Tuning Parameters in the Kalman Filter fit

In principle the Kalman filter should contain no fudge factors. In the past two fudge factors have been introduced:

• the multiple scattering fudge factor
• the average energy loss

Only if all items below are fulfilled, there will be no fudge factors and the covariance matrix should be correct.

• The amount of material seen by a particle traversing the detector must be included in all detail in the reconstruction; i.e. the transportservice must be used with all details.
• The non-Gaussian multiple scattering tail can be neglected
• The magnetic field tracking algorithm has high numerical precision; i.e. a high order stepper should be used
• The covariances (or "weights") of the detector measurement should be correct. This can be a difficult criterion to meet. The error of a Si clusters with 1 or 2 hit strips is not Gaussian. The error depends on the track angle, the Phi strip error depends on R, etc.
• The Kalman filter must be seeded with an initial state which either:
  • is obtained form other measurements as the one used inside the fit,
  • has a covariance of infinite magnitude (in other words: a weight that is negligible in the final fit).

For a fast fit it is conceivable that a fudgefactor is introduced, eg. to compensate for a non precise material description

-- MarcelMerk - 11 Apr 2005