Luminosity measurement at LHCb

The LHCb detector, designed to measure the decays of heavy hadrons, is a forward-arm spectrometer. Its efficiency can be degraded by collisions with high occupancy: therefore, a technique known as "luminosity levelling" has been used since the start of the LHC Run 1, allowing to control and stabilize the instantaneous luminosity with a precision of 5%. During LHC Runs 1 and 2, this technique employed data from the hardware-based trigger level to determine the instantaneous luminosity. These counters are calibrated in dedicated data taking runs a few times per year. The combination of van der Meer scans and of beam profiles obtained in beam-gas interactions, unique to LHCb, allowed LHCb to obtain in Run 1 the most precise luminosity measurement ever achieved at a bunched hadron collider. During LHC Run 3, the upgraded LHCb detector will see a 5x increase of luminosity. Dedicated luminosity detectors have been designed and are being commissioned for use in Run 3 and Run 4. This talk will review the methods used in Run 1 and introduce the new approach being developed for the coming LHC runs.