A little balance is hard to find

A schematic of the sort of collision debris that would hint at supersymmetry: a muon and an electron accompanied by jets and missing energy (invisible particles inferred from the lopsidedness of the rest of the debris). The vast majority of proton collisions produce only jets.

If you smash two protons together what would come out in the debris? In 99.9999 percent of the collisions, the result would be nothing but quarks and gluons, each of which then becomes a jet — a spray of particles made of more quarks and gluons. Though these complex events contain many particles, less than half of the known fundamental particles are represented.

Leptons such as electrons, muons and invisible neutrinos only emerge in the last 0.0001 percent of cases. However, detectors such as CMS are designed to be especially sensitive to leptons. They can even see invisible leptons by measuring of missing energy, the lopsidedness of all the rest of the debris.

Many theories of new physics, some incorporating the supersymmetry principle, result in more balanced event pictures such as the one shown above. This is not true of all supersymmetric theories, but considering the Standard Model’s million-to-one preference for jets over leptons, searching for events with one or two leptons eliminates huge backgrounds from interactions that are already understood. Squarks, sleptons, and other supersymmetric variants of known particles could lead to events containing a little of everything: jets, leptons and missing energy.

CMS physicists recently searched for this kind of particle signature: two or more electrons or muons, two or more jets, and missing energy [paper]. Despite the enormous number of proton collisions in the 2010 dataset, the Standard Model predicts that only about one of them would fit such a strikingly balanced menu. One event was found, confirming the old theory and ruling out some, but not all, supersymmetric models. As the 2011 data pour in, CMS physicists are using this technique to uncover yet more elusive theories.

-Jim Pivarski

These US-based physicists, working in cooperation with physicists across the world, contributed to this analysis.
The CMS forward hadron calorimeter is needed to identify jets close to the beampipe and to measure missing energy more accurately. Pictured above are some of the physicists who recently completed an important upgrade of that system.