Ask a physicist whether the universe is as stable as it appears, and you're likely to get a long pause. When the Large Hadron Collider (LHC)—the world's most powerful particle accelerator, run by CERN in Europe—finishes its upgrade in about four years, scientists hope it will help answer that very question. But a team at the University of Michigan isn't waiting for the giant machine to be switched on. They've just announced the most sensitive method ever for spotting one of the rarest events in physics.
The event in question is a "double-Higgs" signature. The Higgs boson is the famous particle, first confirmed in 2012, that helps explain why other subatomic particles have mass. Now physicists want to know how the Higgs interacts with itself—a process that remains one of the biggest unsolved puzzles of the Standard Model, the best accounting we have of the universe's fundamental particles and forces. "It's kind of the big thing left on the table," said Tom Schwarz, the University of Michigan physics professor who has led this particular analysis for a decade.
"The analysis is the most sensitive in the world to this specific physics," added Greg Myers, a research fellow in the physics department. The new method, built with an advanced AI algorithm, is about 60 times more sensitive than the team's earliest attempts and about 65% better than its immediate predecessor. It was presented at the International Conference on High-Energy Physics in Natal, Brazil, in August.
Here's the challenge: every time the LHC smashes two beams of protons together at nearly the speed of light, it produces a shower of new particles. Higgs bosons are unstable and decay almost instantly into other particles, including electrons, their heavier cousins called muons, and massless particles of light known as photons. A double-Higgs event occurs only about once in every trillion collisions. Finding one is like spotting a single grain of sand on a beach—blindfolded.
The lucky break is that the LHC has already generated vast amounts of data, and the Michigan team, including doctoral student Tamas Baer, former postdoc Kevin Nelson, and recent graduate Dustyn Hofer, is using modern computational techniques to squeeze fresh insights from what's already been collected. The university has been involved in this branch of research since Bing Zhou and Jianming Qian began work in 2010.
Depending on what the team finds, their method could hint at physics beyond the Standard Model—or, more dramatically, suggest whether our universe is more or less stable than we think. As the upgraded LHC comes online, these sharper tools will be ready to make the most of every collision.