Particle masses (on the x-axis) where the solid black line is below the red one are in conflict with the new CMS result and therefore excluded. It is the first time an LHC experiment can constrain this model for dark matter.
What is dark matter made of, the elusive form of matter that accounts for 84.5% of the total mass in the universe? It is fair to say that we do not know, but particle physicists all over the world gather in Venice to discuss exciting new results that successively constrain the properties of the dark matter particles and which will eventually corner it - with a discovery or by excluding it. Until then, we learn from each new hypothesis that is put to a test.
For example, if dark matter is made of particles that interact with the top quarks in the standard model, we can expect to find new hints for the puzzle in the largest dataset collected so far by the CMS experiment in 2016 at the LHC. The experimental signature will mimic that of top quarks, however, the dark matter particles will not interact with the detector material and therefore imbalance the collision events. Such indirect signatures are delicate, because all detector components need to be perfectly understood until any claim for extra invisible particles in the events can be trusted.
Nevertheless, the CMS collaboration has understood the data well enough and hence released new results for the EPS conference currently held in Venice. It is the first LHC result on dark matter coupling to top quarks that is sensitive enough to confront theoretical predictions (See picture). No signs for dark matter were found in this first look, but the hunt continues. Many more models are still allowed by LHC data and astrophysical observations of dark matter do not concretely predict collider signatures. The next step is to combine results from searches different event configurations and include the data collected in 2017. The hiding places for dark matter particles will become fewer and fewer.