The world around us seems concrete and apparent. However, it is in fact governed by what goes on at the sub-atomic level, hidden from immediate experience. We describe this microscopic world by a theory we call the Standard Model (SM). Since its formation in the 1970s, collecting the physics of electromagnetism, the weak and the strong forces, into one coherent theory, it has proven extremely successful. Experimental evidence however, tells us that the Universe consists of 26.8% of dark matter, a type of matter that the SM does not describe. If we had a theory that predicted a symmetry in Nature where each type of particle came in two versions, only separated by their spin, we would have a promising pool of candidate dark-matter particles. Supersymmetry (SUSY) is exactly this type of theory. It even addresses other fundamental short-comings of the SM. Why we have not earlier discovered such particles can be explained by the nature of symmetries, they are often broken, in this case leading to very massive copies of the SM particles. Which means we need large energies in order to discover them.
This thesis performs a search for SUSY in ATLAS, using 4.7 fb-1 of data collected in 2011, at center of mass energy of √s = 7 TeV, focusing on the direct production of charginos, neutralinos, and sleptons. On route to this search, I have worked on the careful estimation of part of the background of a SUSY signal, namely the fake lepton background, using the Matrix Method. Since we can only hope to find SUSY if we understand the SM and the experimental environment well, the prediction of the expected overall background, including the fake leptons, is essential. Two approaches are successfully followed for the fake lepton estimation, a semi data-driven one, as implemented in a publication in Physics Lett. B, and a new fully data-driven approach, with slight improvements, yielding compatible results with the first in the important SUSY signal region.
No SUSY signal was found, and we therefore use the lack of observation to set upper limits v on the cross sections and masses of new physics phenomena such as SUSY. The current √s = 7 TeV mass limits extend earlier SUSY limits.
An important part of my work has been dedicated to education and outreach. How can we let high school students follow along with the discoveries made at the LHC? I describe an educational tool we developed in the framework of the IPPOG International Masterclasses, namely the Z-path. Already, high school students around the world have themselves measured the properties of the Z boson, and discovered the Higgs boson. In the future we hope to discover SUSY, and also bring this discovery out to the public.