Search for the Xb and other hidden-beauty states in the π+π-ϒ(1S) channel at ATLAS

Abstract

This thesis primary concerns a search for a hypothetical state — referred to throughout as the Xb — using the ATLAS detector at the LHC. Specifically, the π+π-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)µ+µ- 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(13DJ) 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 Xb search methodology.