Supersymmetry (SUSY) is an extension of the Standard Model which was introduced to cancel out some divergencies in quantum-mechanical calculations of the Higgs boson mass (mass hierarchy problem). SUSY relates fermions and bosons by transforming fundamental particles into "superpartners" which differ from the original particles by 1/2 unit of spin. The spin-0 partners of quarks and leptons are called squarks and sleptons (short for scalar quarks and scalar leptons). The spin-1/2 partners of gluons are called gluinos, and so on. If SUSY were an exact symmetry, a particle and its superpartner would have the same mass. This is obviously excluded experimentally, and thus SUSY must be a broken symmetry. No superpartners have been found so far. The lightest SUSY particles are believed to be in the mass range ~ 1 TeV or below.

SUSY also provides a framework for attempts to unify electromagnetic, weak, strong, and gravitational interactions. It turns out that the superpartners modify the observed strengths of the electromagnetic, weak, and strong interactions in such a way that their inherent strengths become equal at very short distances (grand unification). It would be really neat if this was indeed the case!

The "minimal" supersymmetric model (MSSM) assumes that all particles carry a conserved multiplicative quantum number, called R-parity, which is +1 for ordinary particles and -1 for superpartners. This would imply that SUSY particles are produced in pairs, and that there is a lightest supersymmetric particle (LSP) which is absolutely stable. The LSP's would have to be neutral and weakly interacting, i.e. much like heavy neutrinos. They would not interact in a detector, leading to apparent missing energy. The LSP's are natural candidates for dark matter, the stuff that supposedly holds galaxies together and accounts for most of the mass in the universe.

     SUSY signal

If SUSY particles exist, the decays of pair-produced sparticles would often result in final states with two or more jets and with two LSP's which would escape the detector. A "classic" SUSY signal would then be large missing energy in association with jets and/or isolated leptons. We will search for such a signal using PDE and dPDE. Numerous other signatures are also possible, of course, and we'll be on the lookout for any kind of new physics.