Searches for direct pair production of third generation squarks, and dark matter, in final states containing $b-$jets and $E_{\mathrm{T}}^{\mathrm{miss}}$ using the ATLAS detector at the LHC

This thesis documents the results of analyses performed searching for BSM processes in final states consisting of $b-$tagged jets and missing transverse energy (${E_{\mathrm{T}}^{\mathrm{miss}}}$) using the ATLAS detector at the Large Hadron Collider. The results of searches for the direct pair production of bottom squarks decaying via ${\tilde{b}} \rightarrow b + {\tilde{\chi}_{1}^{0}}$ are presented. The first search used 3.2 fb$^{-1}$ of Run 2 data at a centre-of-mass energy ($\sqrt{s}$) of 13 TeV and placed exclusion limits on sbottom masses up to 840 GeV for massless neutralinos. The updated search uses a total integrated luminosity of 36.1 fb$^{-1}$ and provides sensitivity to heavier sbottom masses, excluding sbottom masses up to 950 GeV for $m_{{\tilde{\chi}_{1}^{0}}} = 1$ GeV. Two searches for the pair production of third generation squarks decaying asymmetrically in the one-lepton 2-$b-$jets and ${E_{\mathrm{T}}^{\mathrm{miss}}}$ final state are presented. The Run 1 search, using an integrated luminosity of 20.3 fb$^{-1}$ collected at $\sqrt{s}=8$ TeV, placed exclusion limits in pMSSM scenarios with $m_{Ql3}$ excluded between 340 and 680 GeV for $\mu = 110$ GeV. Exclusion limits are also placed in simplified model scenarios, where $m_{{\tilde{t}}}$ is excluded between 300 and 510 GeV for $m_{{\tilde{\chi}_{1}^{0}}} = $110 GeV assuming $\Delta m({\tilde{\chi}_{1}^{\pm}},{\tilde{\chi}_{1}^{0}}) = 5$ GeV. The Run 2 search, performed using 36.1 fb$^{-1}$ collected at $\sqrt{s}=13$ TeV, placed limits in simplified model scenarios, with sbottom masses up to 800 GeV excluded considering neutralinos of mass 110 GeV. A statistical combination of this analysis and the $36.1$ fb$^{-1}$ sbottom analysis extends this exclusion to 880 GeV for $m_{{\tilde{\chi}_{1}^{0}}} = 110$ GeV. The results of these analyses is scheduled to be published in Summer 2017. Finally a search for direct dark matter production in association with $b-$jets, in the 0-lepton, 2 $b-$jets and ${E_{\mathrm{T}}^{\mathrm{miss}}}$ final state using 13.3 fb$^{-1}$ of Run 2 ($\sqrt{s}=13$ TeV) data is presented. Cross section upper limits are placed, in simplified model scenarios, on a range of scalar and pseudoscalar masses assuming a dark matter mass of 1 GeV.