The Large Hadron Collider (LHC) will restart on Friday after a three-year hiatus and is expected to resolve a scientific cliffhanger over whether a mysterious anomaly could indicate the existence of a fifth fundamental force of nature.
The tantalizing discoveries reported last year have rekindled hopes that the 20-mile-long collider could deliver a second blockbuster discovery, more than a decade after the Higgs boson.
Dr Mitesh Patel, a particle physicist at Imperial College London whose team was responsible for last year’s research, said: “We are entering this race with more optimism that a revolution could happen. Crossed fingers.”
So far, everything discovered at the LHC – including the Higgs – has aligned with the so-called standard model. It’s been the guiding theory of particle physics since the 1970s, but it’s notorious for being incomplete because it fails to explain some of physics’ deepest mysteries, such as the nature of dark matter.
However, data collected in the LHCb experiment, one of CERN’s four huge particle detectors in Switzerland, appeared to show that particles behaved in ways that could not be explained by the Standard Model.
The experiment looked at the decay of particles called beauty quarks, which are expected to decay at an equal rate into electrons and their heavier cousins, muons. However, beauty quarks seemed to turn into muons 15% less often, suggesting that an unknown factor – potentially a new force – was tipping the scales. Two of the best candidates include hypothetical force-carrying particles called leptoquarks or Z primes.
“The stakes are extremely high,” Patel said. “If we confirm that, it will be a revolution of the kind we haven’t seen – certainly in my lifetime. You don’t want to mess it up.
Before the LHC was shut down for an upgrade in 2018, the team collected enough data to suggest the odds were around a thousand to one of the outcome happening by chance. But the gold standard for particle physics is a tighter confidence level of 3.5m, which means more data is needed before a discovery can be declared. There is also a lingering possibility that an unknown experimental problem could explain the results.
“When you show this result to particle physicists, the first instinct is, ‘You screwed up’ rather than it’s a new force of nature,” Patel said. “We physicists like to be beyond certainty and on the other side.”
Over the past year, anticipation has been heightened by other intriguing clues to physics beyond the Standard Model seen in other experiments, including recent unexplained discoveries from Fermilab in the United States.
“There now seems to be a series of loose threads,” said Professor Jon Butterworth of University College London, who works on the LHC’s Atlas experiment. “It made me start to think there might be something within reach in this race or the next.”
If the LHC fails to reveal new scientific data beyond the Standard Model, Butterworth said, it would not represent a failure, but would leave the field “in some dilemma” about where to go.
The third run, which is expected to last until 2026, follows an upgrade that included the installation of additional powerful magnets designed to compress the protons inside the collider into finer, denser beams. This will increase the collision rate of particles inside the accelerator, meaning scientists will be able to observe rarer events with greater precision.
Ashutosh Kotwal, experimental particle physicist at Duke University in the United States and co-leader of a research group at the LHC’s Atlas experiment, said: “The potential for discovering new ideas is still so great. It should be noted that the data we have collected so far is only a tenth of the total we expect. It’s too early to be discouraged.