A decade ago, particle physicists surprised the world. On July 4, 2012, 6,000 researchers working with the world’s largest nuclear destroyer, the Large Hadron Collider (LHC) at the European Laboratory of Particle Physics, CERN, announced that they had discovered the Higgs boson, a huge, volatile particle that holds the key forms their obscure explanation of how other fundamental particles get their masses. The discovery fulfilled a 45-year prophecy, completing a theory called the Standard Model and putting physicists in the spotlight.
Then came a long hangover. Before the 27-kilometer-long annular LHC began collecting data in 2010, physicists worried it might be producing Higgs and nothing else, with no idea what lies beyond the Standard Model. So far, that nightmare scenario is coming true. “It’s a little disappointing,” said Barry Barish, a physicist at the California Institute of Technology. “I thought we were going to discover supersymmetry,” the leading extension of the Standard Model.
It’s too early to despair, many physicists say. After 3 years of upgrades, the LHC is now powering up for the third of five planned runs, and a new particle could appear in the billions of proton-proton collisions it will produce every second. In fact, the LHC should operate for another 16 years and, with further upgrades, collect 16 times as much data as it already has. All that data could reveal subtle signs of new particles and phenomena.
Still, some researchers say the writing on the wall represents collision physics. “If they don’t find anything, this field is dead,” said Juan Collar, a physicist at the University of Chicago who hunts for dark matter in smaller experiments. John Ellis, a theorist at King’s College London, says hopes of a sudden breakthrough have given way to the prospect of a long, precarious search for discovery. “It will be like pulling teeth, not teeth falling out.”
Since the 1970s, physicists have been locked in a wrestling match with the Standard Model. It states that ordinary matter is made up of lightweight particles called quarks and down quarks — which bond in trios to make protons and neutrons — along with electrons and feathery particles called electron neutrinos. Two sets of heavier particles lurk in the vacuum and can be blasted into particle collisions in a fleeting existence. They all work together by exchanging other particles: The photon transfers the electromagnetic force, the gluon carries the strong force binding quarks, and the massive W and Z bosons carry the weak force.
The Standard Model describes everything scientists have seen so far in particle accelerators. Yet it cannot be the ultimate theory of nature. It disregards gravity, and it does not include mysterious, invisible dark matter, which appears to outnumber ordinary matter in the universe six to one.
The LHC had to break that deadlock. In its ring, protons circulating in opposite directions collide at energies nearly seven times higher than any previous collider, allowing the LHC to produce particles too heavy to be made anywhere else. Ten years ago, many physicists imagined discovering wonders quickly, including new force-carrying particles or even mini-black holes. “You would drown in supersymmetric particles,” recalls Beate Heinemann, director of particle physics at the German laboratory DESY. Finding the Higgs would take longer, physicists predicted.
Instead, the Higgs appeared in a relatively quick 3 years — in part because it’s slightly less massive than many physicists had expected, about 133 times the mass of a proton, making it easier to produce. And 10 years after that monumental discovery, no other new particle has emerged.
That flaw has undermined two of physicists’ cherished ideas. A concept called naturalness suggested that the low mass of the Higgs more or less guaranteed the existence of new particles within the range of the LHC. According to quantum mechanics, any particles lurking “virtually” in the vacuum will interact with real particles and affect their properties. That’s exactly how virtual Higgs bosons give other particles their mass.
However, that physics cuts both ways. The mass of the Higgs boson should be pulled up dramatically by other Standard Model particles in the vacuum, especially the top quark, a heavier version of the up quark that weighs 184 times the proton. That doesn’t happen, so theorists have reasoned that at least one other new particle with a similar mass and just the right properties — specifically, a different spin — must exist in the vacuum to “naturally” reverse the effects of the top to fight quark.
The theoretical concept known as supersymmetry would provide such particles. For each known Standard Model particle, it represents a heavier partner with a different spin. Lurking in the vacuum, those partners would not only prevent the Higgs mass from running away, but would also help explain how the Higgs field, which permeates the vacuum like an unquenchable electric field, came to be. Supersymmetric particles could even be responsible for dark matter.
But in place of those hoped-for particles, the past decade has revealed tantalizing anomalies — minor discrepancies between observations and predictions from standard models — that physicists will examine over the next three years at the LHC. For example, in 2017, physicists working with LHCb, one of the four large particle detectors powered by the LHC, found that B-mesons, particles containing a heavy bottom quark, are more likely to decay into an electron and a positron than into a particle called a particle. a. muon and an antimuon. The Standard Model says the two velocities should be the same, and the difference could be a hint of supersymmetric partners, Ellis says.
Similarly, experiments elsewhere suggest that the muon may be slightly more magnetic than the Standard Model predicts (Science, April 9, 2021, p. 113). That anomaly could be explained by the existence of exotic particles called leptoquarks, which may already be hiding in the LHC’s output undetected, Ellis says.
The Higgs itself offers other avenues for exploration, as any difference between the observed and predicted properties would signal new physics. For example, in August 2020, teams of physicists working with the LHC’s two largest detectors, ATLAS and CMS, announced that both had seen the Higgs decay into a muon and an antimuon. If the speed of that hard-to-see decay differs from predictions, the anomaly could indicate new particles hiding in the vacuum, said Marcela Carena, a theorist at Fermi National Accelerator Laboratory.
Those searches probably won’t produce a dramatic “Eureka!” moments though. “There is a shift towards very accurate measurements of subtle effects,” Heinemann says. Still, Carena says, “I doubt very much that in 20 years I’ll be saying, ‘Oh, boy, after the discovery of Higgs, we haven’t learned anything new.'”
Others are less optimistic about the chances of LHC researchers. “They’re facing the desert and they don’t know how wide it is,” said Marvin Marshak, a physicist at the University of Minnesota, Twin Cities, who studies neutrinos using other facilities. Even optimists say that if the LHC doesn’t find anything new, it will be harder to convince the world’s governments to build the next bigger, more expensive collider to keep the field going.
For now, many physicists at the LHC are just excited to get back to destroying protons. Over the past 3 years, scientists have upgraded the detectors and reworked the lower-energy accelerators that power the accelerator. The LHC should now operate at a more constant collision rate, increasing data flow by as much as 50%, said Mike Lamont, director of accelerators and jets at CERN.
Accelerator physicists have been slowly tuning LHC jets for months, Lamont says. Only when the beams are sufficiently stable do they turn on the detectors and continue recording data. Those switches should flip on July 5, 10 years and 1 day after the announcement of the Higgs discovery, Lamont says. “It’s good to start with a sustained run.”