Abstract

Despite the great phenomenological successes of the Standard Model (SM), the question whether the SM particles are the only dynamical degrees of freedom within the electroweak energy range is far from established. In fact, there are now very strong indications that the universe is filled with dark matter (DM), so there should be at least one new electrically neutral colorless particle, possibly lighter than the electroweak scale. With this in mind, we construct the most general basis of operators parametrizing a low-scale departure from the SM particle content. As an example application, the resulting set of operators is used to systematically investigate the discovery potential of rare Flavour Changing Neutral Current (FCNC) decays of the K and B mesons with missing energy in the final state. In similar vein we also demonstrate that the discovery channel for DM production at colliders could be through flavour violating interactions resulting in a novel “monotop” signature of a single top and large missing transverse energy. We finally investigate the impact of hypothetical new neutral light particles on the tiny width of a light Higgs boson. Reviewing the possible signatures in the Higgs decay modes with missing energy, in many cases simply preventing these modes from being dominant suffices to set tight model-independent constraints on the masses and couplings of the new light states. We then apply this analysis to Higgs portal models of DM, where DM is light enough to contribute to invisible Higgs decays. We show that DM can be a thermal relic only if there are additional light particles present with masses below a few 100 GeV.