Abstract
An accurate determination of the angle \(\gamma \) 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 \(\gamma \) 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 \(\gamma \) can be extracted in \(B \rightarrow DK\) decays at tree-level, exploiting the interference between \(b \rightarrow c(\bar{u}s)\) and \(b \rightarrow u(\bar{c}s)\) transitions. This interference is sensitive to \(\gamma \) and can give measurable charge asymmetries. In particular, \(\gamma \ne 0\) is required to produce direct \(C \! P\) violation in \(B\) decays and this is the only \(C \! P\)-violating mechanism for the decay of charged \(B^\pm \) mesons. In this thesis, an analysis of \(C \! P\) violation in \(B^{\pm } \rightarrow DK^{\pm }\) and \(B^{\pm } \rightarrow D\pi ^{\pm }\) decays is presented, where the \(D\) meson is reconstructed in the two-body final states: \(K^{\pm }\pi ^{\mp }\), \(K^+K^-\), \(\pi ^+\pi ^-\) and \(\pi ^{\pm }K^{\mp }\). The analysis uses the full 2011 LHCb dataset of 1.0\({\text {fb}}^{-1}\), collected from \(pp\) collisions at \(\sqrt{s}\) = 7\(\mathrm \,TeV \). Several \(C \! P\)-related quantities, e.g. the ratio of \(B \rightarrow DK\) and \(B \rightarrow D\pi \) branching fractions and their charge asymmetries, are measured via a simultaneous fit to the invariant mass distributions of the modes considered. The suppressed \(B^{\pm }\rightarrow DK^{\pm }\) mode is observed for the first time with \(\approx 10\sigma \) significance. Once all measurements are combined, direct \(C \! P\) violation is established in \(B^{\pm }\) decays with a total significance of \(5.8\sigma \). The measured \(C \! P\) observables are summarised here with their statistical uncertainties and assigned systematic uncertainties:
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Notes
- 1.
Although these conditions have been attributed to Sakharov, they were not listed explicitly in his paper in the current form.
- 2.
A more general definition of the \(C \! P\) operator would be \(CP|X^0\rangle = e^{+i\xi }|\bar{X}^0\rangle \) and \(CP|\bar{X}^0\rangle = e^{-i\xi }|{X}^0\rangle \), but the phase \(\xi \) is arbitrary and is chosen to be equal to 0 here for simplicity.
- 3.
The strong \(C \! P\) problem and \(C \! P\) violation in the lepton sector are not treated here.
- 4.
Note that the strong phase in the suppressed \(D\) decay carries a minus sign, unlike the definition in the \(B\) decay.
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Gandini, P. (2014). Introduction. In: Observation of CP Violation in B± → DK± Decays. Springer Theses. Springer, Cham. https://doi.org/10.1007/978-3-319-01029-8_1
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