The Hunt for Dark Matter and dark energy is on at the LHC

With the discovery of the Higgs Boson under their belt, researchers at CERN’s Large Hadron Collider have turned their sights towards two more elusive targets: dark matter and dark energy.

An aerial view of CERN — home of the LHC, which may be key to the discovery of ‘dark particles’ (CERN)

Dark matter and dark energy — the invisible stuff that binds the galaxy together — make up the majority of ‘stuff’ in the Universe. But despite this abundance — no one has been able to directly detect either.

As LianTao Wang, a professor of physics who studies how to find signals in large particle accelerators like the LHC says: “We know for sure there’s a dark world, and there’s more energy in it than there is in ours.”

Wang, and a team of scientists from the University of Chicago and Fermilab, believe they may be able to lead us to its tracks. Their study — published in Physical Review Letters this month — details a new and innovative method for stalking dark matter in the LHC by exploiting a potential particle’s slightly slower speed.

While dark matter and dark energy — make up more than 95 per cent of the universe, scientists only know it exists from its effects. As an example, we know there’s dark matter because we can the gravitational effect it generates preventing galaxies from flying apart.

Theorists suspect that there may be one particular kind of dark particle that occasionally interacts with normal matter. The particle in question should be both heavier and longer-lived — about 1/10 of second — than other known particles. This particle can get caught up in the collisions of protons that the LHC is constantly creating and measuring — albeit incredibly rarely.

Wang believes that the key to discovering more about dark matter and dark energy may be linked to the Higgs Boson, discovered in 2012. He says: “One particularly interesting possibility is that these long-lived dark particles are coupled to the Higgs boson in some fashion — that the Higgs is actually a portal to the dark world.

“It’s possible that the Higgs could actually decay into these long-lived particles.”

The only problem is sorting out these events from the rest. With more than a billion collisions per second in the 27-kilometre LHC — each one sending subatomic shrapnel spraying in all directions — ‘needle in a haystack’ is an understatement.

Wang, UChicago postdoctoral fellow Jia Liu and Fermilab scientist Zhen Liu propose a new way to search by exploiting one particular aspect of such a dark particle. Liu explains: “If it’s that heavy, it costs energy to produce, so its momentum would not be large — it would move more slowly than the speed of light.”

That time delay would set it apart from all the rest of the normal particles. Scientists would only need to tweak the system to look for particles that are produced and then decay a bit more slowly than everything else.

The difference is on the order of a nanosecond — a billionth of a second — or smaller. Fortunately, the LHC already has detectors sophisticated enough to catch this difference — a recent study using data collected from the last run and found the method should work. Also, the detectors are set to get even more sensitive as part of the upgrade that is currently underway.

Liu continues: “We anticipate this method will increase our sensitivity to long-lived dark particles by more than an order of magnitude — while using capabilities we already have at the LHC.”

Experimentalists are already working to build the trap.

When the LHC turns back on in 2021, after boosting its luminosity by tenfold, all three of the major detectors will be implementing the new system, the scientists said.

Liu concludes: “We think it has great potential for discovery.

“If the particle is there, we just have to find a way to dig it out. Usually, the key is finding the question to ask.”

Original research: Enhancing Long-Lived Particles Searches at the LHC with Precision Timing Information, Jia Liu et al, Physical Review Letters(2019). DOI: 10.1103/PhysRevLett.122.131801