The commissioning of the Upstream Tracker for the LHCb upgrade

The LHCb detector has undergone a major upgrade that will enable the experiment to acquire data with an all-software trigger, made possible by front-end readout in real-time and the capabilities of performing the data selection algorithm while the data are acquired. To achieve this goal, almost all detector subsystems have been replaced by new designs mandated by the processing speed requirements. At the heart of the real-time analysis is a fast and efficient track reconstruction, without spurious tracks composed of segments associated with hits from different charged particles. A detector crucial to the charged particle trajectory reconstruction is the Upstream Tracker (UT) (Collaboration, 2014), a 4-plane silicon microstrip detector in front of the dipole magnet. The UT also provides a momentum measurement, as it is in magnet’s fringe field. The UT comprises about 1000 sensors of four different designs, about 4000 dedicated front-end ASICs (SALT chips), performing analog processing, digitization, common-mode subtraction, and zero-suppression. Communications with the DAQ system are coordinated by a set of data control boards that also provide the optical interface. Four firmware algorithms are needed to process the UT data in the TELL40 readout boards because of the different data rates to be dealt with. We focus our report on the challenging task of ensuring that the excellent performance of the various detector components is maintained in the experiment environment and at the high rates expected. The UT was installed in LHCb in early 2023. The first year of commissioning was challenging for data synchronization issues related to specific properties of the GBTx chip. We report the lessons learned during the early commissioning phase and the upcoming run when the detector will be integrated in LHCb.