This book presents a striking demonstration of the synergies that exist between particle-physics experiments. A measurement is reported of the weak-phase ?, which is a fundamental parameter of the Standard Model and can be regarded as quantifying the amount of CP violation that is allowed by the theory. The parameter is measured through a determination of b-meson decays collected by the LHCb experiment at CERN. The b-mesons are reconstructed in a decay to a charm meson and a pion or a kaon, and the charm meson is itself reconstructed in one of two four-body final states (KK?? or ????). This is the first time these decay modes have been used in a ? measurement, which is itself a notable achievement of the thesis study. In order to interpret the measured observables in a theoretically clean manner, it is necessary to have knowledge of the dynamics of the charm-hadron decays. This knowledge is obtained from a study of charm-meson pair production in e+e- collisions at the psi(3770) resonance, using data collected by the BESIII experiment in Beijing. The quantum-correlations present in these decays give unique access to the required information, which is then imported back into the LHCb measurement. The precision obtained on ? is competitive with the most sensitive individual measurements so far performed of this parameter in other decay modes.