Speaker
Mr
Tapio Lampen
(HELSINKI INSTITUTE OF PHYSICS)
Description
Modern tracking detectors are composed of a large number of modules assembled in a
hierarchy of support structures. The sensor modules are assembled in ladders or
petals. Ladders and petals in turn are assembled in cylindrical or disk-like layers
and layers are assembled to make a complete tracking device. Sophisticated
geometrical calibration is essential in these kind of detector systems in order to
fully exploit the high resolution of sensors. The position and orientation of
individual sensors in the detector have to be calibrated with an accuracy better than
the intrinsic resolution, which in the CMS Silicon Tracker is ranging from about 10
um to 50 um. Especially if no hardware alignment system is available (which is the
case for the CMS Pixel), the fine tuning of the sensors needs to be carried out with
particle tracks.
There are about 20000 independent sensors in the CMS tracker and of the order 10^5
calibration constants are needed for the alignment. The alignment algorithm needs to
be computationally practical, especially if considered to provide almost on-line
feedback. We present an effective algorithm to perform fine calibration of
individual sensor positions as well as alignment of composite structures consisting
of a number of pixel or strip sensors. The alignment correction of a composite
structure moves the individual sensors like a rigid body under a rotation and
translation of the structure. Up to six geometric parameters, three for location and
three for orientation, can be computed for each sensor on a basis of particle
trajectories traversing the detector system. The performance of the methods is
demonstrated with both simulated tracks and tracks reconstructed from experimental
data taken with a cosmic rack.
Primary authors
Mr
Tapio Lampen
(HELSINKI INSTITUTE OF PHYSICS)
Dr
Veikko Karimäki
(HELSINKI INSTITUTE OF PHYSICS)
