arXiv2026
The Full Event Interpretation (FEI) algorithm is a central component of the Belle II analysis framework, designed for the efficient reconstruction of exclusive B-meson decays. The FEI performs a hierarchical reconstruction of hadronic and semileptonic final states, using multivariate classification techniques to tag one of the two B mesons produced in electron-positron collisions. The FEI is particularly important for studies of signal-side decays involving missing energy. By reconstructing one B meson with the FEI, the kinematics of the other, signal-side B meson are tightly constrained, leading to improved background rejection. In this contribution, we present the design, implementation, and recent developments of the FEI. We outline the reconstruction strategy, candidate selection, and multistage multivariate training based on gradient boosted decision trees. Recent improvements to the FEI workflow, including improvements in the simulated data used for training, reconstruction of intermediate particles, and improved robustness of the multivariate inputs, are discussed in detail. Beyond algorithmic updates, we also discuss newly developed techniques for the calibration and performance evaluation of the FEI. These include refined procedures to calibrate the efficiency, check the performance across decay channels and data-taking conditions, and application of reweighting techniques to diagnose and mitigate discrepancies between simulation and data. Finally, we briefly discuss ongoing and future developments aimed at further improving reconstruction performance and computational efficiency.