Measurement of time dependent CP asymmetries in charged charmless hadronic two-body B decays at LHCb

The LHCb experiment is one of the four experiments that are installed at the protonproton Large Hadron Collider (LHC) at CERN, Geneva. The experiment is at the latest stage of its setting-up. The first collisions at high energy in LHC are planned to mid-2008, with the first results on the experiments soon after. The LHCb detector is a single-arm spectrometer conceived to pursue an extensive study of CP violation in the B meson system, over-constraining the Standard Model predictions and looking for any possible effect beyond this theory, and to look for rare phenomena in the b quark sector with very high precision. The subject of the present work is the study of the non-leptonic B meson decays into charged charmless two-body final states. This class of decays has been extensively studied and it is still matter of great interest at the B-factories and at Tevatron. In fact the current knowledge of this class of decays in the Bd/Bu sector starts to be quite constrained, but the Bs still remains a field where a rich programme of physics can be pursued. The interest on this class of B meson decays lies in the presence of several loop dominated modes that makes the phenomenology of such processes an ideal ground for increasing our knowledge of CP violation, constraining the Standard Model prediction and possibly detect new physics effects. In addition this class of processes is a very rich source of information for understanding B decays, since all the diagram topologies enter in their amplitudes. LHCb, thanks to the large beauty production cross section at the LHC expected around 500 μb, and to excellent vertexing and triggering capabilities, will be able to collect a large sample of non-leptonic charmless charged two-body B meson decays, of order of 200k per 2 fb$^{−1}$ of integrated luminosity, corresponding to about one year of data taking. Furthermore its particle identification system, composed in particular by two RICH detectors, but also by calorimeter and muon systems, will allow to disentangle the variuos $B^{0}_{s} → h^{+}h'^{-}$ modes with a high purity as well as high efficiency. In this study the non-leptonic B meson decays into charged charmless two-body final states will be exploited to measure the four CP violating terms $A^{dir}_{\pi\pi}$, $A^{mix}_{\pi\pi}$, $A^{dir}_{KK}$ and $A^{mix}_{KK}$. The BABAR and Belle experiments have already provided measurements of the $B_{d} → \pi^{+}\pi^{-}$ asymmetries, but at the moment their estimations are only weakly compatible and it is not yet possible to draw a definitive conclusion. On the other hand, as far as the $B_{s} → K^{+}K^{−}$ asymmetries are concerned, they have not yet been measured. LHCb will provide very important information about these CP asymmetries even after few months of data taking. In the present study for the first time in LHCb the whole procedure to measure the CP asymmetries in the $B_{d} → \pi^{+}\pi^{-}$ and $B_{s} → K^{+}K^{−}$ decays have been performed on Monte Carlo events from the full GEANT simulation, generated with CP violation and reconstructed using realistic pattern recongnition algorithms, rather than on events generated with fast "toy" Monte Carlos. This dissertation is organised in five chapters. In the first one the physics framework of the processes considered will be given, focusing in particular on the motivations for their study. In the second chapter an overview of the LHCb detector design will be given, starting from the LHC proton-proton collider and going on with the description of each LHCb sub-detector and its performaces. In the third chapter the LHCb computing framerwork and the main tools of analysis, such as the track reconstruction, the particle identification and the tagging, will be briefly described. After that the procedure for the event selection will be discussed in depth. The tagging performances will be evaluated, the different background contributions will be estimated and the experimental resolutions obtained will be analyzed. In the fourth chapter the strategy to measure the CP asymmetries will be presented: an unbinned maximum likelihood fit to the sample of the selected and tagged events. The results obtained will be presented and the proper time modelization will be discussed. In the fifth chapter the different sources of background events will be taken into account in order to evaluate their effect on the extraction of the CP asymmetries.