I will present the measurement of the Cabibbo-Kobayashi-Maskawa unitarity triangle angle $ \phi_{3} $ using a model-independent Dalitz plot analysis of $ B^{\pm} \to D(K_{S}^{0}h^{+}h^{-})K^{\pm} $ ($h = \pi$ or $K$) decays, where $D$ can either be a $D^0$ or a $\bar{D^0}$ meson, which currently provides the best sensitivity. The signal events are extracted from the $D$ phase-space by dividing it into bins based on the phase-difference of $D^0$ and $\bar{D^0}$ at each point of the phase-space. This is the first measurement that simultaneously uses Belle and Belle~II experimental data, combining samples corresponding to integrated luminosities of 711~fb$^{-1}$ and 128~fb$^{-1}$, respectively. All data were accumulated from energy-asymmetric $e^+e^-$ collisions at a centre-of-mass energy corresponding to the mass of the $\Upsilon(4S)$ resonance. The $\phi_3$ value is measured to be ($78.4 \pm 11.4 \pm 0.5 \pm 1.0 \right)^{\circ}$), where the first uncertainty is statistical, the second the experimental systematic and the third is from the uncertainties on the strong-phase difference parameters. I will also shade lights on the accurate determination of beam-energy in the centre-of-mass frame at the Belle~II experiment.

Reference: N. Rout, J. Libby, and K. Trabelsi with the Belle II collaboration
"Combined analysis of Belle and Belle~II data to determine the CKM angle $\boldsymbol{\phi_{3}} $ using $\boldsymbol{B^+ \to D(K_{S}^0 h^- h^+) h^+}$ decays}": JHEP 02 063 (2022).