Observation of CP violation in B+/- → DK+/- decays

An accurate determination of the angle γ of the Unitary Triangle is one of the most important goals of the LHCb experiment. The LHCb detector is a single-arm spectrometer at the LHC, optimised for beauty and charm flavour physics. As the angle γ is the least experimentally constrained parameter of the Unitary Triangle, its precise experimental determination can be used to test the validity of the Standard Model. The Unitary Triangle phase γ can be extracted in B → DK decays at tree-level, exploiting the interference between b → c(ūs) and b → u(c̄s) transitions. This interference is sensitive to γ and can give measurable charge asymmetries. In particular, γ ≠ 0 is required to produce direct CP violation in B decays and this is the only CP-violating mechanism for the decay of charged B± mesons. In this thesis, an analysis of CP violation in B± → DK± and B± → Dπ± decays is presented, where the D meson is reconstructed in the two-body final states: K±π∓, K+K−, π+π− and π±K∓. The analysis uses the full 2011 LHCb dataset of 1.0 fb-1, collected from pp collisions at √s = 7 TeV. Several CP-related quantities, e.g the ratio of B → DK and B → Dπ branching fractions and their charge asymmetries, are measured via a simultaneous fit to the invariant mass distributions of the modes considered. The suppressed B± → DK± mode is observed for the first time with ≈ 10σ significance. Once all measurements are combined, direct CP violation is established in B± decays with a total significance of 5.8σ.