ZURICH. The Large Hadron Collider at CERN has returned to its full physics schedule after a winter shutdown that saw engineers replace key accelerator components and retune thousands of superconducting magnets. Beams of protons are once again circulating at close to the speed of light along the 27-kilometre ring beneath the Franco-Swiss border.
The winter work focused on the collimator system, the precision jaws that absorb stray particles before they can quench a magnet. More than 60 collimators were replaced or upgraded, and sections of the cryogenic system, which keeps the magnets at 1.9 degrees above absolute zero, were overhauled for the first time in a decade.
The upgrade matters because the collider now collides protons at intensities that would have been unthinkable at its design stage. Each crossing of the beams produces dozens of simultaneous collisions, and the collimators must protect the machine from the resulting debris without disturbing the beam itself.
For the 10,000-odd researchers who use the facility, the resumption restores the flow of data that feeds hundreds of doctoral theses and dozens of experimental papers each year. Switzerland's universities, including ETH Zurich and the University of Zurich, operate analysis groups that process a significant share of the collisions recorded by the two largest detectors.
The collider remains best known for the 2012 discovery of the Higgs boson, the particle that explains why others have mass. Since then its programme has shifted from discovery to precision: measuring the Higgs so exactly that any deviation from theory would betray new physics. That strategy demands sheer volume of collisions, which is why every winter of maintenance is bought back in beam intensity.
The annual shutdown, which lasts from November to March because electricity is cheaper and demand lower in winter, is also when the experiments themselves are serviced. This year the CMS experiment opened its barrel section to replace photodetectors that had degraded under years of radiation, a task planned down to the hour for six months.
Not everything went to schedule. A vacuum leak discovered in February delayed cool-down by two weeks, and engineers concede that the machine's age is beginning to show: several magnet power supplies are now older than the technicians maintaining them, and spare parts must be manufactured specially. None of this is unusual for a facility entering its third decade, but it concentrates minds on succession planning.
“We ask the machine for more every year, and every winter we pay for it in maintenance,” said the operations coordinator for the accelerator complex. “So far, the bargain still holds.”
The immediate scientific goal is to accumulate the largest possible data set on the Higgs boson before the high-luminosity upgrade transforms the collider at the end of the decade. Rare decays of the Higgs, predicted but not yet observed, should come within reach of the current run's statistics.
The run now under way is scheduled to continue until late 2026, followed by the long shutdown in which the high-luminosity components will be installed. That upgrade, which involves Swiss contributions in superconducting magnet technology and computing, will increase the collision rate tenfold.
CERN remains the reference point for global particle physics, and its schedule effectively sets the calendar for laboratories from Japan to the United States. Proposals for a successor machine, the Future Circular Collider, are now being evaluated by member states, with a decision expected toward 2028.
For now, the older machine continues to justify itself. Each spring's first stable beams are announced in the control room with coffee and a chart of rising luminosity, a ritual that has marked the start of the physics year for a generation of scientists.