B^0 \to D^-1 \pi^+ \pi^0 and B^0 \to D^- \rho^+ at the Belle Experiment: Branching Fraction Measurements and Forward-Backward Asymmetry in Helicity Angle by

Sumitted to PubDB: 2026-05-08

Category: Phd Thesis

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Principal Authors Kim Smith
Date 2024-09-10
Belle II Number BELLE2-PTHESIS-2026-011
Abstract This thesis presents the measurement of the forward-backward asymmetry in the helicity angle of the hadronic $B$ decay $\BtoDpipi$ and the branching fractions of $\BtoDpipi$ and $\BtoDrho$. The analysis is performed on a dataset of \qty{570}{\per\femto\barn} collected at the $\Upsilon(4S)$ resonance by the KEKB asymmetric $e^+e^-$ collider and collected by the Belle detector. The analysis methodology was developed and validated using Monte Carlo simulated samples, in which the signal reconstruction and background suppression was optimised in a fully blind way. To account for the non-uniform relationship between the reconstruction efficiency and the 3-body final state kinematics, an efficiency matrix in bins of the cosine of the helicity angle and the invariant mass of the $\pi^+\pi^0$ was employed. A series of extended maximum likelihood fits in these bins was performed to extract the results from the data. This analysis was first performed on a partial dataset of \qty{74}{\per\femto\barn} before proceeding with the full \qty{570}{\per\femto\barn} dataset. The measured values in partial data are: \begin{align*} \mathcal{B}(\BtoDpipi) &= (9.95 \pm 0.43 \pm 0.56)\times 10^{-3} \\ \mathcal{B}(\BtoDrho) &= (7.87 \pm 0.37 \pm 0.44)\times 10^{-3} \\ \Afb(\BtoDpipi) &= -0.028 \pm 0.005 \pm 0.051 \end{align*} where the first uncertainties are statistical and the second are systematic. The results in full data are: \begin{align*} \mathcal{B}(\BtoDpipi) &= (8.92 \pm 0.23 \pm 0.57)\times 10^{-3} \\ \mathcal{B}(\BtoDrho) &= (7.91 \pm 0.21 \pm 0.47)\times 10^{-3} \\ \Afb(\BtoDpipi) &= -0.048 \pm 0.002 \pm 0.047 \end{align*} This constitutes the world's first measurement of the branching fraction and helicity asymmetry of the $\BtoDpipi$, and the most precise measurement of the $\BtoDrho$ branching fraction.
Institute Melbourne

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