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T: Fachverband Teilchenphysik
T 69: Methods in Particle Physics IV
T 69.5: Talk
Thursday, March 19, 2026, 17:15–17:30, KH 00.020
Reconstruction of Neutrons and Sensitivity to the Decay B → K nn at Belle II — •Timur Lenkeit, Alexander Heidelbach, Thomas Kuhr, and Thomas Lueck — Ludwig-Maximilians-Universität München (LMU), München, Germany
Neutrons are often not taken into account in the detector design or the reconstruction software, but can nevertheless play an important role in understanding physical processes. We examine methods to improve neutron reconstruction at the Belle II experiment. Unlike photons or electrons, neutrons do not interact electromagnetically; their energy is recorded only indirectly through secondary particles from hadronic interactions, which results in irregular and diffuse signatures in the electromagnetic calorimeter (ECL). These features make it challenging to distinguish neutrons from other neutral particles. To address this, we investigate machine-learning*based classifiers that combine ECL variables to enhance neutron particle identification. In addition, we explore the use of variables related to the KL0 and Muon detector to recover information from neutrons that leave no signal in the ECL.
We then assess how these reconstruction strategies affect the background composition and the achievable sensitivity to the decay B → K nn. With these studies, we aim to enable a determination of the corresponding branching fraction.
Keywords: Neutrons; Reconstruction; Machine Learning; Neutrals; Branching Fraction