The search for the dark side of the Universe
After the discovery of the Higgs boson in 2012, the next mission of the LHC, in addition to carry out a detailed study of the new particle, is to put light on the nature of the dark matter in the cosmos, and to determine whether super-symmetry and/or new extra spatial dimensions are realized in nature. Monojet final states (see Figure 1) have been traditionally studied in these contexts.Astronomical observations of galaxies and galaxy clusters have established the existence of an important non-baryonic dark matter (non-luminous) component in the universe. According to the current understanding of cosmology, the dark matter would contribute about 23% of the total mass-energy budget of the universe. The nature of the dark matter remains unknown. It is now commonly accepted that the dark matter should be made of weakly interacting massive particles (WIMPs) acting through gravitational or weak interactions. At the LHC, WIMPs could be produced in pairs leaving the experimental devices undetected. Such events could be identified by the presence of an energetic jet from initial-state radiation, leading again to a monojet signature. The LHC experiments have a unique sensitivity for dark matter candidates with masses below 4 GeV and are complementary to other dark matter searches. Models with large extra spatial dimensions aim to provide a solution to the mass hierarchy problem (related to the large difference between the electroweak unification scale ~102 GeV and the Planck scale ~1019 GeV) by postulating the presence of n extra dimensions such that the Planck scale in 4+n dimensions becomes naturally close to the electroweak scale. In these models, gravitons (the hypothesized particle acting as mediator of the gravitational interaction) are produced in association with a jet of hadrons leading to a monojet signature in the final state.The ATLAS collaboration have searched for new phenomena in monojet final states in the 2011 and 2012 data at 7 TeV and 8 TeV, respectively [1,2]. The data are in good agreement with the SM predictions (see Figure 2). The results have been translated into updated exclusion limits on the presence of large extra spatial dimensions and the production of WIMPs, and new limits on gravitino production (the supersymmetric partner of the graviton) resulting in the best lower bound to date on the gravitino mass.