In the verdant Rhone Valley just west of Geneva, an elaborate game of chance is about to begin.
At stake: the opportunity to glimpse nature’s deepest secrets. The Bet: More than $400 million spent and more than three and a half years of labor by scientists around the world to refurbish and reactivate the Large Hadron Collider, the most powerful particle accelerator ever built.
Ten years ago, the LHC caught the world’s attention when it provided definitive proof of the existence of the Higgs boson, the elusive particle that completes the prevailing theory of matter known as the Standard Model.
Tuesday, a day after the tenth anniversary of that Nobel Prize-winning milestone, researchers turned on their detectors and hoped to reuse the LHC to spin scientific gold.
This time, their goal isn’t just to travel to the edge of the unknown. It’s to use all the energy and trickery they can muster to get past that limit and grab something – anything – that is beyond the unfathomable horizon of the Standard Model.
No one disputes that there is something to be found. There are too many unanswered questions and unexplained phenomena for the current paradigm in particle physics to have the final say. The most glaring example is dark matter, an unidentified substance that permeates the Universe but has no place in the Standard Model.
Whether the LHC can shed light on these or other mysteries is an open question. It’s the gamble thousands of researchers from more than 100 countries made as they gathered this week to resume their exploration of the foundations of reality.
“People are so excited. It’s kind of the beginning of a new era,” said Manuella Vincter, deputy spokesperson for ATLAS, one of four giant particle detectors stationed along the LHC’s 17-mile ring road. Canada is one of 42 countries contributing resources to the detector.
dr. Vincter, a physics professor at Carleton University in Ottawa, has worked at the LHC since the start of the pandemic. She said COVID-19 was slowing progress on upgrades to the accelerator, which has been on a scheduled shutdown since late 2018.
The resulting lack of new data coupled with two years of travel restrictions has brought an entire cohort of young scientists to a standstill who would otherwise have flocked to CERN, the European research center where the LHC is located.
Now, said Dr. Vincter, there’s a newfound sense of exuberance as the collider springs back into action and researchers gather at CERN, eager to share what they’ve been working on.
A special anniversary symposium takes center stage on Monday, featuring many of the senior scientists who played a role in the discovery of Higgs.
There is much to celebrate, though less than physicists had hoped for when the LHC first became operational in late 2009.
The Standard Model then consisted of 16 particles that are the fundamental constituents of matter and energy. Three of them, including the up quark, the down quark (which together make up protons and neutrons), and the electron, make up the atoms of our everyday world.
Most of the remaining particles are too short-lived to be observed, except in physics experiments. Four of them — known as bosons for their quantum properties — transmit the forces that act within and between atoms.
But even when the Standard Model emerged in the 1960s, theorists recognized that something else was needed to explain many of its features, including why some particles have mass and others don’t. The solution includes an additional boson that has the ability to donate mass based on how much it interacts with other particles.
The statement was first elaborated in 1964 by British theorist Peter Higgs and independently by two other research teams. But it is Higgs whose name remains on their proposed particle.
To prove the idea was correct, researchers had to observe the Higgs boson in nature — no easy task, as the particle decays in less than a billionth of a trillionth of a second. It would fuse the LHC, those protons traveling in an accelerator ring at 99.9999991 percent the speed of light, to do the job.
By using the energy temporarily released during such collisions, the laws of physics create particles, including Higgs bosons, with a certain probability. In the giant roulette wheel that is the LHC, protons collide within each detector about 40 million times per second, but only a thousand of those collisions can produce a Higgs boson.
Even then, the particle is too volatile to detect, but its byproducts can be measured. By combining the results of many detections, the properties of the Higgs boson can be deduced to high precision.
The lead-up to the Higgs discovery was an exciting time, said Mike Lamont, CERN’s director of accelerators and technology. Teams working in parallel with ATLAS and CMS, a separate detector, led the hunt.
“They knew something was up at the end of 2011,” he said. “So we came in 2012 and we really worked on getting the machine to run as best we could and keep it there for as long as possible.”
The effort paid off. On July 4, 2012, representatives from both teams gathered in the main hall of CERN to present their results confirming the existence of the Higgs boson in a live-streamed briefing that became the LHC’s signature moment.
Many expected other discoveries to follow soon, including previously unobserved particles that would point the way to a new theory beyond the Standard Model. Instead, the LHC has shown that nature is frugal. Even after the energy of the accelerator’s opposing proton beams doubled for a second run from 2015, no such particles came out.
Since then, the Higgs, once the prize, has become the tool in the search for subtle deviations from the standard model that could be hints of something new. The most surprising thing about the results so far is how few surprises there are to report.
Pierre Savard, a University of Toronto physicist and member of the ATLAS team, said that when he graduated in the 1990s, there were plenty of colleagues who would have staked everything from “their car, their house to their shirt. back” that LHC, once built, would yield something new after the Higgs boson was found.
Instead, he said, “this Higgs boson is very much a Higgs standard model and the theory works very well.”
Still, at least one preliminary clue has surfaced in another detector of the accelerator, called LHCb. The clue boils down to a small imbalance between observations of two ways the lower quark, one of the Standard Model particles, can decay.
The model predicts that the two modes should be equal, but LHCb has seen otherwise. With more data, the anomaly could disappear, just the number of times a tails or tails flips becomes more even the more the tail is tossed. But if the imbalance persists, the LHCb result could represent a true departure from the known laws of physics.
Meanwhile, work has been done to squeeze as much of the performance out of the collider as possible for the third run, which begins this week. After three years of upgrades, collisions will be only marginally more powerful than before. But there are key differences in how the energy from the proton beams will be used and how the detectors will work to extract useful data at a higher rate.
The main challenge is not to generate more collisions – the LHC already makes far more than any computer can store – but to choose the one that is most revealing. For example, to get the most out of ATLAS, scientists have installed new detector components to help distinguish the most interesting collisions.
“We’re actually hopeful that in three years we can double the existing data set,” said Isabel Trigger, an ATLAS team member and research scientist at the TRIUMF particle accelerator in Vancouver.
Looking further, said Dr. Trigger, she and her colleagues are already working on a more ambitious fourth run of the machine called “high luminosity LHC,” which could begin as early as 2027 and keep the accelerator running until about 2040.
dr. Lamont said he has already begun a feasibility study for a 91-kilometer-long accelerator ring that would pick up where LHC left off in the second half of this century. But whether such a machine will be built depends on what physicists can find between now and then to motivate the plan.
Finally he said: “You do everything you can to make it right and then you have to step back and see what happens. Nature doesn’t bend just because we look.”
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