Novel methods in track-based alignment to correct for time-dependent distortions of the ATLAS Inner Detector

Abstract

ATLAS is a multipurpose experiment at the LHC proton-proton collider. Its physics goals require high resolution and unbiased measurement of all charged particle kinematic parameters. These critically depend on the layout and performance of the tracking system and the quality of its alignment. For the LHC Run-II, the system has been upgraded with the installation of a new pixel layer, the Insertable B-layer (IBL). The offline track alignment of the ATLAS tracking system has to deal with about 700,000 degrees of freedom defining its geometrical parameters, representing a considerable numerical challenge in terms of both CPU time and precision. An outline of the track based alignment approach and its implementation within the ATLAS software is presented. Special attention is paid to describe the techniques allowing to pinpoint and eliminate track parameter biases. During Run-II, the ATLAS Inner Detector Alignment framework has been adapted and upgraded to correct very short time scale movements of the sub-detectors. In particular, a mechanical distortion of the IBL staves up to 20 µm has been observed during data-taking. The techniques used to correct for this effect and to match the required Inner Detector performance are presented.