Track reconstruction using a 3-D map of the target magnetic field

Magnetic fields are often applied to the target zone of a particle beam to maintain the polarization of gaseous targets, as with the HERMES experiment. The same field bends the trajectories of charged particles, however, introducing errors in vertex reconstruction.This paper describes a method for accurately describing relativistic charged particle transport within a 3-dimensional (3-D), non-uniform magnetic field. The algorithm is tested on HERMES experimental data, and is shown to substantially improve the Ks0 resonance in the π+π-π+π- invariant mass spectrum. Indeed, corrected data taken with target magnet switched on are as good as data taken with magnet switched off. The method can easily be applied to other experiments given a 3-D magnetic field map of the target region. The relevant code is provided in an Appendix.