Title:

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 singlearm 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 treelevel, 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 CPviolating 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 twobody 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 CPrelated 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.
