Speakers
Jan Amoraal
(NIKHEF)
Wouter Hulsbergen
(NIKHEF)
Description
We report on an implementation of a global chisquare algorithm
for the simultaneous alignment of all tracking systems in the
LHCb detector. Our algorithm uses hit residuals from the
standard LHCb track fit which is based on a Kalman filter. The
algorithm is implemented in the LHCb reconstruction framework
and exploits the fact that all sensitive detector elements have
the same geometry interface. A vertex constraint is implemented
by fitting tracks to a common point and propagating the change
in track parameters to the hit residuals. To remove
unconstrained or poorly constrained degrees of freedom
(so-called weak modes) the average movements of (subsets of)
alignable detector elements can be fixed with Lagrange
constraints. Alternatively, weak modes can be removed with a
cutoff in the eigenvalue spectrum of the second derivative of
the chisquare. As for all LHCb reconstruction and analysis
software the configuration of the algorithm is done in python
and gives detailed control over the selection of alignable
degrees of freedom and constraints. The study the performance
of the algorithm on simulated events and first LHCb data.
