A Correction Term for Transport Jacobian from the Direction Variation

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  • Beomki Yeo


In the track extension model of ACTS, the covariance of a track is updated for every surface intersection by applying a jacobian matrix, the composition of the coordinate transformation and free space transport. In the derivation of the transport jacobian, it is necessary to consider the path difference coming from the track displacement, which appears as a direction variation term with respect to the path length.

The motivation of this white paper is that the author failed in finding any reference explaining this clearly. (Please let the author know if anyone finds it) There are bunch of references which calculate this term with an ideal helix model, but it is not very useful with the runge-kutta based track extension model. Therefore, in this note, we are deriving the term thoroughly and demonstrate how it is implemented in the source code.