First supersymmetry results from the LHC

Collision events in the figure above are arranged according to degree of momentum imbalance: balanced events pile up on the left while unbalanced events are put on the right. One of the supersymmetric models that CMS ruled out is shown in yellow: if this model were true, the physicists would have observed twice as many unbalanced events (See figure 2 in the paper for details).

Editor’s note: This is the first CMS Result written by Jim Pivarski, a CMS collaborator. Fermilab Today will now publish two CMS Results a month. Pivarski will write one of these.

What’s so super about supersymmetry? A lot of things, actually. Originally proposed to relate the particles that make up matter (fermions) to the particles that make up forces (bosons), the theory would also solve many other mysteries.

Supersymmetry predicts an energy scale at which fundamental forces combine (grand unification). It also explains why the Higgs boson mass would need to be much lower than this energy scale for the Higgs mechanism (the mechanism by which particles obtain mass) to work. Additionally, it provides a natural candidate for dark matter, the substance scientists believe is holding the universe together. As a principle of nature, supersymmetry seems to have everything going for it except evidence.

That’s why searches for supersymmetry are among the most anticipated experiments that physicists will perform at the LHC. Though the veil will be drawn back gradually as the dataset grows, the first results on supersymmetry from the LHC were recently posted by CMS.

There are many ways to search for supersymmetry; this week’s featured CMS result looks for two generic features in the aftermath of the proton collisions: unusually energetic jets of debris and invisible particles. The collaboration was able to rule out one model of supersymmetry.

The invisible particle that CMS physicists are searching for is the dark matter candidate predicted by supersymmetry, which is undetectable by definition. Supersymmetric interactions would be incredibly rare, too, so this search would be like looking for a needle in a haystack, except that the needle is invisible.

Fortunately, physicists have a way of distinguishing events with invisible particles from events with only visible particles. Debris from any collision must be balanced in momentum— the speed and mass of debris flying out on one side must be balanced by the speed and mass of debris on the other. An unseen particle would look like a momentum imbalance because we don’t see the particle that balances the momentum. If we arrange the data by the degree of imbalance, putting the hundreds of thousands of well-balanced events on the left and the handful of unbalanced events on the right, we get the plot shown above. The few unbalanced events on the right do contain invisible particles, but only neutrinos, which are a well-known part of the Standard Model. Some supersymmetric models are ruled out by these data, while many more await the additional techniques and larger datasets that are on their way.

Jim Pivarski

These physicists contributed to this first CMS paper on supersymmetry results.
The efforts of these U.S. graduate students in calibrating missing energy made it possible to cleanly identify events with invisible particles.