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Image of Dark matter
NASA, ESA and R. Massey, California Institute of Technology

The search for dark matter at the LHC

When the Large Hadron Collider restarts, it will be an even more powerful dark-matter-hunting machine.

Experiments at the Large Hadron Collider specialize in the discovery of previously undetected particles. So of course they are on the hunt for one of physics’ most coveted prizes: dark matter.

Dark matter outnumbers visible matter—the kind of matter that makes up the Earth, the stars and all of us—five-to-one. Astronomers have seen its effects, and many experiments are working to catch dark matter particles as they pass through the Earth. So far, no one has found it for certain.

At the LHC, however, “we’re not depending on dark matter to find us,” says University of Bergen physicist Heidi Sandaker of the ATLAS experiment. “We’re trying to create it ourselves.”

To produce new particles in the LHC, scientists pump protons full of kinetic energy and then smash them together. The energy in a particle collision can transform into mass in the form of new particles, which then decay into less massive particles and eventually back into energy.

Particles collide in the LHC at a variety of different energies. The amount of energy that goes into a collision determines the kinds of particles that can come out. For example, a collision with about 125 giga-electronvolts of energy can create a Higgs boson. Physicists don’t yet know how much energy it would take to create a dark matter particle.

The CMS and ATLAS experiments at the LHC can detect particles such as Higgs bosons because they decay into other particles that leave signatures in particle detectors. Dark matter particles, however, could be a different story.

Dark matter seems to interact very weakly with ordinary matter. So a dark matter particle made in the LHC could pass right through a particle detector without leaving a trace. The only clue it would leave behind would be its missing energy.

A balanced amount of energy must come out of each side of a particle collision. When there’s an imbalance, scientists know something emerged from the collision that the detector could not see.

That’s how physicists discovered neutrinos, which rarely interact with other matter. They weren’t showing up in particle detectors, but their absence did throw off the balance of energy coming from collisions.

If LHC scientists notice that energy is going missing, they won’t know for sure that they’ve found dark matter. The hidden particles could be something else entirely. But knowing how much energy they had would make it easier to organize a search party.

“We need something to point us in the right direction,” Sandaker says. “Once we know where we’re going, we can take big steps.”

More than 100 physicists of the thousands on LHC experiments are hard at work on the search for dark matter at the LHC. And thousands of other physicists are looking for dark matter through other types of experiments.

“We need information from all different types of experiments,” says CERN theorist Gian Giudice. “If one experiment has a strong signal of dark matter, the other experiments need to have data to support it.”

Dark matter experiments are constantly growing and improving. When the LHC restarts at almost twice its previous luminosity in 2015, it will be another powerful member of the dark matter search team.


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