Embracing complexity

While all particle physics is complicated, certain processes are particularly so. Studying the most common kinds of supersymmetric events predicted to be found at the LHC is like putting together an especially complex puzzle.

Sometimes you just gotta do things the hard way. Today’s article describes an attempt to do just that.

Supersymmetry is a principle that one can incorporate into new or existing theories. At its core, it is nothing more than the rule that matter particles (fermions) and force carrying particles (bosons) in the theory’s equations are indistinguishable from each other. While no evidence for supersymmetry has been observed, it is a very popular idea since it can fill in many physics theory holes. It can provide an explanation for why the mass of the Higgs boson is so small, explain the identity of dark matter, and show how the strong, weak and electromagnetic forces are really different faces of the same thing. Supersymmetry really is a versatile idea.

Supersymmetry itself isn’t a theory. However, many theories include it, and each one makes a very different prediction. The one universal prediction in such theories is that for every known particle, there exists a corresponding, as-yet-undiscovered partner that differs only in its spin. For example, for every known fermion (quark and lepton), there is a cousin supersymmetric boson (squark and slepton). Similarly, for every known boson (photon, W and Z boson, gluon and Higgs), there is a cousin supersymmetric fermion (photino, wino, zino, gluino and higgsino). If supersymmetry is right, we have to find these cousin particles.

Physicists have been looking for these supersymmetric particles for decades now, with no luck at all. As with most searches for new phenomena, scientists looked for the easier signatures first. Because supersymmetric sleptons decay into (among other things) ordinary leptons, and because ordinary leptons are easy to identify, many early searches focused on sleptons.

However, the LHC collides not leptons but protons. That means that, if supersymmetry is real, the most likely supersymmetric particles to come out of the LHC are squarks and gluinos. These particles would decay eventually into more common quarks and gluons, which would make jets — little shotgun-like blasts of particles — in the detector, and often many jets hit the detector simultaneously.

In one particularly challenging situation, a pair of gluinos would each make a top quark-antiquark pair. Since each top quark can decay into three lighter quarks, such an event would have 12 jets. Visually, you can imagine such a collision as 12 randomly oriented shotguns going off simultaneously, with the pellets hitting the detector. Events like these are horribly, horribly complicated.

Complicated though these events may be, CMS scientists went looking for them, and find them they did. The problem is that messy events like these can be created by known physics, and new physics requires that researchers find an excess of events with the above-mentioned characteristics. This analysis is similar to a previous Frontier Science Result but uses collisions in the LHC with higher energy than used in the earlier analysis.

The result of this search was that no evidence was observed for the extra events predicted by supersymmetry. Thus CMS scientists were able to set stringent limits on the mass of the predicted supersymmetric particles. Now it’s back to the drawing board.

Don Lincoln

These US CMS scientists contributed to this analysis. Other contributors (not shown) are: Anwar Bhatti (Rockefeller), Rick Cavanaugh (UIC and Fermilab), Jay Dittmann (Baylor), Daniel Elvira (Fermilab), Bill Gary (UC Riverside), Gheorghe Lungu (Rockefeller), Steve Mrenna (Fermilab) and Jorge Rodriguez (FIU).
These US CMS scientists have been and are now actively working on the offline jet reconstruction (jet algorithms, jet energy corrections, jet energy resolutions, jet software in CMSSW). This effort was a crucial component of this analysis.