After a three-year hiatus for maintenance, consolidation, and upgrading work, the world’s largest and most powerful particle accelerator has reopened. At 12:16 CEST today, two beams of protons circulated in opposing directions around the 27-kilometer ring of the Large Hadron Collider, with 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,” says the Head of CERN’s Beams department, Rhodri Jones. “But first beams represent the successful restart of the accelerator after all the hard work of the long shutdown.”
“The machines and facilities underwent major upgrades during the second long shutdown of CERN’s accelerator complex,” says CERN’s Director for Accelerators and Technology, Mike Lamont. “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.”
At October 2021, pilot beams circulated in the LHC for a short time. However, today’s beams signal not just the end of the LHC’s second long downtime, but also the start of preparations for four years of physics data collection, which is expected to begin this summer.
Until then, LHC experts will labor around the clock to gradually recommission the machine and carefully ramp up the beam energy and intensity before delivering collisions to the experiments at a record energy of 13.6 trillion electronvolts (13.6 TeV).
This third run of the LHC, called 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.
Because of the enormous number of collisions, worldwide teams of physicists at CERN and around the world will be able to examine the Higgs boson in great detail and put the Standard Model of particle physics and its different extensions to the most rigorous testing yet.
The operation of two new experiments, FASER and SND@LHC, is 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 to improve physicists’ knowledge of cosmic-ray physics and the quark-gluon plasma, a state of matter that existed shortly after the Big Bang, are among the other things to look forward to in Run 3.