A decade ago, particle physicists wowed the world. On July 4, 2012, 6,000 researchers working with the world’s largest atom smasher, the Large Hadron Collider (LHC), at Europe’s particle physics laboratory, CERN, announced that they had discovered the Higgs boson, a massive, ephemeral particle associated with the Key to their abstruse explanation is how other fundamental particles get their mass. The discovery fulfilled a 45-year-old prediction, completed a theory called the Standard Model, and put physicists in the spotlight.
Then came a long hangover. Before the 27-kilometer ring-shaped LHC began collecting data in 2010, physicists feared it might produce the Higgs and nothing else, leaving no clue as to what’s behind the Standard Model. So far, this nightmare scenario is coming true. “It’s a bit disappointing,” admits Barry Barish, a physicist at the California Institute of Technology. “I thought we would 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 ramping up for the third of five planned runs, and a new particle could be born in the billions of proton-proton collisions it will produce every second. In fact, the LHC is supposed to run for another 16 years and, with further upgrades, collect 16 times as much data as it already has. All of this data could reveal subtle signs of novel particles and phenomena.
Still, some researchers say the writing for collider physics is on the wall. “If they don’t find anything, that field is dead,” says Juan Collar, a physicist at the University of Chicago who hunts 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, uncertain quest to find it. “It’s going to be like pulling teeth, not teeth falling out.”
Physicists have been wrestling with the Standard Model since the 1970s. It states that ordinary matter is made up of light particles called up quarks and down quarks – which combine in trios to form 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 fleeting existence in particle collisions. All interact by exchanging other particles: the photon carries the electromagnetic force, the gluon carries the strong force that binds quarks, and the massive W and Z bosons carry the weak force.
The Standard Model describes everything that scientists have seen so far about particle accelerators. But it cannot be the ultimate theory of nature. It ignores gravity, and it contains no mysterious, invisible dark matter, which seems to outweigh ordinary matter six to one in the universe.
The LHC should break this impasse. In its ring, protons circulating in opposite directions collide with energies nearly seven times that of any previous collider, allowing the LHC to create particles too massive to be made elsewhere. A decade ago, many physicists dreamed of quickly discovering wonders, including new force-transmitting particles or even mini-black holes. “One would drown in supersymmetric particles,” recalls Beate Heinemann, Director of Particle Physics at the German laboratory DESY. The search for the Higgs would take longer, physicists predicted.
Instead, the Higgs appeared in a relatively short 3 years – partly because it’s a little less massive than many physicists expected, about 133 times the mass of a proton, which made it easier to manufacture. And 10 years after this monumental discovery, no other new particle has appeared.
This deficiency has undermined two of physicists’ cherished ideas. A term called naturalness suggested that the low mass of the Higgs more or less guaranteed the existence of new particles within range of the LHC. According to quantum mechanics, all particles lurking “virtually” in the vacuum interact with real particles and influence their properties. This is exactly how virtual Higgs bosons give other particles their mass.
However, this physics cuts both ways. The mass of the Higgs boson should be pulled up dramatically in the vacuum by other standard model particles – most notably the top quark, a heavier version of the up quark that weighs 184 times the proton. That doesn’t happen, so theorists have argued that at least one other new particle with a similar mass and just the right properties – notably a different spin – must exist in the vacuum to ‘naturally’ counteract the effects of the top quark.
The theoretical concept known as supersymmetry would provide such particles. For every known Standard Model particle it postulates a heavier partner with a different spin. These partners, lurking in the vacuum, would not only prevent the Higgs’ mass from running away, but also help explain how the Higgs field, which permeates the vacuum like an inextinguishable electric field, came about. Supersymmetric particles could even make up dark matter.
But instead of those hoped-for particles, tantalizing anomalies have emerged over the past decade — tiny discrepancies between observations and Standard Model predictions — that physicists will explore in the LHC’s next three-year run. For example, in 2017 physicists working with LHCb, one of four large particle detectors powered by the LHC, found that B mesons, particles containing a heavy bottom quark, decay into an electron and a positron more often than one Particles called a muon and an antimuon. The Standard Model says the two rates should be equal, and the difference could indicate 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). This anomaly can be explained by the existence of exotic particles called leptoquarks, which may already be hiding in the LHC’s output, says Ellis.
The Higgs itself offers other avenues of exploration, as any difference between its 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 discovered the Higgs decaying into a muon and an antimuon. If the rate of this elusive decay deviates from predictions, the discrepancy could indicate new particles hiding in the vacuum, says Marcela Carena, a theorist at Fermi National Accelerator Laboratory.
These searches will likely not result in a dramatic “Eureka!” moments however. “There is a trend towards very precise measurements of subtle effects,” says Heinemann. Still, Carena says, “I very much doubt that in 20 years I’ll be like, ‘Oh man, we didn’t learn anything new after the Higgs discovery.'”
Others are less confident about the LHC experimenters’ chances. “You look at the desert and you don’t know how wide it is,” says Marvin Marshak, a physicist at the University of Minnesota, Twin Cities, who studies neutrinos with other facilities. Even optimists say it will be harder to convince world governments to build the next-largest, more expensive collider unless the LHC finds something new to keep the field going.
Right now, many physicists at the LHC are just excited to start destroying protons again. Over the past three years, scientists have upgraded the detectors and revised the low-energy accelerators that power the collider. The LHC should now run at a more constant collision rate, effectively increasing data flow by up to 50%, says Mike Lamont, director of accelerators and beams at CERN.
Accelerator physicists have been slowly adjusting the LHC’s beams for months, says Lamont. Only when the beams are sufficiently stable do they turn on the detectors and resume data acquisition. Those switches should be flipped on July 5, 10 years and 1 day after the Higgs discovery was announced, Lamont says. “It’s good to get into sustainable running.”