want to investigate the fundamental questions of the Universe. We do not want to search for the ruler all the time. Therefore, they invented new types of detectors that produce electronic signals. These signals can be recorded and analysed by computers. Today, modern particle detectors even combine different detector components that are each specialised to detect a certain set of particle properties. When particles move through these different components, we can measure their properties step by step. In particular, every type of particle leaves a unique pattern of signals in these detector components. And if we combine the information of all detector components, we can identify the pattern and then assign it to a specific type of particle. In this way, we can reconstruct what happened in a particle collision.
The ATLAS detector
We will use one example of a modern particle detector, the ATLAS detector, to explain which type of particle leaves which type of pattern. ATLAS is a so-called multi-purpose detector. That means it is not specialised for only one research question or only one type of particle. Instead, all detector components together can measure many different particles and particle properties. That is why ATLAS is an excellent example to talk about particle detectors in general.
ATLAS is also my favourite particle detector because it is the largest particle detector at CERN. It is 25 metres high, 46 metres long, and it takes about 3000 scientists from around the world to run the detector and to analyse the data it records. The ATLAS detector surrounds one of the interaction points of the LHC, and it comprises several detector components that are arranged in layers around the interaction point. Just like an onion. Well, like a huge electronic version of an onion. Or better, like a gigantic high-tech onion.
We will start at the centre of the ATLAS detector, so, the innermost layer of this high-tech onion and then move our way out from there. The inner part of the detector is responsible for measuring the tracks of electrically charged particles. Therefore, it is simply called the tracker. The tracker has a special feature, a very strong magnetic field. This magnetic field causes a Lorentz force on electrically charged particles. That means that these particles will not move in straight lines through the tracker. Instead, their tracks will be curved, and we can use this curvature to determine the type of the electric charge of a particle.
Let me demonstrate this with my hands, as we did in the previous chapter. Yes, the secret CERN greeting. If a positively electrically charged particle moves in the direction of the thumb of my right hand through a magnetic field in the direction of my index finger, then there will be a force in the direction of my middle finger. If it is a negatively electrically charged particle, the force will be in the opposite direction, and I can use my left hand to show this. So, we basically just need to check whether a particle track is curved to the right or to the left to figure out the electric charge of the particle.
But there is even more we can learn from the curvature. We can determine the momentum of a particle. Just as a reminder, the momentum of a particle depends on its mass and its velocity. Imagine a positron and a proton, both moving with the same velocity. Since the mass of the positron is way smaller than the mass of the proton, the momentum of the positron is smaller than the momentum of the proton. Now, what does momentum have to do with the curvature of tracks in magnetic fields? Well, the tracks of particles with a small momentum are curved much stronger than tracks of particles with a high momentum. Therefore, the track of the positron, which has a small momentum, is curved a lot. It can look almost like a half-circle. In contrast, the track of the proton, which has a high momentum, is curved only slightly. This is the third important message of this chapter. Particle detectors use magnetic fields to determine the electric charge and the momentum of particles.
Nice, so now we already know that the ATLAS detector can measure the movement of electrically charged particles, the type of their electric charge, and their momentum. But this is not enough yet to clearly identify particles, and we did not measure the energy of the particles yet. So, let us move further outwards in our high-tech onion because the next two layers of the ATLAS detector can help us with this problem. They are called the electromagnetic calorimeter and the hadronic calorimeter. Wow, these are fancy names. But actually, it is not difficult to understand what these layers do.
Calorimeters are detector components that measure the energy of particles. You might remember that "calory" is one of the energy units, which you can find, for example, on chocolate bars. So, it makes sense that something that measures energy is called a calorimeter. There is one problem with calorimeters, though. You can only measure the energy of a particle precisely if you stop the particle. And that is the reason why we always measure the track first and only afterwards the energy. When moving through a calorimeter, particles will transfer all their kinetic energy to the detector material along their way. And we can measure this energy that was deposited in the detector material.
Now, the two types of calorimeters can help us to distinguish between elementary particles, such as electrons or positrons, and composite particles systems, such as protons or neutrons. It is quite easy. Electrons and positrons are stopped already in the first calorimeter layer, the electromagnetic calorimeter. That means their unique pattern looks as follows: a curved track in the tracker and a strong signal in the electromagnetic calorimeter. A proton, on the other hand, is stopped only inside the second layer, the hadronic calorimeter. That makes sense, because the proton is, well, a hadron. That means a proton pattern looks differently: a curved track in the tracker, a low signal in the electromagnetic calorimeter, but a strong signal in the hadronic calorimeter. Indeed, all composite particle systems are stopped only inside the hadronic calorimeter. This applies to mesons, which are made of one quark and one anti-quark, and also to baryons, which are made of three quarks or three anti-quarks. You might remember that we use the word "hadron" to summarise mesons and baryons. And that is exactly where the name "hadronic calorimeter" comes from, a calorimeter for hadrons.
The calorimeters also help us to identify particles or particle systems with no electric charge, such as the photon and the neutron. This is super useful since particles without an electric charge do not leave a track in the tracker and are also not deflected in magnetic fields. Instead, the photon produces only a strong signal in the electromagnetic calorimeter, and the neutron produces only a strong signal in the hadronic calorimeter. There are no tracks in the tracker leading up to their signal. We just have to look for these isolated signals in the calorimeters to know if a photon or a neutron were detected.
Now with these components, a tracker with a strong magnetic field and calorimeters, you already have a pretty good particle detector. And these are the components all multi-purpose detectors have in common. However, the ATLAS detector has another special feature in its outermost layer, the so-called muon chambers. And the name says it all. This layer is made of chambers that are responsible for detecting muons and anti-muons. In particular, the muon chambers are designed to precisely measure the momenta of muons and anti-muons. Muons and anti-muons already leave a track in the tracker because of their electric charge. However, they only interact occasionally with the detector material. You might remember the shape of muon tracks in our cloud chamber. There, the muons just moved straight through the detector, leaving only very thin tracks. In the ATLAS detector, the situation is quite similar. They just move through the calorimeters. And they also move through the rest of the detector without interacting much with the detector material on their way. This is how we recognise muons and anti-muons. They leave a track in the tracker, they might leave tiny signals in the calorimeters, and then they leave a track in the muon chambers. To distinguish between muons and anti-muons, we use the same trick as for the electrons and positrons. We study the curvature of their tracks.
Now, what about neutrinos? Their average lifetime is sufficiently long, but neutrinos only very rarely interact with other particles. That makes it super difficult to detect them. And indeed, the ATLAS detector cannot detect neutrinos directly. But no worries, there is a way to prove that neutrinos were created in a collision. Actually, you already know how this works if you remember our discussion of the weird kink in the cloud chamber in one of the previous chapters of this course. If we sum up the momenta of all particles that are created in a particle collision, sometimes this does not add up. But we know that momentum is always conserved. Therefore, we know that at least one neutrino was created in the collision as well, which accounts for the missing momentum.
To summarise, multi-purpose detectors such as the ATLAS detector consist of different detector components. If we combine the information from all components, we can clearly identify all particles and their properties. This is the fourth important message of this chapter. We identify particles based on the unique patterns they leave in different layers of detector components.
Higgs transformation into two photons
That was a lot of information about particle detectors. But now, you know their basic principles, and we can apply our knowledge to investigate, how the ATLAS detector, together with our second multi-purpose detector, the so-called CMS detector, proved the existence of the famous Higgs boson. As you remember, Higgs bosons have a very short lifetime, and they transform into other particles super quickly. For example, a Higgs boson can transform into two photons. This is one way a Higgs boson can transform. There are other possibilities, but we will focus on this one to explain the basic principles.
If protons collide in the centre of a particle detector, many new particles can be created. The different types of particles leave different unique patterns in the detector, which allow us to identify them. Now, we want to study one particular process, the transformation of a Higgs boson in exactly two photons. You probably remember the patterns photons leave in the detector. They do not leave a track in the tracker because they do not have an electric charge. But, they leave a strong signal in the electromagnetic calorimeter. So, these are the signals we are looking for.
But there is one more trick we can use: momentum conservation. Let us assume that the Higgs boson was at rest when it was created. That means its momentum was zero. Therefore, the momenta of the two photons also need to add up to zero. And we already know how this works. The photons need to move into exactly opposite directions. So that is what we are looking for: no tracks in the tracker, but then two strong signals in the electromagnetic calorimeter on opposite sides of the detector.
If we find something like this, we use one more of our amazing conservation laws, the conservation of energy. Once we sum up the energies of the two photons, we get the energy of the Higgs boson that transformed into these two photons. Since our Higgs boson was at rest before the transformation, its kinetic energy was zero as well. That means that the only energy the Higgs boson had before its transformation was the energy associated with its mass. Yes, we even need Einstein's E=mc2 to detect particles. Therefore, we can calculate the mass of the Higgs boson based on the signals of two photons in our particle detector. It is pretty amazing that we can figure out the mass of a particle in this way, although we cannot detect it directly. Today, we know that the mass of the Higgs boson is equivalent to an energy of 125 GeV.
However, there is one more problem Nature put in place for us. We never really know whether there was a Higgs boson, that transformed into these two photons, or not. Because there are also other options how these two photons can be created. Therefore, particle physicists had to analyse huge amounts of data for several years. Only in this way, they could really prove that the Higgs boson exists. So, it was no surprise that thousands of particle physicists around the world started crying of joy when the discovery of the Higgs boson was finally announced on the 4th of July, 2012. Almost 50 years after this particle was predicted, they had finally proven the existence of the famous Higgs boson.
And that is it. Now you know the whole story of particle detectors and how they are used to make awesome discoveries. The only information that is missing is how to build your own particle detector to make your own discoveries. And this is precisely what the next chapter is all about.