Ten years after the Higgs, physicists face the nightmare of finding nothing else

Ten years ago, particle physicists rocked the world. On July 4, 2012, 6,000 researchers working with the world’s largest atom destroyer, the Large Hadron Collider (LHC) at the European particle physics laboratory, CERN, announced that they had discovered the Higgs boson, a massive, fleeting particle, key to their obscure 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 limelight.

Then came a long hangover. Before the 27-kilometre-long ring-shaped LHC began collecting data in 2010, physicists feared it was producing the Higgs and nothing else, leaving no clues as to what lies beneath. beyond the standard model. So far, this nightmarish scenario is coming true. “It’s a little disappointing,” admits Barry Barish, a physicist at the California Institute of Technology. “I thought we were going to discover supersymmetry”, the main extension of the Standard Model.

It’s too early to despair, say many physicists. After 3 years of upgrades, the LHC is now kicking off for the third of five planned runs, and new particles could emerge in the billions of proton-proton collisions it will produce every second. In fact, the LHC is expected to operate for another 16 years and, with further upgrades, collect 16 times more data than it already has. All of this data could reveal subtle signs of new particles and phenomena.

Still, some researchers say the writing is on the wall for collider physics. “If they don’t find anything, that field is dead,” says Juan Collar, a University of Chicago physicist who hunts for dark matter in small experiments. John Ellis, theorist at King’s College London, says hopes of a sudden breakthrough have given way to the prospect of a long and uncertain road to discovery. “It will be like pulling teeth, not like teeth falling out.”

Since the 1970s, physicists have been locked in a wrestling match with the Standard Model. He argues that ordinary matter is made up of light particles called up quarks and down quarks, which bind together in trios to form protons and neutrons, as well as electrons and featherweight particles called electron neutrinos. Two sets of heavier particles lurk in the void and can be propelled into fleeting existence through particle collisions. All interact by exchanging other particles: the photon conveys the electromagnetic force, the gluon conveys the strong force which binds the quarks and the massive bosons W and Z convey the weak force.

The Standard Model describes everything scientists have seen so far in particle colliders. Yet this cannot be the ultimate theory of nature. It leaves out the force of gravity and does not include the mysterious and invisible dark matter, which seems to outweigh ordinary matter six times in the universe.

The LHC was supposed to get out of this impasse. In its ring, protons traveling in opposite directions collide at energies nearly seven times higher than at any previous collider, allowing the LHC to produce particles too massive to be made anywhere else. A decade ago, many physicists envisioned the rapid spotting of wonders, including new force-carrying particles or even mini black holes. “We would drown in supersymmetric particles,” recalls Beate Heinemann, director of particle physics at the German DESY laboratory. Finding the Higgs would take longer, physicists predicted.

Instead, the Higgs appeared in a relatively quick 3-year timeframe, partly because it’s somewhat less massive than many physicists expected, about 133 times heavier than a proton, which facilitated its production. And 10 years after this monumental discovery, no other new particle has emerged.

This shortage has undermined two of the ideas that physicists hold dear. A notion called naturalness suggested that the low mass of the Higgs more or less guaranteed the existence of new particles within reach of the LHC. According to quantum mechanics, all particles that “virtually” hide in the vacuum will interact with real particles and affect their properties. This is exactly how virtual Higgs bosons give their mass to other particles.

This physics goes both ways, however. The mass of the Higgs boson is expected to be pulled up considerably by other Standard Model particles in vacuum, in particular the top quark, a heavier version of the up quark that weighs 184 times as much as the proton. This 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 a vacuum to “naturally” counter the effects of the quark. top .

The theoretical concept known as supersymmetry would provide such particles. For each known Standard Model particle, it postulates a heavier partner with a different spin. Hidden in the vacuum, these partners would not only prevent the mass of Higgs from getting away, but would also help explain how the Higgs field, which pervades the vacuum like an inextinguishable electric field, came into being. Supersymmetric particles could even represent dark matter.

But instead of those hoped-for particles, which have emerged over the past decade, are tantalizing anomalies — small discrepancies between observations and Standard Model predictions — that physicists will explore over the next three years of research. operation of the LHC. For example, in 2017 physicists working with LHCb, one of the Big Four particle detectors powered by the LHC, discovered that B mesons, particles that contain a heavy quark, more often decay into an electron and a positron than into a particle called a muon and an antimuon. The Standard Model says the two rates should be the same, and the difference could be a clue to supersymmetric partners, Ellis says.

Similarly, experiments elsewhere suggest that the muon may be very slightly more magnetic than predicted by the Standard Model (Science, April 9, 2021, p. 113). This anomaly can be explained by the existence of exotic particles called leptoquarks, which could already be hiding undetected in the LHC production, explains 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 they had both spotted the Higgs decaying into a muon and an antimuon. If the rate of this hard-to-see decay differs from predictions, the discrepancy could indicate that new particles are lurking in the vacuum, says Marcela Carena, a theorist at Fermi National Accelerator Laboratory.

These searches will probably not return “Eureka!” moments though. “There is a move towards very precise measurements of subtle effects,” says Heinemann. Still, Carena says, “I highly doubt that in 20 years time I’ll be saying, ‘Oh my God, after the discovery of the Higgs, we haven’t learned anything new. “”

Others are less optimistic about the chances of the LHC experimenters. “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 doesn’t find anything new, it will be harder to convince the world’s governments to build the next bigger and more expensive collider to keep the field going.

For now, many LHC physicists are happy to get back to squashing protons. Over the past 3 years, scientists have improved the detectors and reworked the low-energy accelerators that power the collider. The LHC should now operate at a more constant collision rate, increasing data throughput by up to 50%, says Mike Lamont, Director of Accelerators and Beams at CERN.

Accelerator physicists have been slowly tuning the LHC beams for months, Lamont says. Only when the beams are stable enough do they turn on the detectors and resume taking data. Those switches are expected to flip on July 5, 10 years and 1 day after the Higgs discovery was announced, Lamont said. “It’s good to get into a sustained run.”

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