Beam-Energy Calibration at Belle II

The SuperKEKB colliding-beam accelerator provides \(e^{+}e^{-}\) collisions at an energy corresponding to the mass of the \(\Upsilon (4S)\) resonance, the products of which are being recorded by the Belle II detector. The mass of the \(\Upsilon (4S)\) meson ( \(\sim \) 10.58 GeV) is just above the threshold for decay into B-meson pairs. We measure the beam energy in the center of mass frame using fully reconstructed neutral and charged B decays from various final states. The measurement is performed using the beam-constrained mass of the reconstructed B decays. An accurate determination of the beam energy is important as a kinematic constraint in many analyses, as well as being an input for calibrations of the momentum and energy scale. We have calculated the average center-of-mass energy along with its uncertainty and the width of the distribution of the center-of-mass energy for the data recorded by Belle II detector from 2020 to 2022.