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We consider a high momentum
track originating from the interaction point (IP) region.
When we take our local coordinate system in such a way that
the z axis is along the magnetic field, the y axis in the radial
direction which is approximately along the transverse momentum of the track,
and the x axis makes the overall system right-handed,
the helical track can be approximated by
where we have introduced the following short-hand for the
Notice that the last approximation (),
which is justified by our choice of the coordinate system
makes the r- and the r-z fittings decouple from each other.
We will, thus, the r- and the r-z fittings separately
in what follows.
Notice also that the problem then becomes a linear one, which
simplifies the necessary calculations considerably.