Searching for Top Squarks at the Large Hadron Collider

This dissertation describes the search strategies we developed for the lighter top squark (called stop, or ?t) at the Large Hadron Collider (LHC). When the lighter top squarks are produced from the cascade decay of gluino and squark, the analysis is performed in the stop-neutralino coannihilation region where stop decays into a charm quark and the stable lightest supersymmetric particle (called lightest neutralino, or X ?_1^0). We develop observables through the endpoint measurements to determine the stop masses.

When the lighter top squarks are produced from the direct production processes of stop pairs (?t?t*), three scenarios are investigated. In the fully hadronic final state scenario, we investigate the identification of stops which decay predominantly into a top quark and the stable lightest neutralino. A simple kinematical variable, M3, is used to reconstruct two top quarks which are pair-produced from the stops in the fully hadronic channel. We identify kinematical variables to reduce the standard model (SM) background. The expected mass reach of stop is shown at 8-TeV LHC (LHC8).

In the Bino-Higgsino dark matter scenario, the lightest neutralino is a mixture of Bino and Higgsino, satisfying the thermal dark matter relic density. Stop can decay into a top quark plus the second or third lightest neutralino (called X ?_2^0, X ?_3^0), and the second or third lightest neutralino can decay into 2 leptons plus the lightest neutralino via an intermediate slepton (?light selpton? case) or Z boson (?heavy slepton? case). The final states have at least 2 jets, 2 opposite-sign same flavor leptons and missing energy. The opposite-sign same flavor dilepton mass distribution after subtracting the opposite-sign different flavor distribution shows a clear edge in the case of light slepton. We also calculate the significance at LHC8 for discovering such a scenario in both light slepton case and heavy slepton case.

In the compressed scenario where the mass difference between stop and the lightest neutralino is approximately equal to the mass of the top quark, stop does either the two-body decay of a top quark, and the lightest neutralino (?two-body decay? case, when mass difference is slightly greater than the top quark mass), or the three-body decay of a bottom quark, a W boson and the lightest neutralino (?three-body decay? case, when the mass difference is smaller than the top quark mass). We perform the study for both two-body and three-body decay cases in the final state of two b-jets, one lepton, large missing energy, and two high energetic Vector Boson Fusion tagging jets with large separation in pseudo-rapidity, in opposite hemispheres, and with large dijet mass. The expected experiment discovery and exclusion limits of such a compressed scenario are shown at (14)-TeV LHC (LHC(14)) for both cases.