A decade ago, particle physicists thrilled the world. On 4 July 2012, 6000 researchers working with the world’s biggest atom smasher, the Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, announced they had discovered the Higgs boson, a massive, fleeting particle key to their abstruse explanation of how other fundamental particles get their mass. The discovery fulfilled a 45-year-old prediction, completed a theory called the standard model, and thrust physicists into the spotlight.
Then came a long hangover. Before the 27-kilometer-long ring-shaped LHC started to take data in 2010, physicists fretted that it might produce the Higgs and nothing else, leaving no clue to what lies beyond the standard model. So far, that nightmare scenario is coming true. “It’s a bit disappointing,” allows 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 powering up for the third of five planned runs, and some new particle could emerge in the billions of proton-proton collisions it will produce every second. In fact, the LHC should run for another 16 years, and with further upgrades should collect 16 times as much data as it already has. All those data could reveal subtle signs of novel particles and phenomena.
Still, some researchers say the writing is on the wall for collider physics. “If they don’t find anything, this 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 grind toward discovery. “It’s going to be like pulling teeth, not like teeth falling out.”
Since the 1970s, physicists have been locked in a wrestling match with the standard model. It holds that ordinary matter consists of lightweight particles called up quarks and down quarks—which bond in trios to make protons and neutrons—along with electrons and featherweight 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 conveys 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 scientists have seen at particle colliders so far. Yet it cannot be the ultimate theory of nature. It leaves out the force of gravity, and it doesn’t include mysterious, invisible dark matter, which appears to outweigh ordinary matter in the universe six to one.
The LHC was supposed to break that impasse. In its ring, protons circulating in opposite directions crash together at energies nearly seven times as high as at any previous collider, enabling the LHC to produce particles too massive to be made elsewhere. A decade ago many physicists envisioned quickly spotting marvels including new force-carrying particles or even mini–black holes. “One 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 speedy 3 years—in part because it is somewhat less massive than many physicists expected, about 133 times as heavy as a proton, which made it easier to produce. And 10 years after that monumental discovery, no other new particle has emerged.
That dearth has undermined two of physicists’ cherished ideas. A notion called naturalness suggested the low mass of the Higgs more or less guaranteed the existence of new particles within the LHC’s grasp. According to quantum mechanics, any particles lurking “virtually” in the vacuum will interact with real ones and affect their properties. That’s exactly how virtual Higgs bosons give other particles their mass.
That physics cuts both ways, however. The Higgs boson’s mass ought to be pulled dramatically upward by other standard model particles in the vacuum—especially the top quark, a heavier version of the up quark that weighs 184 times as much as 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—in particular, a different spin—must exist in the vacuum to “naturally” counter the effects of the top quark.
The theoretical concept known as supersymmetry would supply such particles. For every known standard model particle, it posits a heavier partner with a different spin. Lurking in the vacuum, those partners would not only keep the Higgs’s mass from running away, but would also help explain how the Higgs field, which pervades the vacuum like an unextinguishable electric field, came into being. Supersymmetric particles might even account for dark matter.
But instead of those hoped-for particles, what have emerged in the past decade are tantalizing anomalies—small discrepancies between observations and standard model predictions—that physicists will explore in the LHC’s next 3-year run. For example, in 2017, physicists working with LHCb, one of four large particle detectors fed by the LHC, found that B mesons, particles that contain a heavy bottom quark, decay more often to an electron and a positron than to a particle called a muon and an antimuon. The standard model says the two rates should be the same, and the difference might be a hint of supersymmetric partners, Ellis says.
Similarly, experiments elsewhere suggest the muon might be very slightly more magnetic than the standard model predicts (Science, 9 April 2021, p. 113). That anomaly can be explained by the existence of exotic particles called leptoquarks, which might already be hiding undetected in the LHC’s output, Ellis says.
The Higgs itself provides 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 biggest detectors, ATLAS and CMS, announced that both had spotted the Higgs decaying to a muon and an antimuon. If the rate of that hard-to-see decay varies from predictions, the deviation could point to new particles hiding in the vacuum, says Marcela Carena, a theorist at Fermi National Accelerator Laboratory.
Those searches likely won’t yield dramatic “Eureka!” moments, however. “There’s a shift towards very precise measurements of subtle effects,” Heinemann says. Still, Carena says, “I very much doubt that in 20 years, I will say, ‘Oh, boy, after the Higgs discovery we learned nothing new.’”
Others are less sanguine about LHC experimenters’ chances. “They’re facing the desert and they don’t know how wide it is,” says Marvin Marshak, a physicist at the University of Minnesota, Twin Cities, who studies neutrinos using other facilities. Even optimists say that if the LHC finds nothing new, it will be harder to convince the governments of the world 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 smashing protons. During the past 3 years, scientists have upgraded the detectors and reworked the lower energy accelerators that feed the collider. The LHC should now run at a more constant collision rate, effectively increasing the flow of data by as much as 50%, says Mike Lamont, director of accelerators and beams at CERN.
Accelerator physicists have been slowly tuning up the LHC’s beams for months, Lamont says. Only when the beams are sufficiently stable will they turn on the detectors and resume taking data. Those switches should flip on 5 July, 10 years and 1 day after the announcement of the Higgs discovery, Lamont says. “It’s good to be going into some sustained running.”
Science, 13 June 2022