Читать книгу A Day at CERN - Gautier Depambour - Страница 11

If there is one place in the world devoted to science that beats records, it is CERN! Imagine: an international collaboration with 22 Member States in 2016 has built a ring of 27 km in circumference, at an average depth of 100 m, to cause tiny constituents of matter (such as protons) to collide at a vertiginous speed almost equals to that of light. When you work at CERN and repeat this description almost mechanically, you tend to get used to it and forget that... it’s just amazing! But behind such exploits lies a whole history, whose story deserves to be told.

As you may know, the first half of the 20th century deeply changed our view of physics: first with Einstein, who formulated his theories of special relativity and general relativity in 1905 and 1915 respectively, and then with the rise of quantum mechanics, which developed considerably from the 1920s onwards. However, all these conceptual revolutions did not take place just anywhere: they took place in Europe. Heisenberg, Pauli, Dirac, Schrödinger, de Broglie, Born, Bohr, Einstein: almost all the great physicists who founded quantum physics were European.

But the Second World War, which led to a brain drain to America, put an end to this golden age, and fundamental research in Europe came to a standstill. To remedy this, Louis de Broglie — among other personalities such as the Italian Eduardo Amaldi and Pierre Auger and Raoul Dautry of France — proposed in 1949 the creation of a European laboratory for high-energy physics, with UNESCO’s support, which encouraged the creation of scientific collaborations without a military aspect. The idea was to bring together young researchers from all over Europe, to put them in contact with each other, then to send them back to their home universities with a high level of excellence and thus restore the reputation of basic science in Europe. Eleven countries agreed on the principle, but it was still necessary to decide where the new laboratory would be located.

One group of theorists made a strong argument for Copenhagen. This is where the iconic Niels Bohr Institute of Theoretical Physics was (and still is) located, an essential place in the advent of quantum mechanics, which witnessed an intense intellectual ferment in the 1920s and 1930s. Niels Bohr himself, still alive at the time, was involved in the creation of CERN.

There were other possible choices: France, Italy... But Switzerland, with its central position in Europe and its tradition of peace, was the ideal candidate. The question of location was settled in Amsterdam in 1952, at a conference where it was decided that the Laboratory would be located in Meyrin, in the open countryside near Geneva — a decision endorsed by a referendum in 1953 by which the local population accepted the project. And the following year, 12 countries signed the CERN Convention, officially recording its birth: Belgium, Denmark, France, Germany (in fact, the FRG), Greece, Italy, Norway, the Netherlands, the United Kingdom, Sweden, Switzerland and Yugoslavia. In parallel, the theoretical section of CERN, of which Niels Bohr was a member, was initially established in Copenhagen, but this did not last long, given the distance from Switzerland.

The first objective of the collaboration was to build particle accelerators to unlock the secrets of matter. Several circular accelerators thus emerged one after the other, each larger than the one before. In the 1950s, the Proton Synchrotron was built, which, among other things, allowed the study of very strange particles that we will have the opportunity to discuss again: neutrinos. About twenty years later, the Super Proton Synchrotron was born — from then on, the PS was converted into an SPS injector, so that the protons arriving in the SPS had already acquired a certain energy, as I explained earlier.

The SPS, which sent protons against antiprotons, had its moment of glory in 1983 by allowing the discovery of the Z⁰, W⁺ and W^- bosons, which earned Carlo Rubbia and Simon van der Meer the Nobel Prize in 1984. These are the mediating particles of one of the four fundamental forces of nature: the weak nuclear force, involved in beta radioactivity processes. Today, the mass of the boson Z⁰ is known with great precision, and for this reason it acts as a standard when calibrating the detectors.

Proton collisions are conducive to the discovery of new particles; indeed, because they have a composite structure, they can produce a wide variety of collisions, allowing a wide range of energy to be explored and thus very different particles to be observed. On the other hand, when we want to study a known particle in a narrow and well-defined energy range, electron collisions (which, to our knowledge, do not have an internal structure) are more suitable because they are “cleaner.” Thus, after the discovery of the Z and W bosons, CERN decided to change its strategy by producing collisions no longer of protons, but of electrons, in order to study these new particles in detail. But when electrons circulate in an accelerator, they emit radiation called “synchrotron radiation” and lose energy because of this phenomenon. The same is true for protons, but in much smaller proportions: for the same energy, electrons lose about 10,000,000,000,000,000 times more energy than protons through synchrotron radiation. In short, it had become necessary again to build an accelerator even larger than the SPS.

This is what gave birth to the LEP, the Large Electron–Positron collider of 27 km of circumference powered by the SPS, which produced collisions between electrons and their antimaterial congeners (if I may say so), positrons. LEP made important discoveries, such as studying the decay of boson Z into other particles, in perfect accordance with the theoretical hypotheses formulated since the 1950s.

From the 1980s onwards, physicists decided to return to proton collisions, keeping the tunnel of the LEP but changing the accelerator inside. This led to the construction of the LHC, the Large Hadron Collider, in the place of the LEP. A hadron is a particle subjected to strong nuclear interaction, which ensures the cohesion of atomic nuclei, and this is the case of the components of the proton (quarks and gluons — we will come back to this later).

The LHC achieves such high energies that it recreates at the collision points the physical conditions that prevailed just after the Big Bang (in the case of collisions between lead ions). This is already remarkable, but we still hope to increase energy to highlight new particles, whether predicted or not by current theories.