Meyrin (Switzerland) (AFP)

A facelift to hope to find new particles or even explain dark matter, one of the great puzzles of the Universe: six years after the historic discovery of the Higgs boson, the largest particle accelerator in the world takes a break to boost its power.

Proton collisions fell silent in the bowels of the LHC of CERN (European Organization for Nuclear Research), the world temple of the infinitely small: the gigantic 27 km ring, buried 100 meters underground at the French border. Switzerland, has been on technical shutdown since December 2018.

But in the basement as well as on the surface, it's effervescence. The major improvement works undertaken during this rest period must be completed before the restart of the LHC (Large Hadron Collider) in spring 2021.

"We cross our fingers, everything must be ready at the same time!", Says Christophe Delaere, Belgian researcher at the Fund for Scientific Research (FNRS), proudly showing the new parts of CMS, one of the four detectors installed at different locations of the acceleration tunnel.

Nestled in a cave among a Dantesque network of cables, this immense 14,000-ton particle hunter trains "blank" on muons, cosmic rays accelerated in their natural state.

CERN teams can thus test advances in the resolution of the machine, which functions like a giant camera.

- "It will spit!" -

The detectors must be ready to read a much larger mass of data, the objective being to multiply by ten the number of collisions within the LHC.

This ramp-up will be gradual: a first phase from 2021 to 2025, then, after a new shutdown, a drastic increase in the energy of the infrastructure from 2027. A project called "high brightness", the cost of which amounts to around 1.5 billion euros.

"We're going to make a gigantic jump ... it's going to spit!" Enthuses Frédérick Bordry, director of accelerators and technology at CERN.

Protons (particles of the atom's nucleus) have so far traveled in bundles of 100 billion through the pipes of the LHC, "compacted in a volume smaller than a hair", explains Jean-Philippe Tock, engineer.

But the collision rate, at 13 TeV (teraelectronvolts), was low. With high brightness, CERN targets 180 billion protons per packet, at 14 TeV.

"The more collisions we have, the more we can highlight rare phenomena," explains Frédérick Bordry.

- Supersymmetric particles -

Because after the revolutionary discovery of the Higgs boson, keystone of the standard model of particle physics, the LHC begins a new phase of exploration.

He must first learn to detail the mechanism of this ultimate particle which gives mass to all the others. "It is not because we discovered the boson that we know it well", analyzes Gaëlle Boudoul, researcher at the CNRS.

But the future LHC aims above all to establish a new physics, beyond the standard model.

"This model works well but there are reasons to believe that it is insufficient and that there would be new observable particles", decrypts Laurent Vacavant, of the Institute of particle physics.

The dream for researchers would be to access the Grail of fundamental physics: supersymmetric particles, predicted by theory but never revealed. They could in particular transport dark matter, a great unknown in the Universe - an invisible mass populating our galaxies but with unexplained effects.

Undoubtedly too heavy to have been observed at 13 TeV, the supersymmetric particles will be more likely to be at 14 TeV. "It would be an opening as big as the boson!" Hopes Gaëlle Boudoul.

"But by building the LHC, we knew in which energy range the Higgs boson could be. With the supersymmetric particles, we are more in the blur. If they are at masses on the scale of 100, the LHC will never find them, "she said.

© 2020 AFP