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Belle II
Abstract:
To explain the origin of the observed matter-antimatter asymmetry in the universe, new sources of charge-parity (CP) symmetry violation beyond the standard model of particle physics are required. The oscillation frequency Delta m in the neutral B meson system is an important parameter in overconstraining standard model predictions for CP violation in the quark sector. We aim for a precision measurement of the lifetime tau_B0 and oscillation frequency Delta m at the Belle II experiment. At the asymmetric electron-positron collider SuperKEKB, B0B0-bar meson pairs are produced in an entangled coherent quantum state. We reconstruct B mesons in three hadronic signal decay channels B0 -> pi+D(*)- and employ flavor tagging algorithms to determine the flavor eigenstate of the accompanying B meson decay. From a time-dependent fit to the mixing asymmetry in the decay time difference of both B mesons, in seven bins of the flavor tag figure-of-merit, we extract the oscillation frequency and lifetime. From simulation, we analyse the (peaking) background compositions in all three signal channels. We develop multi-dimensional fit strategies to separate signal and backgrounds and to extract the background composition from Belle II data. We fit in the energy difference Delta E of the reconstructed B mesons and the output of a continuum suppression boosted decision tree, trained on simulated event samples. In simulation the fit consistently tells apart signal and backgrounds with deviations of less than 10%. Pseudo-experiment indicate a stable and un-biased fit and the fit result on 176.9 +/- 12.6 fb^-1 of Belle II data agrees with the expectations from simulation.
