Desperately seeking SUSY
These Feynman diagrams show two possible ways that three leptons can be made in a supersymmetric model. A chargino and the second heaviest neutralino eventually decay into three charged leptons, a neutrino and two of the lightest neutralinos.
Many of the most intriguing possible extensions to the Standard Model of particle physics include the principle of supersymmetry (SUSY). All models incorporating SUSY predict particles beyond those we have already observed. These SUSY particles are generally unstable, except for the lightest supersymmetric particle (LSP). Since supersymmetry is preserved in particle decay, all decaying SUSY particles must have a supersymmetric daughter particle. However, since the LSP is by definition the lightest supersymmetric particle, there is no particle into which it can decay. We know that the LSP must be electrically neutral; otherwise we would have observed it by now. The LSP is the leading candidate for dark matter.
Scientists from the DZero experiment have just completed a study searching for events that decay into LSPs. They searched for events in which a chargino and the second-lightest neutralino are created, concentrating on ways they can decay that are specific and easy to identify. These particles are the SUSY partners of the Standard Model force-carrying particles and Higgs boson. In these decay modes, the SUSY particles decay only into leptons (three charged and one a neutrino) and LSPs. Both the neutrino and LSPs escape detection entirely.
Because there are three types of electrically-charged leptons (electrons, muons and taus), which scientists expect to play a role in this type of process, DZero collaborators searched for all physically-possible combinations of leptons in events that also had a large amount of missing energy. The experimenters found the number of events predicted by the Standard Model.
Since the measurement found no events beyond those predicted by the Standard Model, it is possible to rule out certain models that incorporate the principle of supersymmetry. For those models that cannot be completely ruled out, certain parameters for that model can be excluded. While supersymmetry has not been discovered, we now know more about what isn't true. With extra data expected over the next couple of years, we will continue to explore what events that have three charged leptons and missing energy can tell us about supersymmetry.
More information on these analyses can be found here.
This measurement rules out the green region for a particular SUSY model called mSUGRA. Earlier limits shown were set by the LEP experiments (gray) and the CDF experiment (black line). This new measurement is a significant improvement. [A version with more technical information is available in the links.]
These scientists played a crucial role in performing this analysis.
The tracking algorithms group studies the performance of the tracks used in this analysis and pursues improvements that would benefit future results.
-- Don Lincoln