Inside CERN's Large Hadron Collider

The following sections is included:

• Contents

Always men struggled to make sense of the surrounding world. Are the stars in the sky the results of old fights between gods and heroes? Are the mountains, the rocks, the rivers, coming from primordial creatures? Is mankind at the centre of the world? Could the complexity of the world surrounding us be expressed by complex interactions of simple elements?…

Men of different cultures and traditions developed models of the surrounding world that helped them to make sense of what was happening, and to predict the consequences of their actions. Often these models were inaccurate, connected to unproven myths. Most of them are lost, with perhaps some echoes surviving through the oral tradition…

The name CERN is often associated with advanced technology and state-of-the-art science, but the history of this lab is quite long, and CERN is one of the first inter-European organisations created after the Second World War. Let us go back to those times: Europe was rebuilding itself from the debris of this terrible war where neighbouring countries have been invading and bombarding each other for years. Resources for fundamental science were scarce, and Europe, especially the countries where dictatorships were in power before the war, was plagued by a strong brain drain: many scientists, some very famous ones but also many others that constitute the fundamental scientific “humus” for the advancement of research, had left the continent because of political or racial persecutions. But fortunately a few enlightened scientists had remained (among them Bohr, Amaldi and Auger), and they understood something very important: if the various European countries wanted to avoid becoming irrelevant in the new post-war world, they had to forget the rivalries and start to build a common project. So, something incredible happened: countries that had been bombarding each other until a few years earlier began to collaborate in a very ambitious peaceful scientific endeavour…

The largest scientific instrument in the world is located about 100 metres underground, between Geneva airport, in Switzerland, and the Jura mountain range, in France. The tunnel hosting it was built in the eighties for the LEP accelerator; LEP was then completely removed from the tunnel in the year 2000, and then the new machine was built. The idea of having a proton collider inside the LEP tunnel existed since a very long time, even before the start of LEP itself, and for some years it was thought possible to have the two machines co-existing in the same tunnel, to also perform electron–proton collision; however the excessive weight of the LHC magnets made it impossible to have them placed above LEP, as initially thought, and the cryogenic system ended up requiring much more space than initially foreseen, forcing the removal of LEP…

While the LHC machine can accelerate protons at energies over 7000 times the value of their mass, and collide them creating about 40 million “small Big Bangs” at four collision points, it tells us nothing about what are the results of these collisions. For that, huge particle detectors are placed around the collision points: very large and sophisticated instruments measuring the results of these collisions with an extreme accuracy…

Let us now look in some detail at the main physics topics that are studied at the LHC…

The Higgs boson has been for decades the Holy Grail of particle physics, the missing piece of the puzzle of the electroweak theory. This very successful model, able to explain an enormous quantity of experimental observations, is based on symmetry principles, basically stating that all fermions have the same intrinsic mass, zero. But we do know that most of the particles have nonzero mass, and actually their mass difference explains why our universe is full of electrons, but there are very few muons and taus, particles very similar to the electron but much heavier. The electroweak theory has a very elegant explanation for the mass difference between all these particles: the underlying symmetry is “broken” by an interaction with an external field, called the Higgs field…

Who are the scientists analysing the LHC data, studying the Higgs bosons and running the detector? Forget about the stereotype of bearded professors in white lab coats. The usual question that visitors ask when they see people taking shifts in the control room of ATLAS or CMS is: but are they scientists? They look so young, casually dressed, perhaps just hopped off their bike… Yes, they are scientists, even though perhaps a good fraction of them are PhD students, graduates who are preparing a doctoral thesis…

The basic research made in particle physics has the aim of better understanding nature, not directly of producing technologies of practical use. The Higgs discovery for instance cannot be used to produce new forms of energy, or clean up the environment: it is “just” a deeper understanding of the concept of mass. However if we look back, often in the history of science big scientific discoveries have been followed by technological revolutions that changed the life of billions. For instance the electromagnetic unification, having understood that electricity and magnetism are indeed the same thing, is at the basis of the use of electromagnetic waves for long-distance communications, which is used for radio, TV, etc. Similarly quantum mechanics, the theory describing the behaviour of tiny particles like electrons and protons, is at the basis of all modern electronics (which is used in computers, cell phones, digital cameras, just to give a few examples), lasers, solar panels, quantum microscopes, etc…

The following sections are included:

• Conclusions
• Index