Impact parameter resolution is calculated as
Let us consider again a tracking device having
(n+1) equally sapced sampling points with an idential spatial
resolution .For simplicity, we further assume that multiple scattering in
the tracking volume is negligible.
Then the error matrix is given by Eq.B.7, where
the pivot is located at y0 = 0 which is the -th hit position.
When the -th hit is at r = Rin, we need to transform
the error matrix by , using
Ignoring the material between the -th hit and the interaction point
(by setting ES = 0), we obtain Fig.3.2 which
plots as a function of
the ratio of the extrapolation length and the lever arm length:
Notice that the impact parameter resolution here is much worse in general than that expected from a straight-line approximation. This is because of the curvature error. If we have an external tracking device which provides additional curvature information, we can thus improve the impact parameter resolution significantly. We can do this by modifiying , according to