Search for exotic particles with correlated leptons in the final state with the ATLAS detector

The Large Hadron Collider (LHC) – the world’s highest-energy particle accelerator at CERN, Geneva is pushing High Energy Physics into a brand new territory. It presents an unprecedented opportunity to probe the realm of New Physics in the TeV region, shed light on some of the core unresolved issues of Particle Physics, and go further in search of understanding. These issues include the possible constituent of dark matter, the possible existence of extra gauge groups. The current era may bring discoveries of unprecedented magnitude, delivering validation or extreme disappointment to the Physics theories of the previous decades. LHC aims to produce particles in the center-of-mass energy up to 14 TeV – above those that are already known. At the same time, there are exciting possibilities for new physics in the low-mass range that may have gone unnoticed until now. Many models of new physics beyond the Standard Model contain a ‘light’ hidden sector (dark sector) with a colorful spectrum of new particles, which can be probed at the LHC. Recently, it has been shown that the hidden sector can give rise to unique signatures at colliders when the mass scale in the hidden sector is well below a TeV, as in Hidden Valleys, Stueckelberg extensions, and Unparticle models. These physics models produce unique signatures of collimated leptons at the LHC energies. By studying these ephemeral particles we hope to trace the history of the Universe. Our present theories lead us to believe that there is something new just around the corner, which should be accessible at the energies made available at the LHC. In the first part of this work, I consider the interesting astrophysical evidence that motivates a search for lepton jets and focuses our attention on a Hidden Valleys model with a GeV-scale dark sector that produces this exciting signature. Results from the recent underground experiments are also considered. In the next part of the thesis, I focus on the search strategy using the A Toroidal LHC ApparatuS (ATLAS) detector for evidence of dark matter in events containing prompt lepton-jets, formed by electrons or muons, produced in 7 TeV proton-proton collisions at the LHC in the association with W and Z boson. A novel lepton-jet algorithm is employed in order to find any evidence of an excess of such events with respect to the rate predicted by the Standard Model and interpret the non-null result in terms of recently developed theoretical models of dark matter. In doing so, I severely constrain the theoretical model and its parameters with the actual data from the LHC. In addition, I report the first evidence of production of associated W/Z + lepton jets, collimated electrons or muons (in this thesis prompt lepton-jets), a new physical process at the current energy frontier. However, before jumping to conclusions, one must examine the accuracy of theoretical predictions. The value of the present thesis for the future is in its careful presentation of the techniques needed to make these lepton-jet measurements, in particular in understanding that we have a realistic evaluation of experimental uncertainties and thus a reliable estimate of the significance of new effects.