Multi-Modal Track Reconstruction using Graph Neural Networks at Belle II

High backgrounds and detector ageing impact the track finding in the Belle II central drift chamber, reducing both track purity and track efficiency in events. This necessitates the development of new track finding algorithms to mitigate detector performance degradation. Building on our previous success with an end-to-end multi-track reconstruction algorithm for the Belle II experiment at the SuperKEKB collider (arXiv:2411.13596), we have extended the algorithm to incorporate inputs from both the drift chamber and the silicon vertex tracking detector, creating a multi-modal network. We employ graph neural networks to handle the irregular detector structure and object condensation to address the unknown, varying number of particles in each event. This approach simultaneously identifies all tracks in an event and determines their respective parameters. We demonstrate the algorithm's effectiveness using a realistic full detector simulation, which incorporates beam-induced backgrounds and noise modelled from actual collision data. The simultaneous reconstruction of the information from the two detectors yields a track efficiency improvement from 48.0 % to 74.7 % for uniformly displaced particles up to 100 cm, while increasing the track purity by 5.5 percentage points. We provide a detailed comparison of its track-finding performance against the current Belle II baseline across various event topologies.