Physicists have a massive problem as Higgs boson refuses to misbehave

By Leah Crane

Max Brice/CERN

Physicists have spotted the Higgs boson performing a new trick, but one that brings us no closer to understanding the workings of fundamental particles.

The Higgs boson, discovered at the CERN particle physics laboratory near Geneva, Switzerland, in 2012, is the particle that gives all other fundamental particles mass, according to the standard model of particle physics. However, despite the work of thousands of researchers around the world, nobody has been able to figure out exactly how it does that or why some particles are more massive than others.

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

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

Muons are much less massive than the other types of particles we’ve seen the regular Higgs interact with, so the new discovery makes it more likely there is only one Higgs. That behaviour is exactly what we expect from the standard model. Adam Gibson-Even at Valparaiso University in Indiana, who wasn’t involved with this work, says that it is an instance of “Higgs boson, exactly as ordered”.

But that leaves the mystery of why particles have different masses completely unanswered. While this result may not be surprising, Gibson-Even says, it is somewhat frustrating because we know the standard model is incomplete – in addition to not explaining why particles have different masses, it also doesn’t account for dark matter or dark energy. Nevertheless, experimental results have been entirely in line with the model.

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

The next step, Blekman says, 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 those the LHC can provide, which is part of the argument for building a more powerful “Higgs factory” collider, she says.

“We have removed scenarios, but we don’t have an explanation yet,” says Blekman. “But this is what particle physics is about – we have tens of thousands of predictions and we have to eliminate them.”