Before we dive deep into the Standard Model of particle physics, we want to first spend some time to discuss what particle physics actually is. Well, obviously, the simple answer is: particle physics is the area of physics that deals with particles. This was easy. But how does particle physics fit together with quantum physics or high-energy physics? In some textbooks, these terms are used as separate categories, while others use them interchangeably. This can get confusing really quickly. So, let us briefly go through it and come up with a working definition of particle physics that we will then use throughout this particle physics course.

We will start with the first important message of this chapter. Particle physics is the research field of physics that deals with particles. Indeed, we consider particle physics as the big overarching category. So, all research that has to do with elementary particles or composite particle systems fits within the field of particle physics. Depending on the energies of the particles, however, we then distinguish between low-energy particle physics and high-energy particle physics. This is the second important message of this chapter. The field of particle physics can be divided into low-energy particle physics and high-energy particle physics.

Low-energy particle physics is usually called quantum physics, and this is probably the side of particle physics that you know best from your high-school education. It is a fascinating topic, but also quite challenging since objects in the quantum world behave differently from our everyday world. Indeed, it is the topic of physics, where quantum effects, such as the uncertainty principle and quantum entanglement, play an essential role.

Speaking of quantum effects – you might already know that in physics, the term "quantum" describes the minimum amount of something. But depending on the context, the term quantum is also used to describe a particle. Here, the photon is the most famous example, which is usually defined as the smallest quantity of light. And while this is true, we will see later on that this particle has way more important roles than just bringing visible light to our life.

High-energy particle physics, on the other hand, is essentially what we do here at CERN, the largest particle physics laboratory in the World. We use powerful particle accelerators to bring particles to super-high energies and then have them collide so that they transform into new particles. Both quantum physics and high-energy physics are different sides of particle physics. But you see, they just focus on different energies of the particles. This is the third important message of this chapter. Quantum physics and high-energy particle physics investigate the same types of particles but at different levels of energy.

So, why do we distinguish between quantum physics and high-energy particle physics after all? I mean, if both fields are investigating the same types of particles, we could simply call everything particle physics, right? Well, it actually makes a lot of sense to keep this distinction between low-energy particle physics and high-energy particle physics. Because, yes, we always use the same types of particles, but performing experiments with low-energetic particles is fundamentally different from performing experiments with high-energetic particles.

First, you need to know that every experimental observation of a particle has an effect on the particle. Specifically, the interaction with the experimental setup can change the properties of a measured particle. For example, the interaction can change the energy of the particle. This effect of the observation might not be large, but if the particle's initial energy is low, even a small effect caused by an observation can have dramatic consequences.

At high energies, on the other hand, the effect of an observation is not that relevant because the particle's energy is already extremely high compared to the effect of the observation. That means that changes in the properties of a measured particle can often be neglected. This makes high-energy particle physics a little bit easier to think through because, when it comes to observations, high-energetic particles behave similarly to everyday objects. But no worries, there are still plenty of challenging concepts and ideas that we will encounter throughout this course. :)

Energy range

Ok, by now we have heard a lot about the distinction between low-energy particle physics and high-energy particle physics. But what do we actually mean by that? What do we consider to be a low-energetic particle, and how energetic are high-energetic particles? For this, we will have a quick look at the energy unit used in particle physics to sort everything out. There are different energy units. You might already know the unit "Joule", which is the official unit of energy in the International System of Units. Another energy unit is "calorie", which you can find, for example, when checking the nutrition information on a bar of Swiss chocolate. In particle physics, a different energy unit is used, which is called "electronvolt". This is the fourth important message of this chapter. In particle physics, "electronvolt" is used as the unit of energy.

From the name of this energy unit you can already derive a lot. First, this unit has something to do with electrons. Second, this unit has something to do with the "volt", which is the unit of voltage, or if you want to be more precise the unit of an "electric potential difference". Now you are probably wondering how voltage and electrons fit together. Let me show you how to get one electron to an energy of one electronvolt. Here, I have a 1 Volt battery with copper wires connected to both ends. Imagine I place this setup in a vacuum, and I place one electron here at rest at the loop of copper wire that is connected to the minus pole. Then the electron would accelerate towards the loop of copper wire that is connected to the plus pole. That means the electron is at rest first, but then it gains speed. And this speed contributes to its energy of movement, which you might know as kinetic energy. Once it reaches the loop of copper wire connected to the plus pole of the battery, the electron would reach a kinetic energy of exactly one electronvolt. To summarise, one electronvolt is equivalent to the amount of kinetic energy gained by a single electron that is accelerated by a voltage of one volt in vacuum. As a symbol for the energy unit electronvolt, we use "eV". So, the energy of this electron at one electronvolt would be 1 eV.

In particle physics, you see almost all energies expressed in this unit. However, usually, eV is accompanied by scary-sounding prefixes like kilo, mega, giga, or sometimes even tera. Do not worry. These prefixes simply help us to express extremely large energies in a compact way. You do not need to remember any of them. To be honest, I have to look up the meaning of some of these prefixes myself from time to time. In this chapter, we just want to introduce you to these units for the first time. In this way, you will recognise them in future chapters.

Now, let us start with one electronvolt. One electronvolt, or 1 eV, is not a lot of energy. That is about the amount of energy photons have in the visible range of light. For example, the photons of a red laser pointer have an energy of only a little bit more than 1 eV. Many quantum physics experiments use photons at this energy range, although these experiments use more advanced laser devices. So this would be an example of low-energy particle physics. Did you ever want to do experiments with low-energetic photons yourself? Then we have good news for you because we prepared detailed instructions on how to 3D-print and assemble your own quantum physics laser lab. You can find the link in the video description below.

If you go a bit higher in energy and use electrons, you can already do really cool physics experiments. Maybe you have seen this setup already in your school – it is called an "electron gun". Do you see the beautiful orange line? This is where electrons with an energy of 200 eV go through the tube. If you show this to physicists at CERN, they will all ask "Do you have a magnet I could borrow?". Indeed, we do like magnets at CERN, let me show you why. I promise, we will come back to this experiment in a future chapter, and we will explain in detail what just happened there. But let me continue going higher up in energy.

Let us go to one thousand electronvolts, which is one kilo-electronvolt, in short, 1 keV. The prefix "kilo" is easy to remember. It is the same prefix as in "kilo-gram" or "kilo-metre". When it comes to energy, X-ray machines, for example, produce photons at the "keV" energy range. Let us go to one million electronvolts, which is one Mega-electronvolt, in short, 1 MeV. Now, this energy starts getting interesting for high-energy particle physics. Indeed, the very first particle accelerator at CERN that came into operation in 1957 was able to accelerate particles up to an energy of 600 MeV. Now, we will literally go up to a "giant amount of energy", one Giga-electronvolt or 1 GeV. The prefix Giga really stems from the Greek word for giant and 1 GeV equals one thousand million electronvolts. That is not bad at all, but at CERN we can go even higher in energy than that. We like to work with "monster amounts of particle energies". Yes, the Greek word for monster is "teras", and this is the origin of the prefix "Tera". One Tera-electronvolt, in short, 1 TeV equals one million million electronvolts.

Now hopefully, you are impressed by tera-electronvolt. But how much energy is that really? It is a huge "monster" amount of energy for one single particle. However, in our daily life, this energy is actually not that much, and we can demonstrate this with a very different type of monster – a mosquito. In fact, a mosquito flying around your head has about the kinetic energy of one tera-electronvolt.

DIY Experiment

3D-printable Laserlab