ATLAS Inner Detector alignment towards Run 3

The algorithm used in the alignment of the Inner Detector of the ATLAS experiment is based on the track-to-hit residual minimization in a sequence of hierarchical levels mimicking the detector assembly: from large mechanical structures to individual modules. It aims to describe the detector geometry and its changes in time as accurately as possible, such that the resolution is not degraded by an imperfect positioning of the signal hit in the track reconstruction. The ID alignment during Run 2 has proven to describe the detector geometry with a precision at the level of µm. Moreover, the hit-to-track residual minimization procedure is not sensitive to deformations of the detectors that affect the track parameters while leaving the residuals unchanged. Those geometry deformations are called weak modes. The minimization of the remaining track parameter biases and weak mode deformations has been the main target of the alignment campaign of the reprocessed Run 2 data. New analysis methods for the weak mode measurement have been therefore implemented, providing a robust geometry description, validated by a wide spectrum of quality-assessment techniques. These novelties are foreseen to be the new baseline methods for the Run 3 data-taking, in which the higher luminosity would allow an almost real-time assessment of the alignment performance.