The CP-violating phase φ_s is measured in B_s → J/ψφ decays, using data from the 2010 to 2012 run periods of the ATLAS detector at the LHC at CERN. This measurement potentially provides a means to falsifying the standard model, known to provide insufficient levels of CP-violation to account for the observable universe. This thesis focuses on the trigger selections used in the analysis of the ATLAS data and includes material produced for public papers and ATLAS CONF notes. These include a study of a shift in the reconstructed mass of J/ψ candidates, an optimisation of trigger selections and cuts applied to 2011 data, and a full description of a correction for application to a trigger bias present in the early 2012 data. The measurements presented in this thesis complement and are competitive with measurements made at other experiments around the world.
Results were obtained from the final 2012 fit of combined corrected L2StarA and unbiased L2StarB data and the main parameters extracted, where Γ_s is the lifetime, ∆Γ is the width difference, and φ_s is the weak phase, and the errors shown are statistical and systematic respectively:
Γ_s = 0.6706±0.0024±0.0022
∆Γ = 0.0816±0.0086±0.0031
φ_s = -0.2340±0.0718±0.0190