It’s been calculated that dark matter is around five times more common than regular matter – and yet, we still haven’t directly detected it. Many different types of experiments are trying to find it, and now CERN has joined the hunt, testing whether the famous Higgs boson could decay into dark matter.
The Large Hadron Collider (LHC) probes the secrets of the universe by smashing particles together at incredible speeds. In doing so, new and exotic types of particles are often created, giving scientists a fleeting opportunity to study things that would be virtually impossible to come across naturally.
One of the most groundbreaking discoveries made by the LHC is the Higgs boson, in 2012. This long-hypothesized particle was the last remaining puzzle piece in the Standard Model of particle physics, believed to create the means by which other elementary particles gain mass.
Since its discovery, scientists have used the Higgs boson as a tool to probe other particle physics mysteries. The boson quickly decays into other particles, and it’s predicted that some may not be directly detectable by the equipment.
But in this case, a non-detection is more exciting than a detection would be. Some types of particles don’t interact with regular matter very much, so if the Higgs produces such particles, they would kind of just float away, ignoring the walls of the collider. The scientists would then notice energy missing from the debris, and be able to infer some things about the “invisible” particles.
Only one invisible decay product fits the Standard Model – if the Higgs decays into four neutrinos – but that’s extremely unlikely, at a likelihood of about 0.1 percent. That means that if non-detections happen with any regularity, we might have stumbled onto new particles.
And one of those invisible particles could be dark matter. This strange stuff is said to permeate the universe, effectively holding it all together – and yet it remains ever elusive. Its gravitational influence is clear, but it appears to not reflect or emit any kind of light.
Given the Higgs boson’s role in “giving” particles mass, and dark matter only being detectable through its mass, the two should interact with each other. So for the new study, scientists with the ATLAS collaboration at CERN set out to check whether the Higgs boson may be decaying into dark matter.
The team examined the entire dataset from the second run of the LHC, which took place between 2015 and 2018. That’s about 100 quadrillion collisions, for those keeping score. And in all that data, the researchers found no excess of invisible particle events over the background number that you’d expect from known processes in the Standard Model.
From that, the team was able to narrow down the upper limit for how often the Higgs boson decays into invisible particles – no more than 13 percent of the time. That may still sound like a lot, but it’s coming down from previous models that suggested it could happen as often as 30 percent of the time.
The researchers say that even though they didn’t find any signs of dark matter this time, the work still helps put constraints on the properties of the stuff. Between this and the many other experiments attempting to hunt it down, dark matter may be running out of places to hide. Or perhaps, we're just getting closer to realizing it doesn't exist, and our models need to be tweaked. Either way, the search continues.
The research was presented at an LHC seminar, and published online.
Sources: CERN, ATLAS collaboration
