Beyond Standard Model searches in jets plus missing transverse energy final states with the ATLAS experiment at the LHC

Dark Matter (DM) is currently one of the most challenging goal in the LHC programme: if DM exists it can be pair-produced in proton-proton collisions. Since its weakly-interacting nature, final signatures with high missing momentum and Standard Model (SM) particles are employed in these searches. This thesis presents results on signatures involving bottom quarks in final states, described in models where DM production occurs via massive spin-0 mediators (scalar or pseudoscalar) with a coupling to SM particles proportional to their masses. These collider searches provide an interesting complementarity to DM direct and indirect detection experiments, covering the parameter space with low DM masses. The results shown in the thesis are obtained on the data collected by the ATLAS experiment in 2015 and 2016.