Search for $W' \rightarrow tb \rightarrow qqbb$ with Novel Techniques at the ATLAS Experiment

This thesis presents the first search for a massive $W'$ gauge boson in the decay channel of $tb \rightarrow Wbb \rightarrow qqbb$, performed with data corresponding to $20.3~{\rm fb}^{-1}$ of integrated luminosity collected by the ATLAS detector at the LHC in $pp$ collisions at a center-of-mass energy of $\sqrt{s} = 8~TeV$. This analysis searches for $W'$ with masses above $1.5~TeV$, where the $W'$ decay products have large momentum. Novel jet substructure techniques are used to identify jets from high momentum top quarks to ensure high sensitivity, independent of $W'$ mass, up to $3~TeV$; $b$-tagging is also used to identify jets originating from bottom quarks. The data is consistent with the Standard Model background-only expectations, and 95% Confidence Level limits are set on the $W' \rightarrow tb$ cross section times branching ratio ranging from $0.16\;\mathrm{pb}$ to $0.33\;\mathrm{pb}$ for left-handed $W'$ bosons, and ranging from $0.10\;\mathrm{pb}$ to $0.21\;\mathrm{pb}$ for right-handed $W'$. In addition, 95% Confidence Level limits are set on the $W'$-boson coupling to $tb$ as a function of the $W'$ mass using a model independent effective field theory approach.