BESIII track extrapolation and matching

A GEANT4 based, Object-Oriented package, TrkExtAlg, is developed for extrapolating the BESIII MDC track into outer sub-detectors. The magnetic deflection and ionization loss of the particle in the BESIII detector is considered, the algorithm supplies the position and the momentum of the track in all the outer sub-detectors, as well as the error matrix at any given hit point with the multiple scattering effect taken into account. Through comparing the results of track extrapolation with the results of a full simulation and checking the results and application of track matching with hits in outer sub-detectors, TrkExtAlg is proved to be reliable.