Lepton Jets in the ATLAS Experiment

Two searches for a light boson -- predicted by Hidden Valley supersymmetric models -- that decays into a final state of collimated electrons, denoted "lepton-jets." The first analysis uses 5 $\mathrm{fb}^{-1}$ of $\sqrt{s}$ = 7 TeV proton--proton collision data recorded by the ATLAS detector at the Large Hadron Collider to search for pairs of lepton-jets with two or more prompt electrons. This study finds no statistically significant deviation from the Standard Model prediction and places 95% confidence-level exclusion limits on the production cross section times branching ratio of light bosons for several parameter sets of a Hidden Valley model. The second analysis uses 20 $\mathrm{fb}^{-1}$ of $\sqrt{s}$ = 8 TeV proton--proton collision data recorded by the ATLAS detector at the Large Hadron Collider to search for pairs of lepton-jets with at least one electron-positron pair from which a displaced vertex can be reconstructed. No statistically significant deviation from the Standard Model prediction is found and 95% confidence-level exclusion limits are placed on the production cross section times branching ratio of light bosons in a Hidden Valley model.