TPC Track Reconstruction: Generalized Least Squares Fit

Track fitting in the HARP TPC must account for the fact that, because of the rotational symmetry of the TPC, both x and y cluster coordinates have errors at the same level of importance. Conventional fit algorithms which have only one coordinate with error while all other coordinates are error-free, are not appropriate. A generalized least-squares method is described which is symmetric in all coordinates, and applied in a 3D helix fit of TPC tracks.