The world’s largest particle accelerator will restart next week when scientists resume exploring the mysteries of the universe after a three-year shutdown to work to improve the machine’s performance and precision.
The restart of the Large Hadron Collider at the Cern laboratory near Geneva coincides with the 10th anniversary of its researchers’ celebrated discovery of the Higgs boson, a long-sought fundamental particle that gives mass to other subatomic components of the Universe.
Scientists hope that increasing the energy and frequency at which protons collide in the LHC experiments after being accelerated to almost the speed of light in a 27 km long underground ring will provide evidence of ‘new physics’ – fundamental forces and particles that go beyond the borders. so-called standard model, which the Higgs boson put the finishing touches to.
Thousands of physicists work at the LHC at Cern’s headquarters near the Swiss-French border and far from universities around the world. Among other things, they hope to find out why matter and not antimatter dominates the universe, and to uncover the nature of “dark matter” – invisible to all scientific instruments developed so far – which is known to be more abundant than conventional matter.
Some physicists have expressed concern that the excitement of the Higgs discovery and its recognition with a Nobel Prize the following year may have misled the public into believing that the discovery of new particles is the pinnacle of particle physics – although it is well deserved – and has led to disappointment that nothing quite as spectacular has emerged since 2012.
“Of course it would be fantastic to see unequivocal evidence of new physics, and we always hope when we analyze data that this is the moment we observe something. . . like a new elementary particle,” said Tara Shears, Cern researcher and physics professor at the University of Liverpool in the UK.
“But it could be that the new physics shows up indirectly, causing a pattern of differences in particle behavior that our theory can’t explain and that would take longer to collect and understand the evidence for,” she said. “It all depends on what the nature of the new physics is – and since we don’t know that, we’ll have to try every way we can think of to find it.”
Gavin Salam, a physics professor at Oxford University, pointed out that over the past 10 years, scientists have learned a lot about the Higgs boson from LHC data. “Our exploration of Higgs and its interactions has far exceeded our initial expectations,” he said.
LHC experiments have shown that the boson is responsible for the mass of an increasing number of other particles, expanding the scope of the Standard Model.
The supercharged LHC would push this process even further, Salam said. A key question he hopes to answer is whether the Higgs boson is really an indivisible elementary particle or is composed of other particles.
Several clues to new physics from previous experiments will be explored in the next LHC run. One result was an unexpected discrepancy between the behavior of the electron and its heavy cousin, the muon, which appears to contradict the Standard Model, although the data wasn’t enough for the researchers to be sure.
Another was an observation by the now-defunct Tevatron particle accelerator at Fermilab in the US, which revealed that another subatomic particle, the W boson, had an unexpectedly large mass that was inconsistent with the Standard Model. LHC experiments will add enough statistical power to refute or confirm this inconsistency.
“It’s important to add that the lack of evidence for new physics doesn’t mean you haven’t learned anything from the search—quite the opposite,” Shears said.