Physicists have a huge problem because the Higgs boson refuses to misbehave

By Leah Crane

CMS experiment at CERN studies the Higgs boson

Max Brice / CERN

Physicists have spotted the Higgs boson performing a new trick, but one that doesn’t get us any closer to understanding how fundamental particles work.

The Higgs boson, discovered at CERN’s Particle Physics Laboratory near Geneva, Switzerland, in 2012, is the particle that gives mass to all other fundamental particles, according to the Standard Model of particle physics. However, despite the work of thousands of researchers around the world, no one has been able to understand exactly how this works or why some particles are more massive than others.

The only way to try to solve this problem is to observe how the Higgs interacts with other particles using the Large Hadron Collider (LHC). For the first time, the two major groups that use it – the CMS and ATLAS collaborations – have observed the Higgs decay into two muons, a kind of particle that we’ve never seen directly interact with before. Members of the collaborations presented this work at the Virtual International Conference on High Energy Physics.


Some researchers have suggested that particles have different masses because there is more than one type of Higgs boson, with each type of Higgs being coupled to a different mass range of other particles.

Muons are much less massive than the other particle types we’ve seen regular Higgs interact with, so the new finding makes it more likely that there is only one Higgs. This behavior is exactly what we would expect from the standard model. Adam Gibson-Even of the University of Valparaiso in Indiana, who was not involved in this work, says it is a “Higgs boson, exactly as ordered” instance.

But that leaves the mystery of why particles have different masses unanswered. While this result is not surprising, says Gibson-Even, it is somewhat frustrating because we know the Standard Model is incomplete – besides not explaining why particles have different masses, it also ignores dark matter or dark energy. However, the experimental results were fully consistent with the model.

“It’s a problem in the sense that we know that the Higgs boson as it is doesn’t explain these things,” says Freya Blekman, a CMS researcher at the Free University of Brussels, Belgium. If the same Higgs interacts with both muons and heavier particles, this is another way to solve the closed mass issue.

The next step, says Blekman, is to take even more precise measurements of the Higgs interacting with a range of different particles. Many of these measurements need to be more precise than what the LHC can provide, which is part of the argument for building a more powerful “Higgs factory” collider, she says.

“We’ve removed the scenarios, but we don’t have an explanation yet,” Blekman says. “But that’s what particle physics is – we have tens of thousands of predictions and we have to get them out of the way.”

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