GeneralBrokenLines  Rev:70
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General information


For a track with an initial trajectory from a prefit of the measurements (internal seed) or an external prediction (external seed) the description of multiple scattering is added by offsets in a local system. Along the initial trajectory points are defined with can describe a measurement or a (thin) scatterer or both. Measurements are arbitrary functions of the local track parameters at a point (e.g. 2D: position, 4D: slope+position). The refit provides corrections to the local track parameters (in the local system) and the corresponding covariance matrix at any of those points. Outliers can be down-weighted by use of M-estimators.

The broken lines trajectory is defined by (2D) offsets at the first and last point and all points with a scatterer. The prediction for a measurement is obtained by interpolation of the enclosing offsets and for triplets of adjacent offsets kink angles are determined. This requires for all points the jacobians for propagation to the previous and next offset. These are calculated from the point-to-point jacobians along the initial trajectory.

Additional local or global parameters can be added and the trajectories can be written to special binary files for calibration and alignment with Millepede-II. (V. Blobel, NIM A, 566 (2006), pp. 5-13).

The conventions for the coordinate systems follow: Derivation of Jacobians for the propagation of covariance matrices of track parameters in homogeneous magnetic fields A. Strandlie, W. Wittek, NIM A, 566 (2006) 687-698.

Calling sequence

  1. Initialize trajectory:
    CALL gblini(..)
  2. For all points on initial trajectory:
    • Create points and add appropriate attributes:
      • CALL gbladp(..)
      • CALL gbladm(..)
      • Add additional local or global parameters to measurement:
        • CALL gbladl(..)
        • CALL gbladg(..)
      • CALL gblads(..)
  3. Optionally add external seed:
    CALL gbladx(..)
  4. Construct and fit trajectory, get Chi2, Ndf (and weight lost by M-estimators):
    CALL gblfit(..)
  5. For any point on initial trajectory:
    • Get corrections and covariance matrix for track parameters:
      CALL gblres(..)
  6. Optionally write trajectory to MP binary file:
    CALL gblmp2(..)


Linear algebra routines are taken from Millepede-II (by V. Blobel, University Hamburg). Only 2D (or 1D) measurements are implemented.