Читать книгу A Day at CERN - Gautier Depambour - Страница 10

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 in the PS, instead of being injected into the SPS, leave for the antimatter factory, which we will visit later; others leave for a lead target, producing high-energy neutrons studied by the n-ToF experiment, with applications in both astrophysics and medicine. The same is true for the SPS: some protons, if not injected into the LHC, are directed to the AWAKE experiment, which employs another plasma-based particle acceleration technique; others leave for the COMPASS experiment, which studies quarks and gluons within the atomic nucleus. So, even if we focus today on the ATLAS experiment, there are many others.

Follow me as I move to the other side of this room in the Globe. I would like to show you a little curiosity — which might even be raised to the status of an object of worship. Look into that sphere.

The first Web server

This is the first Web server, which was developed by Tim Berners-Lee in 1989. Take in what I’m about to tell you: the World Wide Web and the HTTP transfer protocol were invented at CERN! If you still had doubts about the usefulness of CERN, keep repeating to yourself: without CERN, no Facebook, no Google, no YouTube, no Wikipedia, no Amazon, no Twitter... Imagine your life today without the Web. Well, that would be a life without CERN.

Next to this server, you can see a document: it is the article that Tim Berners-Lee submitted to his superior in presenting his project. The reaction of the superior: “Vague... but exciting!” Initially, the Web project was intended to allow data sharing between physicists: we know what it is today.

The article at the origin of the Web (Copyright: CERN)

But the Web is not the only contribution of CERN to IT! It is also at CERN that the idea of the touch screen was invented. So, once again, imagine a world without a touch screen: it’s a world without CERN. Last but not least, it is at CERN that the prototype of what can be considered as the ancestor of the computer mouse was developed. That was in 1972, a certain Bent Stumpe had to design a system for the SPS control room — which I was telling you about a moment ago — to move a cursor on a control screen. He then ordered 12 bowling balls and developed the precursor of the ball mouse. The bowling ball was chosen for reasons of stability and fluidity of movement, but fortunately, all this could be miniaturized.

To anyone who would tell you that CERN has never been used for anything and that it is expensive, I now count on you to tell him or her that without CERN there would be no Web, no touch screen, no computer mouse. And if this person persists in his scepticism, sincerely, it is because he or she is acting in bad faith.

Before leaving the Globe, I have two more little surprises. Come on...

The first particle accelerator

Equations which describe the infinitely small

In this sphere, you can contemplate the very first circular particle accelerator, a few centimetres in diameter. Don’t you find the experience moving? And in this other sphere, here, you can admire the beauty of the equations that rule particle physics today. There’s no rule against getting a little bit ecstatic!

Let’s leave the Globe. Now you’re ready for the discussion we’re going to have with Arnaud Marsollier. Arnaud is part of the communication department, and I made an appointment with him to talk about money issues: how much did the LHC cost? And the detectors? What are the various economic and societal benefits? I think it is useful to understand from the beginning of the visit that the money invested in CERN is not lost money, far from it. But first, on the way to his office, let me tell you a little bit about the history of CERN, so that you can feel the weight of history as you walk through its corridors...