The Alignment of the LHCb Vertex Detector: Performance in Run 2 and studies for the Upgrade

The LHCb detector at the LHC is a general purpose detector in the forward region designed to reconstruct decays of b and c hadrons. The Vertex Locator (VELO), consisting of a series of silicon strip sensors placed along the beam direction, allows the reconstruction of primary and secondary vertices and precise lifetime measurements. In Run 2 (2015-2018), a realtime alignment and calibration procedure was developed. Data collected at the start of each LHC fill are processed to evaluate the alignment using a Kalman filter method. An overview of the alignment algorithm and the real-time procedure is presented emphasizing the performance and its stability during the Run 2 data taking period. The VELO operates in a secondary vacuum with a bi-phase CO$_2$ running through cooling blocks attached to the module base, with heat conducted through a TPG core. The cooling temperature is -30 degrees and the sensors operate at -8 degrees. Tests involving a change of the operational temperature to higher values up to -20 degrees were made to take dedicated samples at each temperature. These samples were analysed to evaluate the dependency of the module position as a function of the VELO temperature and were compared with measurement on a single module. For Run 3 (2021-2023), a new tracking system is being built, including a new Vertex Locator based on silicon pixels (VP) and an improved alignment procedure is under development, building upon the success of the Run 2 strategy. From recent tests, misalignments can arise from potential distortions of the VELO upgrade modules when cooled to their operating temperature. A study on simulated data has been performed to evaluate the effect of the distortions on the physics performance. A broad variety of detector movements are simulated and the residual distortions are determined by the alignment procedure. Results of these studies and the on-going optimisation of the upgrade VELO alignment procedure are presented