Forward decay chain reconstruction with displaced vertices in intense magnetic fields

Abstract

Precise reconstruction of particle decay chains is essential for a broad range of analyses in particle physics experiments, particularly those targeting flavour physics and spectroscopy. We present a standalone, modular and extensible decay tree reconstruction framework designed to handle complicated topologies, accounting for tightly constrained particle decays and long-distance trajectory extrapolations through regions of intense and inhomogeneous magnetic field, as well as limited momentum resolution. Implemented in C++, the framework is optimised for configurability and performance. Developed primarily within CERN's LHCb experiment, its focus is on decay chains with vertices located several metres from the LHC proton-proton interaction point, where the dipole magnetic field is maximal and non-uniform. Even so, it remains flexible: it can accommodate these special cases as well as the approximations commonly used in standard decay tree fitting tools, and the geometries of general-purpose collider detectors. Benchmark studies using simulated data demonstrate significant improvements in reconstruction efficiency, invariant mass resolution, and vertex position resolution with respect to existing tools, opening new opportunities for precision measurements and searches involving highly displaced decays.