[en] Cosmological and astrophysical observations indicate the existence of Dark Matter. Many beyond Standard Model theories predict associated production of Dark Matter particles with a Higgs boson at energies that can be probed at collider experiments. The signature of the presented analysis consists of a b anti b pair from a Higgs boson decay recoiling against missing transverse momentum from Dark Matter particles, h(→b anti b)+E^miss_T. The presented h(→b anti b)+E^miss_T search is performed using proton-proton collisions at a centre-of-mass energy of 13 TeV with an integrated luminosity of 36.1 fb^-1 recorded with the ATLAS detector at the Large Hadron Collider. Several new refined event selections were developed, which increase considerably the experimental sensitivity of the search to beyond Standard Model theories resulting in a h(→b anti b)+E^miss_T signature. Discovery significance along with its gains from the new refined event selections are presented. The observed data are found to be in agreement with the Standard Model predictions. Upper limits on the production cross-section of h+E^miss_T events times the h→b anti b branching ratio in a two-Higgs-doublet model with an extra Z^' boson scenario are presented at 95% confidence level. The improvement of the limits from the new refined event selections is quantified. A new topological algorithm is suggested for future analyses and its associated increase in experimental sensitivity is also presented.

[en] The recent discovery of the Higgs boson motivates collider-based searches for new physics models, where Dark Matter is produced in association with a Higgs boson. The signature of this search is a pair of b quarks from the Higgs boson decay, recoiling against missing transverse momentum from Dark Matter particle(s). To increase the sensitivity of the search, various studies are performed and applied in the analysis. The dominant background from t anti t production is reduced by about 70% for a signal loss of about 10%; as a result the irreducible background from Z(νν)+jets production became the dominant background contribution. These improvements result in a substantial increase of sensitivity of up to 50%. In case of no significant excess, limits at 95% confidence level over a parameter phase space of signal models will be provided using 36 fb-1 of data.

[en] The Level-1 Calorimeter Trigger is a crucial part of the ATLAS trigger effort to select only relevant physics events out of the large number of interactions at the LHC. In Run II, in which the LHC will double the centre-of-mass energy and further increase the instantaneous luminosity, pile-up is a limiting key factor for triggering and reconstruction of relevant events. The upgraded L1Calo Multi-Chip-Modules (nMCM) will address this problem by applying dynamic pile-up corrections in real-time, of which a precise simulation is crucial for physics analysis. Therefore pile-up effects are studied in order to provide a predictable parametrised baseline correction for the Monte Carlo simulation. Physics validation plots, such as trigger rates and turn-on curves are laid out.

[en] The increased centre-of-mass energy of 13 TeV in Run 2 of the LHC provides an excellent ground to search for resonances in the diboson invariant mass spectrum above 1 TeV, where the sensitivity in Run 1 was limited by data statistics. Studies of such resonances decaying into WZ or ZZ boson pairs, where one Z decays into a pair of charged leptons and the other W or Z decays hadronically, are presented. In the kinematic boosted regime considered, the hadronic decay products of W or Z bosons form a single large-R jet, which is identified as a boson jet using jet substructure properties. The invariant mass distribution of the system of such jet and highly energetic lepton pair from a Z boson decay is used as discriminator for the diboson resonance search. Studies on further kinematic distributions for SM backgrounds and various beyond-SM signal models are investigated in order to potentially improve the background rejection.