Status and upgrade of LUCID-2: the ATLAS luminosity detector from Run-3 to the HL-LHC

Abstract The LUCID-2 detector serves as the primary luminometer of the ATLAS experiment and is the only one capable of delivering reliable luminosity measurements across all beam configurations, luminosity regimes, and at the bunch-crossing level. During LHC Run-2, ATLAS achieved a luminosity precision of 0.8%, the most accurate determination ever obtained at a hadron collider. LUCID-2 continues to provide luminosity measurements for ATLAS in LHC Run-3, and preliminary results based on the collected datasets will be shown. The ATLAS physics program at the High-Luminosity LHC demands a luminosity precision of 1%. Meeting this requirement in an environment with up to 140 simultaneous interactions per bunch crossing (and up to 200 in the ultimate scenario) necessitates the use of several dedicated luminosity detectors. LUCID-3, the planned upgrade of LUCID-2, is among these. This contribution presents two LUCID-3 design options currently under investigation: one relying on photomultiplier tubes (PMTs), similar to LUCID-2 but positioned farther from the beam pipe to reduce acceptance and prevent detector saturation; and a second detector that uses optical fibers as Cherenkov radiators, read out by PMTs placed in a low-radiation region. All PMTs will be calibrated and monitored with a 207 Bi radioactive source to ensure long-term stability better than 1%. The main results obtained with both type of detector in the 2024 data taking period will be discussed, with emphasis on the implications for the final LUCID-3 design.