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Abstract

The Large Hadron Collider has recently began colliding proton beams at a record center-of-mass energy $\sqrt{s}=13~\TeV$. This allows the LHC experiments to drastically improve searches for heavy new particles. However at the same time, the more complicated environment means an increase in the trigger thresholds. Consequently a viable set of theories at lower energies remains unexplored. This thesis describes three ATLAS analyses that used different techniques to target new particles with sub-TeV mass. The first analysis searches for the Higgs boson produced via vector-boson fusion and decaying to two bottom quarks in in $\sqrt{s}=8~\TeV$ dataset. The presence of the extra jets from vector-boson fusion and $b$-tagging is used to reduce the event rate to a manageable level. The second analysis searches for generic resonances decaying to two $b$-quarks with masses between $600~\GeV$ to $1.2~\TeV$. The use of $b$-tagging in the trigger allows it to set unique limits in the sub-TeV mass range. The third analysis searches for hadronic resonances produced in associtaion with energetic initial state radiation (ISR). The use of the ISR as a trigger decorrelates the resonance mass from the trigger efficiency. The result is world-class limits on new particles with masses between $200~\GeV$ to $500~\GeV$. This space is interesting for particles mediating between the Standard Model and the dark sector. The dark matter relic density prefers $\order{100~\GeV}$ mediators, yet the most stringent constraints in this region previously came from the pre-LHC era.

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