The Standard Model of particle physics, a theory that describes much of the nature of particle interactions we’ve seen in experiments, is considered wildly successful. We’ve discovered every one of its predicted elementary particles. It is, nevertheless, far from a complete theory of the universe. For instance, the equations of the Standard Model don’t quite seem to work at the very high energies that were prevalent in the early universe; it doesn’t describe dark matter, which seems to be the most pervasive type of matter in the universe; and it doesn’t describe the force of gravity.
For decades now, physicists have been postulating theories to address these issues, and a common feature in many of these theories is an idea known as supersymmetry. In a nutshell, supersymmetry inherently relates two particle families to each other: fermions (the building blocks of matter) and bosons (the particles that deal out the forces of the universe). This results in each fermion having a boson as a partner and vice versa. Supersymmetric theories then describe how the forces act at very high energy, explain why the Higgs boson is the mass it is, provide a candidate for dark matter and even do your taxes for you. OK, maybe not that last one.
There is one major problem with these supersymmetric theories. Although they all predict the existence of partner particles, or “superpartners,” no superpartners have been discovered in over three decades of searching. Right off the bat we know that the superpartners must be more massive than their Standard Model brethren. Otherwise they would have been seen, just like the Standard Model particles we know and love.
The top quark, the most massive Standard Model particle, is predicted in most supersymmetric theories to have a partner, the “stop,” with a mass low enough to have been easily seen at the LHC. Some supersymmetric theories get around this contradiction by letting their superpartners violate a rule called “R-parity.”
In physics, switching the parity of a particle transforms it into its mirror image. Switching a particle’s R-parity can be thought of as a supersymmetry version of this, transforming an object into its superpartner. Thus, every particle, super or not, ends up with a value for R-parity: Standard Model particles get an R-parity of +1 while their superpartners get an R-parity of -1.
If physics processes conserve R-parity, you can never start out in a state with R-parity +1 and end up in a state with -1, or vice versa. You can think about an R-parity-conserving scenario as a basketball game: The players on the court at the end of a timeout are not necessarily the ones there at the beginning, but the number of players from each team (five) remains the same. An R-parity-violating scenario is one you might expect in a Harlem Globetrotter game: At the end of a timeout, you just might have no Washington Generals players on the court and 10 Globetrotters.
If R-parity is conserved, a supersymmetric particle can never decay into only normal particles. In this case, some fraction of the resulting supersymmetric particles escape the detector unseen, leaving an energy imbalance, or “missing” energy. If R-parity is violated, all bets are off, and our erstwhile unseen stop could decay entirely into known Standard Model particles.
The CMS collaboration recently performed a search for such decays, looking specifically for events with no missing energy that could have been caused by an R-parity-violating stop decaying into Standard Model particles. Using the full LHC data set accumulated through 2012, CMS physicists saw no obvious signs of such decays and established a lower bound of about 1 TeV for the mass of an R-parity-violating stop, approximately six times the mass of a top quark.
While no evidence for supersymmetry was found, it remains the most popular proposed extension to the Standard Model, and scientists are already looking for it using the recently upgraded LHC.