Search for new phenomena in jets plus missing transverse energy final states at the LHC

This Thesis presents a search for new phenomena in $pp$ collisions at $\sqrt{s} = 8$ TeV recorded with the ATLAS detector at the LHC collider. The final state under investigation is defined by the presence of a very energetic jet, large missing transverse energy, a maximum of three reconstructed jets, and no reconstructed leptons, leading to a monojet-like configuration. The monojet final state constitutes a very clean and distinctive signature for new physics processes. After the discovery of the Higgs and the constraints on the masses of first and second generation squarks and gluinos up to the TeV scale, much attention has been put to searches for third generation squarks. These searches are motivated by naturalness arguments, which point to relatively light stops and sbottoms, and therefore allowing their production at the LHC. The monojet analysis is interpreted in terms of pair production of stops and sbottoms, and in terms of inclusive searches for pair production of squarks, and gluinos. In particular, this final state has large sensitivity to supersymmetric models involving a very compressed mass spectra of the superpartners in the final state (also known as "compressed scenarios"). Monojet final states have been used traditionally to search for large extra dimensions and the production of Dark Matter (DM). In this context, limits on the parameters of models involving the direct production of Kaluza- Klein towers of gravitons, neutralinos, or light gravitinos in gauge mediated supersymmetry breaking scenarios, are also considered.