Publication

Measurement of time-dependent CP violation in B0->Dpi decays and optimisation of flavour tagging algorithms at LHCb

Vincenzo Battista
2018
EPFL thesis
Abstract

This thesis presents the results of a time-dependent analysis of B0Dπ±B^0\to D^{\mp}\pi^{\pm} decays using 3 fb13~\rm fb^{-1} of proton-proton collision data collected with the LHCb detector at CERN's Large Hadron Collider during Run 1 with a centre-of-mass energy of 77 (2011) and 88 (2012) TeV. The LHCb experiment is dedicated to the study of the properties of bb-flavoured hadrons, in particular CPCP violation in the BB meson system. The Standard Model of Particle Physics describes very precisely the mechanism and the amount of CPCP violation expected in the Universe. However, the observed matter-antimatter asymmetry is larger by several order of magnitude compared to the predictions. This could be explained by the existence of a new source of CPCP violation, originating in New Physics beyond the Standard Model. The time-dependent analysis of B0Dπ±B^0\to D^{\mp}\pi^{\pm} decays provides constraints on the angle γ\gamma of the Unitarity Triangle, one of the fundamental parameters of the Standard Model related to CPCP violation. Since no sizeable high-order Standard Model processes are expected to contribute, any deviation from the predictions would be an unambiguous signature of New Physics. The current experimental precision on γ\gamma is significantly lower than that of theoretical predictions. This motivates the effort for new experimental determinations of γ\gamma in order to reduce its uncertainty. The analysis of \Bz\Dmp\pipm\Bz\to\Dmp\pipm decays, although not as sensitive as that obtained from decays of charged BB mesons into D()0K()+D^{(*)0}K^{(*)+} final states, represents an independent and uncorrelated estimation of γ\gamma that contributes to the global combination of all γ\gamma measurements. The result obtained in this thesis is more precise than previous determinations from other experiments (BaBar, Belle) using B0Dπ±B^0\to D^{\mp}\pi^{\pm} decays. Although based on a very large sample of about half a million signal events, it is still dominated by statistical uncertainties, indicating good prospects for future improvements in precision with more data from Run 2 and beyond. In addition to the B0Dπ±B^0\to D^{\mp}\pi^{\pm} analysis, this thesis also summarizes the studies to improve the performances of the flavour tagging algorithms used by the LHCb collaboration to infer the flavour of neutral BB mesons in time-dependent analyses. The performance of these algorithms, being correlated with the kinematics of the reconstructed particles as well as the complexity of the event (number of tracks and primary vertices), showed a significant decrease on Run 2 data (2015--2018), which were collected at a centre-of-mass energy of 13 TeV13~\rm TeV. Thanks to new implementations, these algorithms now have a performance similar to that obtained with Run 1 data.

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