Large Hadron Collider restarts after three-year pause

Situated in Geneva, Switzerland, and operated by the European research facility CERN (its name derived from the French, Conseil Européen pour la Recherche Nucléaire), the Large Hadron Collider (LHC) is the world’s largest and most powerful particle accelerator. The restart today (22 April) follows more than three years of upgrade work.

At 12:16 CEST, two beams of protons circulated in opposite directions around the Large Hadron Collider’s 27km ring at their injection energy of 450 billion electronvolts (450 GeV).

“These beams circulated at injection energy and contained a relatively small number of protons. High-intensity, high-energy collisions are a couple of months away,” said Rhodri Jones, the head of CERN’s Beams department, “but first beams represent the successful restart of the accelerator after all the hard work of the long shutdown.”

Mike Lamont, CERN’s director for accelerators and technology, added: “The machines and facilities underwent major upgrades during the second long shutdown of CERN’s accelerator complex.

“The LHC itself has undergone an extensive consolidation programme and will now operate at an even higher energy and, thanks to major improvements in the injector complex, it will deliver significantly more data to the upgraded LHC experiments.”

Pilot beams circulated in the LHC for a brief period in October 2021. However, the beams that circulated today mark not only the end of the second long shutdown for the LHC but also the beginning of preparations for four years of physics-data taking, which is expected to start this summer.

Until then, LHC experts will work around the clock to progressively recommission the machine and safely ramp up the energy and intensity of the beams before delivering collisions to the experiments at a record energy of 13.6 trillion electronvolts (13.6 TeV).

This third run of the LHC, dubbed 'Run 3', will see the machine’s experiments collecting data from collisions not only at a record energy but also in unparalleled numbers. The 'ATLAS' and 'CMS' experiments can each expect to receive more collisions during this physics run than in the two previous physics runs combined, while 'LHCb', which underwent a complete revamp during the shutdown, can hope to see its collision count increase by a factor of three.

Meanwhile, 'ALICE', a specialised detector for studying heavy-ion collisions, can expect a fifty times increase in the total number of recorded ion collisions, thanks to the recent completion of a major upgrade.

The unprecedented number of collisions will allow international teams of physicists at CERN and across the world to study the Higgs boson in great detail and put the 'Standard Model' of particle physics and its various extensions to the most stringent tests yet.

Run 3 will also include the operation of two new experiments, 'FASER' and 'SND@LHC', which are designed to look for physics beyond the Standard Model; special proton-helium collisions to measure how often the antimatter counterparts of protons are produced in these collisions, and collisions involving oxygen ions that will improve physicists’ knowledge of cosmic-ray physics and the quark–gluon plasma, a state of matter that existed shortly after the Big Bang.

In 2015, E&T visited CERN to speak to the scientists manning the Large Hadron Collider, as they prepared to reboot the system following its first upgrade since it went online in 2008.

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