Measuring Belle II's track-finding efficiency with radiative Bhabha scattering

Sumitted to PubDB: 2026-04-08

Category: Master Thesis

Tags: ECL

Principal Authors Katharina Korf, Stephan Paul, Daniel Greenwald, Lukas Grussbach
Date 2026-02-18
Belle II Number BELLE2-MTHESIS-2026-011
Abstract This thesis determines the tracking efficiency of Belle II in radiative Bhabha scattering events, $e^+e^-\to e^+e^-\gamma$, as a function of charge, momentum magnitude, and polar angle. The events are reconstructed using a tag electron and two energy clusters reconstructed in the electromagnetic calorimeter, which do not inherit any information about the second track. From the well-known decay topology of radiative Bhabha events, the cluster originating from the charged particle is extracted. The purity and resolution of the dataset are improved by implementing a boosted decision tree that classifies events with the higher-energy cluster originating from the charged particle and those with the lower-energy cluster originating from the charged particle. The overall results are computed for a binning in the cluster energy and the cluster polar angle of the higher-energy cluster. The tracking efficiencies, calculated using an unfolding procedure that accounts for residual background contamination, are above \SI{99}{\percent} with uncertainties below \SI{0.01}{\percent}, yielding results from experimental data and simulation. For the correction factor between experimental data and simulation, the uncertainty is smaller than \SI{0.01}{\percent} in all bins, which is a clear improvement over the previous tracking efficiency result at Belle II, which is \SI{0.27}{\percent}. This result is determined for a lower energy region than our result from the radiative Bhabha study.
Institute TUM

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