Search for the $X_b$ and other hidden-beauty states in the $\pi^+ \pi^-ϒ$(1S) channel at ATLAS

This thesis primary concerns a search for a hypothetical state — referred to throughout as the $X_b$ — using the ATLAS detector at the LHC. Specifically, the $\pi^+\pi^-ϒ$(1S) channel was reconstructed in the mass ranges between 10.05-10.31 GeV and 10.40-11.00 GeV using the $ϒ(1S) \to \mu^+ \mu^-$ decay. In the analysis of the 2011 datasets, the method of candidate selection, whereby only a single dipion combination was used for each ϒ(1S), was adopted. Fits were performed every 10 MeV to test for the presence of a signal peak, but nothing statistically significant was found. The analysis was re-optimised for the 2012 dataset, with the candidate selection method being abandoned in favour of a binning approach. As for the results of the search in the 2011 dataset, nothing statistically significant was observed. This was also the case for dedicated searches for the $ϒ(1^3D_J)$ triplet, the ϒ(10860) and the ϒ(11020), which were performed in both the 2011 and 2012 analyses. Consequently, CLs upper limits were calculated at the 95% confidence level, with values for the relative production rate, with respect to the $ϒ(2S)$, of between 0.8% and 4.0%. Also included are a technical-based study of the performance and operation of the SCT sub-detector, and an investigation of the feasibility of using the dielectron final state for quarkonium studies at ATLAS. The latter of these had a direct influence on the $X_b$ search methodology.