Speaker
Dr
Markus Stoye
(Inst. f. Experimentalphysik, Universitaet Hamburg)
Description
The CMS silicon tracker comprises about 17000 silicon modules. Its radius
and length of 120 cm and 560 cm, respectively, make it the largest silicon tracker
ever built. To fully exploit the precise hit measurements, it is necessary to
determine the positions and orientations of the silicon modules to the level of mum and
murad, respectively.
Among other track based alignment algorithms, the CMS collaboration studied
MillePede II, developed by V. Blobel. This experiment independent program offers
several methods to solve the large system of linear equations which arises from
a global chi^2-minimisation.
Studies show that MillePede II is indeed capable to align the about 45000
degrees of freedom of the CMS silicon tracker that have sensible influence on track
reconstruction. This result is achieved utilising complementary data sets like
muons from Z- or W-decays and cosmic rays, vertex and mass constraints. A hierarchical
parametrisation allows to make full use of survey measurements accomplished
during construction.
In a realistic case study, all elements of the tracker have been aligned
simultaneously. The precision reached is close to 1 mum for the pixel detector
and about 20 mum in the endacps of the strip detector. Remarkably, using the GMRES
method to solve the matrix equation takes less than 2 hours on a standard 64-bit
PC and requires only 2 GB of memory.
| Submitted on behalf of Collaboration (ex, BaBar, ATLAS) | CMS Offline group |
|---|
Author
Dr
Markus Stoye
(Inst. f. Experimentalphysik, Universitaet Hamburg)
