The souped-up Large Hadron Collider is back to take on its weightiest questions yet

What happens where the LHC's beams meet tells us how the universe works.
The Large Hadron Collider's magnet chain.
A chain of magnets inside the tunnel at the Large Hadron Collider. Samuel Joseph Hertzog via CERN

The bleeding edge of physics lies in a beam of subatomic particles, rushing in a circle very near the speed of light in an underground tunnel in Central Europe. That beam crashes into another racing just as fast in the other direction. The resulting collision produces a flurry of other particles, captured by detectors before they blink out of existence.

This is standard procedure at the Large Hadron Collider (LHC), which recently switched on for the first time since 2018, its beams now more powerful than ever. The LHC, located at the European Organization for Nuclear Research (CERN) near Geneva, is the world’s largest particle collider: a mammoth machine that literally smashes subatomic particles together and lets scientists watch the fountain of quantum debris that spews out.

That may seem unnecessarily violent for a physics experiment, but physicists have a good reason for the destruction. Inside those collisions, physicists can peel away the layers of our universe to see what makes it tick at the smallest scales.

The physicists behind the machine

The “large” in the LHC’s name is no exaggeration: The collider cuts a 17-mile-long magnetic loop, entirely underground, below the Geneva suburbs on both sides of the ragged French-Swiss border (home of CERN’s headquarters), through the shadows by the eastern slopes of France’s Jura Mountains, and back again.

Assembling such a colossus took time. First proposed in the 1980s and approved in the mid-1990s, the LHC took over a decade to build before its beam first switched on in 2008. Construction took $4.75 billion, mostly coming from the coffers of various European governments.

LHC consumes enough electricity to power a small city. Even before its current upgrades, LHC’s experiments produced a petabyte of data per day, enough to hold over 10,000 4K movies—and that’s after CERN’s computer network filtered out the excess. That data passes through the computers of thousands of scientists from every corner of the globe, although some parts of the world are better represented than others.

[Related: The biggest particle collider in the world gets back to work]

Time, money, and people power continue to pour into the collider as physicists seek to answer the universe’s most fundamental questions. 

For instance, what causes mass to exist? Helping to answer that question has been one of the LHC’s most public triumphs to date. In 2012, LHC scientists announced the discovery of a long-sought particle known as the Higgs boson. The boson is the product of a field that, when particles interact with the field, gives those particles mass.

The discovery of the Higgs boson was the final brick in the wall known as the Standard Model. It’s the heart of modern particle physics, a schematic that lays out about a dozen subatomic particles and how they neatly fit together to give rise to the universe we see.

But with every passing year, the Standard Model seems increasingly inadequate to answer basic questions. Why is there so much more matter in the universe than antimatter, its opposite? What makes up the massive chunk of our universe that seems to be unseen and unseeable? And why does gravity exist? The answers are anything but simple.

The answers may come in the form of yet-undiscovered particles. But, so far, they’ve eluded even the most powerful particle colliders. “We have not found any non-Standard Model particles at the LHC so far,” says Finn Rebassoo, a particle physicist at Lawrence Livermore National Laboratory in California and an LHC collaborator.

Upgrading the behemoth

Although the COVID-19 pandemic disrupted the LHC’s reopening (it was originally scheduled for 2020) , the collider’s stewards have not sat by idly since 2018. As part of a raft of technical upgrades, they’ve topped up the collider’s beam, boosting its energy by about 5 percent. 

That may seem like a pittance (and it certainly pales in comparison to the planned High-Luminosity LHC upgrade later this decade that will boost the number of collisions). But scientists say that it still makes a difference.

“This means an increase in the likelihood for producing interesting physics,” says Elizabeth Brost, a particle physicist at Brookhaven National Laboratory on Long Island, and an LHC collaborator. “As a personal favorite example, we will now get 10 percent more events with pairs of Higgs bosons.”

The Standard Model says that paired Higgs bosons should be an extremely rare occurrence—and perhaps it is. But, if the LHC does produce pairs in abundance, it’s a sign that something yet undiscovered is at play.

“It’s a win-win situation: Either we observe Higgs pair production soon, which implies new physics,” says Brost, “or we will eventually be able to confirm the Standard Model prediction using the full LHC dataset.”

The enhancements also provide the chance to observe things never before seen. “Every extra bit provides more potential for finding new phenomena,” says Bo Jayatilaka, a particle physicist at Fermilab in suburban Chicago and an LHC collaborator.

It wasn’t long ago that one potential fodder for observation emerged—not from CERN, but from an old, now-shuttered accelerator at Fermilab, outside Chicago. Researchers poring over old data found that the W boson, a particle responsible for causing radioactive decay inside atoms, seemed to have a heavier mass than anticipated. If that’s true, it could blow the Standard Model wide open.

Naturally, particle physicists want to make sure it is true. They’re already planning to repeat that W boson measurement at CERN, both with data collected from past experiments and with new data from experiments yet to come.

It will likely take time to get the LHC up to its newfound full capacity. “Typically, when the LHC is restarted it is a slow restart, meaning the amount of data in the first year is not quite as much as the subsequent years,” says Rebassoo. And analyzing even that data it produces takes time, even for the great masses of scientists who work on the collider.

But as soon as 2023, we could see results—taking advantage of the collider’s newfound energy boost, Jayatilaka speculates.