Alignment of the ATLAS silicon tracker

ATLAS is one of the four experiments currently under preparation at Large Hadron Collider. Charged particle track reconstruction in the ATLAS Inner Detector is performed both with silicon and drift-tube-based detectors. The alignment of the ATLAS tracking system is one of the challenges that the experiment must overcome in order to achieve its physics goals. This requires the determination of almost 35 000 degrees of freedom. The precision required for the most sensitive coordinate of the silicon devices is of the order of few microns. This precision will be attained with a combination of two techniques: a hardware system with Frequency Scan Interferometers, and track-based alignment. The latter requires the application of complex alignment algorithms that can be both CPU and memory intensive due to the possible requirement of large matrix inversion or many iterations. The alignment algorithms have been already exercised on several challenges such as a Combined Test Beam, cosmic ray runs and large scale computing simulation of physics samples. This note reports on the methods, their computing requirements and preliminary results. (C) 2008 Elsevier B.V. All rights reserved.