Luminosity Measurements for the CMS Experiment

The physics program of the CMS experiment at the Large Hadron Collider depends on precise luminosity measurements. During Run 2 (2015–2018), CMS achieved a relative uncertainty of less than 1% in its luminosity measurements, setting a benchmark for accuracy. The CMS experiment employs a suite of independent instruments, including the Pixel Luminosity Telescope (PLT), which uses silicon pixel detectors with fast trigger capability to measure the instantaneous luminosity for each LHC bunch crossing. It also provides fast feedback on beam conditions and backgrounds. The PLT is rebuilt for the 2025/26 run. The calibration of luminosity detectors is based on beam-separation scans using the Van der Meer method, requiring additional corrections to account for beam-related and detector-specific effects. This talk will discuss the systematic studies required to achieve high precision and provide an overview of the performance of the CMS luminosity detectors during Run 2 and Run 3 (2022 - present). Additional improvements in calibration techniques and corrections are expected to make the luminosity measurements even more precise.