Operating luminosity and collision energy are the two fundamental parameters that determine the discovery reach and precision potential of a particle collider experiment. While luminosity in astrophysics describes the brightness of a star, instantaneous luminosity in particle physics characterizes the intensity of an accelerator. Specifically, it is proportional to the collision rate and, consequently, to the production rate of any process with a given cross section. Integrated over time and summed over all colliding bunch pairs, it determines the total number of events produced for a given cross section and thus quantifies the size of the dataset delivered to an experiment. The Large Hadron Collider (LHC) has delivered hundreds of inverse femtobarns of data, establishing itself not only as a discovery machine but also as a precision instrument, provided that the luminosity is measured with sufficient accuracy.
At hadron colliders such as the LHC, luminosity determination presents a diverse set of experimental challenges. The measurement is carried out with dedicated detector elements calibrated using the van der Meer method. The colloquium discusses these challenges and presents the methodology behind the combined Run 2 luminosity determination at the Compact Muon Solenoid (CMS) experiment. The methods introduced allow for a relative uncertainty of 0.73%, marking the most precise luminosity measurement to date at a bunched-beam hadron collider. The talk concludes with an outlook on future challenges and potential solutions.
