Search for Scalar Top Quarks and Higgsino-Like Neutralinos

This book reports a last-gasp effort to search for theoretically "natural" SUSY. The data collected with the ATLAS detector in 2012 corresponding to 20/fb of an integrated luminosity have been analyzed for stop pair production in proton-proton collisions at a center-of-mass energy of 8 TeV at the LHC in the scenario of higgsino-like neutralino.§§The author focuses on stop decaying into bottom quark and chargino. In the scenario of higgsino-like neutralino, mass difference between chargino and neutralino is expected to be small and observable final-state particles are likely to have low-momentum (soft).The author develops a dedicated analysis with a soft lepton as a probe of particles from chargino decay, which suppresses the large amount of backgrounds. As a result of the analysis, no significant SUSY signal is observed. The 95% confidence-level exclusion limits are set to masses of stop and neutralino assuming m = 20 GeV. The region with M (the mass difference between stop and neutralino) 120 GeV up-to 600 GeV of stop mass with neutralino mass of less than 280 GeV. The author clearly shows very little remaining parameter spaces for light stop (e.g. topology of stop decay is extremely similar to the SM top quark) by combining his results and previous ATLAS analyses. His result gives a strong co§nstraint to searches for new physics in the future.§§