Low-latency alignment software for sensor calibration in the CMS-PPS detector at the CERN laboratory

The Precision Proton Spectrometer (PPS) is a subsystem of the Compact Muon Solenoid (CMS), which is one of the main detectors at the CERN's Large Hadron Collider (LHC). The PPS sensors are hosted in vessels called Roman Pots (RPs), which are frequently inserted and retracted from the LHC beam pipes. Thus, their shifts with respect to the beam are not constant and have to be calculated regularly, in a procedure called 'alignment'. It has already been designed and implemented but is not automated and can work only a sizeable time after collecting the data. In the LHC Run 3, beginning in 2022, PPS will need a faster way of obtaining the alignment results. The main goal of this project was to reimplement the alignment procedure so that it could operate within 48 hours after data acquisition. To accomplish that, the software uses the Prompt Calibration Loop (PCL) mechanism, and the code follows the structure of the Data Quality Monitoring (DQM) software. At first, the particle tracks detected in selected RPs are preprocessed in parallel in the worker module. Then, the data are merged, and the harvester module produces the alignment results. The software has been integrated into the CMS software framework – CMSSW, and the alignment configuration is effectively delivered via the CMS database – Conditions DB. The final product meets all the requirements. It has been thoroughly tested and reproduced the results from the old software.