Alignment of the ATLAS Inner Detector

The Run-2 of the LHC has presented new challenges to track and vertex reconstruction with higher energies, denser jets and higher rates. In addition, the Insertable B-layer (IBL) is a fourth pixel layer, which has been deployed at the centre of ATLAS during the longshutdown-1 of the LHC. The physics performance of the experiment requires a high resolution and unbiased measurement of all charged particle kinematic parameters. In its turn, the performance of the tracking depends, among many other issues, on the accurate determination of the alignment parameters of the tracking sensors. The offline track based alignment of the ATLAS tracking system has to deal with more than 700,000 degrees of freedom (DoF). This represents a considerable numerical challenge in terms of both CPU time and precision. During Run-2, a mechanical distortion of the IBL staves up to 20um has been observed during data-taking, plus other short time scale movements. The talk will also describe the procedures implemented to detect and remove the alignment weak modes. Finally, a summary of the alignment aspects that may be relevant to the deployment of the future experiments, like the ones at ILC, will be discussed.