Studies of CP violation in the Bs →J/ψϕ channel, and prospects for future measurements at the ATLAS detector

This thesis presents an analysis of the B0s →J/ψϕ decay, using the Full Run-2 proton-proton collision dataset at 13 TeV collected by the ATLAS detector at the LHC, corresponding to 139 fb−1 of integrated luminosity. The following quantities of physics interest are measured: the average decay width of the heavy and light B0s meson states, Γ, the width difference, ∆Γs, between the B0s meson eigenstates, and the mass difference between light and heavy states ∆ms. ∆Γs = 0.0620 ±0.0034 (stat.) ±0.0016 (syst.)ps−1 Γ = 0.6695 ±0.0011 (stat.) ±0.0011 (syst.)ps−1 ϕs =−0.069 ±0.030 (stat.) ±0.020 (syst.)rad ∆ms = 17.889 ±0.060 (stat.) ±0.061 (syst.)ps−1 At the time of writing, these results are not finalised and the exact values may change before official publication, and are ATLAS internal results. Work presented in this thesis looks to develop methods with the goal of improving these results and builds on an ongoing analysis by the “BsJPsiPhi” team. There are three investigations presented in this thesis; one into the choice of the mass sidebands, a second into the use of sPlot statistical weighting as a background rejection model, and the third, a study of mass-time correlations in B0s meson background. A separate study into the material budget at ATLAS, including consideration of the material map variation systematics is also presented.