The question — indeed the very existence — of dark matter is a hot topic these days in cutting-edge science. Dark matter is thought to be a ghostly cousin of ordinary matter that floats throughout the cosmos, adding to the gravitational tug that keeps galaxies and even clusters of galaxies bound together instead of literally scattering to the corners of the universe.
While dark matter is thought to exist, so much in fact that it appears to be five times more common than ordinary matter, it has never been observed. And that’s not for lack of trying. Experiments located deep underground sniff around for the ephemeral wind of dark matter that is thought to be passing through the solar system, while others peer into the heart of the Milky Way, looking for hints that dark matter particles are colliding together and signaling their existence.
But both of those attempts to study dark matter are purely observational approaches. To get the most precise characterization of the properties of dark matter, scientists would like to make it here on Earth. Assuming that dark matter interacts even very weakly with ordinary matter, there is every reason to hope that we can make and discover dark matter in the same way we did the Higgs boson and the top quark — by converting the energy carried in the beams of particle accelerators into the mass of particles that we can detect. Einstein’s famous equation E = mc2 has helped us in the past, and maybe it will be our path to finding and characterizing dark matter.
Scientists in the CMS collaboration use the super-powerful beams of the LHC to look for this hypothetical substance. There are many possible approaches, but one recent study collided pairs of protons and searched for pairs of jets of particles exiting the collision. They even invented new measurement variables, called razor variables, to enhance their capability to identify events containing dark matter. The variables both estimated the mass of the physical process that occurred in each collision and identified the topology, which means the orientation of the particles as they exit the interaction. In addition, this study made a special attempt to identify jets containing bottom quarks. This choice was made because observations of an excess of gamma rays emerging from the center of the Milky Way have been explained by a proposal involving dark matter interactions and bottom quarks.
The CMS measurements were consistent with the Standard Model and therefore did not show hints of dark matter. While a discovery would have been more exciting, this technique is a new one and shows great promise as the LHC moves forward and collects more data and at higher collision energies than was ever before possible. The end of this story hasn’t been told. We’re only just getting started.