Particle physicists are rather philosophical when describing their work.
“Whatever we find out, that is what nature chose,” Kyle Cranmer, a physics professor at New York University, tells me. It’s a good attitude to have when your field yields great disappointments.
For months, evidence was mounting that the Large Hadron Collider, the biggest and most powerful particle accelerator in the world, had found something extraordinary: a new subatomic particle, which would be a discovery surpassing even the LHC’s discovery of the Higgs boson in 2012, and perhaps the most significant advance since Einstein’s theory of relativity.
And yet, nature had other plans.
In August, the European Organization for Nuclear Research (CERN) reported that the evidence for the new particle had run thin. What looked like a promising “bump” in the data, indicating the presence of a particle with a unique mass, was just noise.
But to Cranmer — who has analyzed LHC data in his work — the news did not equate failure. “You have to keep that in mind,” he says. “Because it can feel that way. It wasn’t there to be discovered. It’s like being mad that someone didn’t find an island when someone is sailing in the middle of the ocean.”
What’s more, the LHC’s journey is far from over. The machine is expected to run for another 20 or so years. There will be more islands to look for.
“We’re either going to discover a bunch of new particles or we will not,” Cranmer says. “If we find new particles, we can study them, and then we have a foothold to make progress. And if we don’t, then [we’ll be] staring at a flat wall in front figuring out how to climb it.”
This is a dramatic moment, one that could provoke “a crisis at the edge of physics,” according to a New York Times op-ed. Because if the superlative LHC can’t find answers, it will cast doubt that answers can be found experimentally.
From here, there are two broad scenarios that could play out, both of which will vastly increase our understanding of nature. One scenario will open up physics to a new world of understanding about the universe; the other could end particle physics as we know it.
The physicists themselves can’t control the outcome. They’re waiting for nature to tell them the answers.
Why do we care about new subatomic particles anyway?
The LHC works by smashing together atoms at incredibly high velocities. These particles fuse and can form any number of particles that were around in the universe from the Big Bang onward.
When the Higgs boson was confirmed in 2012, it was a cause for celebration and unease. The Higgs was the last piece of a puzzle called the standard model, which is a theory that connects all the known components of nature (except gravity) together in a balanced, mathematical equation. The Higgs was the final piece that had been theorized to exist but never seen.
After the Higgs discovery, the scientists at the LHC turned their hopes in a new direction. They hoped the accelerator could begin to find particles that had never been theorized nor ever seen. It was like going from a treasure hunt with a map to chartering a new ocean.
They want to find these new subatomic particles because even though the standard model is now complete, it still can’t answer a lot of lingering questions about the universe. Let’s go through the two scenarios step by step.
Scenario 1: There are more subatomic particles! Exciting!
If the LHC finds new subatomic particles, it lend evidence to a theory known as supersymmetry. Supersymmetry posits that all the particles in the standard model must have a shadow “super partner” that spins in a slightly different direction.
Scientists have never seen one of these supersymmetrical particles, but they’re keen to. Supersymmetry could neatly solve some of the biggest problems vexing physicists right now.
1) No one knows what dark matter is
One of these particles could be what scientists call “dark matter,” which is theorized to make up 27 percent of the universe. But we’ve never seen dark matter, and that leaves a huge gaping hole in our understanding of the how the universe formed and exists today.
“It could be that one particle is responsible for dark matter,” Cranmer explains. Simple enough.
2) The Higgs boson is much too light
The Higgs discovery was an incredible triumph, but it also contained a mystery to solve. The boson — at 126 GeV (giga electron volts) — was much lighter than the standard model and the math of quantum mechanics suggests it should be.
Why is that a problem? Because it’s a wrinkle to be ironed out in our understanding of the universe. It suggests the standard model can’t explain everything. And physicists want to know everything.
“Either nature is sort of ugly, which is entirely conceivable, and we just have to live with the fact that the Higgs boson mass is light and we don’t know why,” Ray Brock, a Michigan State University physicist who has worked on the LHC, says, “or nature is trying to tell us something.”
It could be that a yet-to-be-discovered subatomic particle interacts with the Higgs, making it lighter than it ought to be.
3) The standard model doesn’t unify the forces of the universe
There are four major forces that make the universe tick: the strong nuclear force (which holds atoms together), the weak nuclear force (what makes Geiger counters tick), electromagnetism (you’re using it right now, reading this article on an electronic screen), and gravity (don’t look down.)
Scientists aren’t content with the four forces. They, for decades, have been trying to prove that the universe works more elegantly, that, deep down, all these forces are just manifestations of one great force that permeates the universe.
Physicists call this unification, and the standard model doesn’t provide it.
"If we find supersymmetry at the LHC, it is a huge boost to the dream that three of the fundamental forces we have [all of them except gravity] are all going to unify,” Cranmer says.
4) Supersymmetry would lead to more particle hunting
If scientists find one new particle, supersymmetry means they’ll find many more. That’s exciting. “It’s not going to be just one new particle that we discovered, and yay!” Cranmer says. “We’re going to be finding new forces, or learn something really deep about the nature of space and time. Whatever it is, it’s going to be huge.”
Scenario 2: There are no new subatomic particles. Less exciting! But still interesting. And troubling.
The LHC is going to run for around another 20 years, at least. There’s a lot of time left to find new particles, even if there is no supersymmetry. “This is what always blows my mind,” Brock says. “We’ve only taken about 5 percent of the total planned data that the LHC is going to deliver until the middle 2020s.”
But the accelerator also might not find anything. If the new particles aren’t there to find, the LHC won’t find them. (Hence, the notion that physicists are looking for “what nature chose.”)
But again, this doesn’t represent a failure. It will actually yield new insights about the universe.
“It would be a profound discovery to find that we’re not going to see anything else,” Cranmer says.
1) For one, it would suggest that supersymmetry isn’t the answer
If supersymmetry is dead, then theoretical physicists will have to go back to the drawing board to figure out how to solve the mysteries left open by the standard model.
“If we’re all coming up empty, we would have to question our fundamental assumptions,” Sarah Demers, a Yale physicist, tells me. “Which is something we’re trying to do all the time, but that would really force us.”
2) The answers exist, but they exist in a different universe
If the LHC can’t find answers to questions like “why is the Higgs so light?” scientists might grow to accept a more out-of-the-box idea: the multiverse.
That’s the idea where there are tons of universes all existing parallel to one another. It could be that “in most of [the universes], the Higgs boson is really heavy, and in only in very unusual universes [like our own] is the Higgs boson so light that life can form,” Cranmer says.
Basically: On the scale of our single universe, it might not make sense for the Higgs to be light. But if you put it together with all the other possible universes, the math might check out.
There’s a problem with this theory, however: If heavier Higgs bosons exist in different universes, there’s no possible way to observe them. They’re in different universes!
“Which is why a lot of people hate it, because they consider it to be anti-science,” Cranmer says. “It might be impossible to test.”
3) The new subatomic particles do exist, but the LHC isn’t powerful enough to find them
In 20 years, if the LHC doesn’t find any new particles, there might be a simple reason: These particles are too heavy for the LHC to detect.
This is basic E=mc2 Einstein: The more energy in the particle accelerator, the heavier the particles it can create. The LHC is the most powerful particle accelerator in the history of man, but even it has its limits.
So what will physicists do? Build an even bigger, even smashier particle collider? That’s an option. There are currently preliminary plans in China for a collider double the size of the LHC.
Building a bigger collider might be a harder sell for international funding agencies. The LHC was funded in part because of the quest to confirm the Higgs. Will governments really spend billions on a machine that may not yield epic insights?
“Maybe we were blessed as a field that we always had a target or two to shoot for. We don’t have that anymore,” says Markus Klute, an MIT physicist stationed at CERN in Europe. “It’s easier to explain to the funding agencies specifically that there’s a specific endpoint.”
The LHC will keep running for the foreseeable future. But it could prove a harder task to make the case to build a new collider.
Either way, these are exciting times for physics
“I think we have had a tendency to be prematurely depressed,” Demers says. “It’s never a step backward to learn something new,” even if the news is negative. “Ruling out ideas teaches us an incredible amount.”
And she says that even if the LHC can never find another particle, it can still produce meaningful insights. Already, her colleagues are using it to help determine why there’s so much more matter than antimatter in the universe. And she reminds me the LHC can still teach us more about the mysterious Higgs. We will be able to measure it to a more precise degree.
Brock, the MSU physicist, notes that since the 1960s, physicists have been chasing the standard model. Now they don’t quite know what they’re chasing. But they know it will change the world.
“I can’t honestly say in all those 40 years, I’ve been exploring,” Brock says. “I’ve been testing the standard model. The Higgs boson was the last missing piece. Now, we have to explore.”