A Day at CERN. Gautier Depambour
that you should know in globish, plot. Here, when you see what looks like a histogram, a graph, a diagram or something similar, you don’t look for the precisely appropriate word, you just say: plot. This can save you from tricky situations, such as when you start an emphatic sentence with: “Oh! Very nice, your... your uh...” And then, since you don’t know the exact word, you let the worst escape: “...your drawing.” So if you have any doubts about what is represented, do as everyone else does; don’t take any risks and say “plot.”
With plot, you already reach the semi-pro grade, but if you want to appear immediately as a true professional, I have a trick to suggest: season all your sentences with basically. “Basically” is a word that, in English as in globish, is meaningless but very effective in making you shine. If you are asked a question, especially if it concerns particle physics, assume a very inspired look, and start by saying in your most beautiful globish “Basically, I think that....” You will give considerable weight in advance to your words, I assure you.
We’re arriving at the entrance of the Globe — after you.
The exhibition “Universe of Particles”
As you will see, the effect is quite striking when you discover the exhibition room.
Impressive, isn’t it? I am always amazed by the beauty of this room. I particularly appreciate its cosmic aspect: in the middle of these big luminous spheres, one feels projected into interstellar space.
The exhibition “Universe of Particles”
This room thus sends to everyone an essential subliminal message, which Bernard has already mentioned earlier and which I find magnificent: it is by observing the smallest things that we can understand the biggest. What a beautiful idea! Perhaps we will find answers to astrophysical mysteries, such as dark matter or dark energy, by studying the smallest components of matter. At CERN, therefore, we do not limit ourselves to understanding the infinitely small: we try to understand the universe on every scale.
The spheres you see in this room — at least those close to the ground — each contains a curiosity relating to CERN’s history, to accelerators or particle detectors, or to a point of theoretical physics. We’re going to discover some of them, but just before that, I have to take out my secret weapon: My beautiful drawing of CERN’s accelerator chain! With this visual aid, I will be able to describe the path of the protons: after being accelerated, some of them will frontally meet other protons coming from the opposite direction. The energy released during such a collision will allow, by virtue of the equivalence between mass and energy formalized by Einstein, the production of new particles that can be studied using particle detectors.
But let’s not go too fast, and let’s start at the beginning: where do these protons come from? Don’t expect a staggering answer, because the protons simply come from a hydrogen bottle. This is an opportunity to show you the first sphere — come closer.
A hydrogen bottle
Hydrogen is the simplest atom that can be found: its nucleus consists of only one proton of positive electrical charge +1, around which gravitates an electron of negative electrical charge −1. More precisely, if we do not want to stick to this classical image, which dates back more than a century, it is better to imagine the electron as an electronic cloud which represents its probability of presence around the nucleus. However, there is no need to discuss the electrons, since they are separated from their respective protons as soon as they leave the hydrogen bottle, thanks to an electric field.
The protons thus obtained are first injected into a straight-line accelerator: LINAC 2, which stands for LINear ACcelerator 2. Its cousin, LINAC 3, which you can also spot on the diagram, accelerates not protons but Pb29+ lead ions, i.e., lead atoms that have been stripped off some of their electrons. However, most of the time, accelerated particles are protons — let’s focus on them for now. They will go through a succession of increasingly powerful accelerators, which will make them go faster and faster, giving them more and more energy.
Why do we need several accelerators in a chain, and why don’t we send the protons directly into the largest accelerator? For a simple reason: because you don’t immediately switch from a local road to the highway. It is as if you were asked to arrive on the highway at 40 km/h, then accelerate to 130 km/h: this is not possible; cars that are already there are going too fast. The protons therefore pass through successive insertion ramps. In the life of an accelerator, there is no retirement: when you are supplanted by a bigger one, you become the insertion ramp of the newcomer.
First, in LINAC 2 — note this! — protons are grouped in packets. Then these packets of protons are sent to the Booster, which accelerates them and sends them to the PS — the Proton Synchrotron with its 628 m of circumference. The PS (which is located on the surface) accelerates them and then sends them 40 m underground to the SPS, the Super Proton Synchrotron, with its 7 km of circumference. The SPS accelerates them and then sends them to the LHC, the Large Hadron Collider, with its 27 km of circumference. But here there is a subtlety: the SPS sends half of the proton packets clockwise into the LHC, and the other half counterclockwise for the purpose of producing collisions!
The tunnel of the LHC
As I said at the outset, the LHC is now the largest and most powerful accelerator in the world. There are even more ambitious projects under consideration, which we will discuss at the end of the day in offering a perspective on the future, but which remain at an embryonic stage for the time being. At the moment, collisions produced in the LHC reach an energy of 13 tera-electronvolts. If this number doesn’t tell you much, just consider that the energy contained in a particle at full speed in the LHC is equivalent to that of a mosquito in flight — except that at CERN, this energy is concentrated in a tiny proton, not distributed among the billions of billions of protons that make up the mosquito!
Collisions occur in the four detectors I mentioned earlier: ATLAS (A Toroidal LHC ApparatuS), CMS (Compact Muon Solenoid), ALICE (A Large Ion Collider Experiment) and LHCb (Large Hadron Collider beauty). CMS research, like that of ATLAS, focuses largely on the study of the Higgs boson. The ALICE experiment is intended for the study of quark–gluon plasmas, a kind of primordial magma that, by cooling, gives rise to other particles of matter. To do this, collisions within ALICE involve heavy ions, i.e., large chemical elements; there are periods when the LHC doesn’t provide collisions between protons, but between much heavier elements, such as lead. The LHCb experiment, for its part, focuses on the quark b, known as “beauty” or “bottom” — a fleeting particle (because its lifetime before decay is very short) that helps physicists to understand the differences between matter and antimatter. All these detectors have different technologies adapted to their research programmes. At the risk of repeating myself later, I would like to stress that ATLAS and CMS, although having very similar programmes, do not use the same technologies for particle detection, or, therefore, the same methods. This is very important, because observing the same phenomenon with two different detectors ensures the reliability of the results and limits errors.
The detectors record the data from the collisions, which are then processed, in comparison with the simulated data, until an experimental result is announced, such as the discovery of a new particle. But one thing at a time: so far, the important thing has been to give you an overview of the path of a proton from the hydrogen bottle to the various detectors of the LHC.
One remark must be made here: this path is in fact only used by less than 0.08% of the protons! It should not be forgotten that many other experiments take place between the Booster and the LHC. For example, some accelerated protons