Searches for new physics involving massive invisible particles

One of the most obvious problems with the Standard Model is the lack of any explanation for dark matter: a form of matter predicted from astrophysical observations but not compatible with any known subatomic particle. Many theories of new physics predict dark matter candidates which could be produced in 13TeV collisions at the LHC. The primary experimental signature expected to accompany the production of a dark matter candidate is an imbalance in measurements of transverse momenta, referred to as $E_\mathrm{T}^\mathrm{miss}$. This thesis first covers the $E_\mathrm{T}^\mathrm{miss}$ trigger, a system responsible for selecting events with large $E_\mathrm{T}^\mathrm{miss}$ for later use in analyses and a vital component in many analyses. This signature suffers from very large backgrounds from QCD events with little true $E_\mathrm{T}^\mathrm{miss}$ and the high thresholds that would be required to control pass rates pose a risk to the efficiencies of analyses. A technique is presented that is shown to be able to reduce the rate by a factor of almost 4 while retaining high efficiencies in analysis signal regions. This thesis then documents a search for new physics using 36.1 fb$^{−1}$ of data collected by the ATLAS detector. The search targets final states with no leptons and very high jet multiplicities, ranging from $\geq$ 7 to $\geq$ 11 in the tightest signal regions, which are characteristic of long decay chains which can be predicted by certain new physics models, especially supersymmetry. By using this signature the analysis is able to use a looser selection on $E_\mathrm{T}^\mathrm{miss}$, gaining sensitivity to areas of model spaces which may not be accessible to analyses reliant on the $E_\mathrm{T}^\mathrm{miss}$ trigger. No significant deviation from the Standard Model is observed and the analysis sets strong limits on the masses of supersymmetric particles, excluding gluino masses up to 1.8TeV at the 95% confidence level.