When Paul Glaysher was approaching the end of his master's degree in 2012, everyone was talking about the Higgs boson. After two years of smashing protons together, CERN’s Large Hadron Collider was about to bring the mysterious particle---it helps explain how the universe got its mass---out of the theoretical realm. Students who landed a spot on a LHC research team had a chance to aid the biggest discovery in modern physics.
Glaysher bit. Then, two months before he started his PhD program with the University of Edinburgh’s CERN team, the LHC’s ATLAS and CMS experiments announced they had found the Higgs boson.
“It was a bit sad,” Glaysher says. “They waited 50 years to find it, and couldn’t wait the extra two months until I was part of the party.”
The three years that followed were a champagne-fueled hangover. Further data confirmed the Higgs discovery, and then the collider shut down for a two-year upgrade that more than doubled its particle-smashing power.
This summer, the LHC’s long-awaited restart came with a new promise: the chance to spot larger particles never before created in a human-made particle accelerator. Physicists believe they might glimpse the particles that make up dark matter—the unknown substance thought to make up a quarter of the universe—or even hints of other dimensions.
But despite the chance to study exotic new particles, Glaysher finds himself three and a half years later still studying the Higgs boson for the ATLAS experiment. Instead of spending his entire life chasing a specter, he’s examining something very real.
“Discovery---as exciting as it is, as Nobel-prize-generating as it may be---it’s actually just the first step,” Glaysher says. Theorists and other researchers at the collider agree with him. They think the Higgs could find them some new physics yet.
The Higgs was, in a way, the end of the line. At the heart of particle physics is what’s known as the Standard Model: a group of 17 elementary particles and the rules for how they should interact. Up until the Higgs discovery, physicists had observed 16 of these particles---and the field was desperate for a 17th that would push the model in new directions. But the Higgs turned out to be totally ordinary. It acted just like the model said it would act, obeyed every theorized rule.
The physicists, in other words, had done too good of a job with their predictions. “With the Higgs, we thought we had touched the bottom,” says Andre David, a CMS research physicist leading the effort to characterize the boson.
But with a newly-upgraded LHC, the ATLAS researchers---along with their counterparts at CMS and theoretical physicists---think the Higgs could yet lead to new insights about the nature of the world. “It’s like you’ve pierced the bottom and there must be a new bottom,” says David. “You just have to keep digging.”
So far, the scientists have some juicy theories for the Higgs. When you’re part of the process responsible for giving the universe mass, it’s likely you’re mixed up in some other interesting business. This month marked the completion of the LHC’s first round of observing proton collisions at a higher energy, and the data collected could play into some of physics’ biggest questions.
One of physicists’ greatest hopes for the new LHC is to not upend the Standard Model with new observations, but to extend it---by finding a partner for each of its 17 particles, validating a theory called supersymmetry. The Standard Model has a good explanation for the weak force, which allows one particle to turn into another. But physicists don’t know why the weak force is able to overpower gravity. Theories that explained that weirdness called for a Higgs with a huge mass, but the boson discovered in 2012 was relatively light. Observing supersymmetric particles that are also light could account for the discrepancies.
The Higgs could play a role in another unobserved particle, too: dark matter. It’s possible that the Higgs likes to turn into dark matter, or play some other role in its behavior. The LHC’s huge detectors measure what happens after collisions by detecting the energies of the resulting particles---and if part of the energy disappears, it could be a hint that dark matter appeared.
Then there’s matter and antimatter. While physicists have documented both, they aren’t sure what happened right after the Big Bang, when the universe was still made of equal parts matter and antimatter. The two have a tendency to destroy each other and turn into pure energy when they collide. But something caused an imbalance, leading to a modern universe that has far more matter than antimatter. Physicists believe the Higgs’ interactions with itself could have played a part---so they plan to study what happens when two Higgs meet in the LHC.
Finally, physicists believe they could find even more Higgs particles. One prominent theory holds that instead of one type of Higgs boson, there are five. Some of them are much heavier than the Higgs found in 2012, which means the LHC may not have been powerful enough to create them. Until now.
Those are all tantalizing possibilities. Still, the LHC's most intriguing results could come from seeing something that nobody predicted. The Higgs discovered in 2012 happens to have a mass that is suspiciously compatible with a huge number of particle interactions. That could be a coincidence. Or---hope beyond hope---it could lead to an underlying principle that physicists have missed until now. The end goal, as always, is to find a string that, when tugged, rings a clarion bell that draws physicists toward something new.
“It’s not guaranteed we have thought everything that can be thought of. It might just be we are not imaginative and creative enough,” David says. “We might be going in a direction where new physics could be subtle. It’s not like a new particle in your face.”
Scientists are, once again, starting the clock on a nebulous waiting period. Peter Higgs theorized the boson in 1964---and then the particle went unobserved for 50 years. CERN’s teams don’t know whether their current collider is powerful enough to provide the answers they seek, or if they will have to wait for a major energy upgrade years or even decades from now.
“We have lots of questions. We have indirect evidence that they might be answered by the experiments we’re doing,” says ATLAS researcher Elliot Lipeles. “We might come up empty, or we might find a shocking discovery literally next month.”
It’s tedious and generally unglamorous work. Glaysher’s group at the University of Edinburgh spends its days analyzing instances of the Higgs decaying into several specific types of particles. To uncover the Higgs’ secrets, it’s up to physicists to spend thousands of hours combing through the unfathomable number of particle collisions produced each day in the LHC. And if Glaysher is lucky, his team might be the one to find out physics has got the Higgs all wrong.
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