Thesis BELLE2-MTHESIS-2021-076

Background Analysis for $B^0$ Lifetime and Oscillation Frequency Measurement using Hadronic $B^0\rightarrow D^{(*)-}\pi^+$ Channels at Belle II

Caspar Schmitt ; Christian Kiesling ; Thibaud Humair

2021
Max-Planck Institute for Physics Munich

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 $\tau_{B^0}$ and oscillation frequency $\Delta m$ at the Belle II experiment. At the asymmetric electron-positron collider SuperKEKB, $B^0\overline{B}^0$ meson pairs are produced in an entangled coherent quantum state. We reconstruct B mesons in three hadronic signal decay channels $B^0\rightarrow D^{(*)-}\pi^+$ 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 $176.9 \pm 12.6 \textrm{fb}^{−1}$ of Belle II data agrees with the expectations from simulation.

Note: Presented on 13 09 2021
Note: MSc

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