Searches for dark matter and lepton-jets with the ATLAS detector

A search is performed for Higgs boson decaying to hidden sector and subsequently into highly collimated jets of electrons, known as electron-jets. The hidden sectors and lepton-jets are predicted in the new theories of dark matter, motivated by the recent proliferation of astrophysical anomalies, observed by cosmic-ray and dark matter direct-detection experiments. The search is performed with 2.04 $fb^{−1}$ of data collected in 2011 with the ATLAS detector at the LHC in proton--proton collisions at $\sqrt{s}$=7 TeV. To select electron jets, various jet identification parameters are exploited, based on the combined calorimeter and tracking information, providing good discrimination against background sources, and avoiding sensitivity to the detailed topology of the electrons within the electron-jet. Background contamination in the signal region is determined using a completely data-driven technique, and is cross-checked with two alternate methods of background evaluation. Systematic uncertainties for the signal selection efficiency are estimated using data-driven methods, by examining the electrons from $Z\to ee$ decay and photons that have converted in the detector into highly collimated electron-positron pairs. One event satisfying the signal selection criteria is observed after the final selection, which is consistent with the expected background rate. Consequently, 95\% confidence level limits are set on the Higgs boson production cross section times the branching ratio into electron-jets, assuming the two benchmark models of a hidden sector and the condition of a dark photon mass below 210 MeV. The prospects are discussed for further lepton-jet searches, using the full data set collected by the ATLAS detector in 2012 at $\sqrt{s}$=8 TeV. The work presented in the thesis has been published as a scientific article on behalf of the ATLAS collaboration.