A new tool for constrained vertex fitting in ATLAS

The precise reconstruction of trajectories of charged and neutral particles and their decay vertices is crucial for many physics analyses. Studying the tracking performance on well-known benchmark channels helps to understand the properties of the ATLAS detector during the initial phase of the LHC. In order to exploit the correlations between reconstructed parameters of final state tracks having the same mother particle, a new tool for vertex fitting with the possibility of simultaneous application of kinematic constraints has been developed. Using this tool with a mass constraint on a benchmark channel such as J/ψ → μ⁺μ^- helps to correct shifts in the reconstructed curvature induced by systematic deformations of the detector.