LUCID-3: the upgrade of the ATLAS luminosity detector for High Luminosity LHC

The ATLAS physics program at the High Luminosity LHC (HL-LHC) calls for a precision in the luminosity measurement of about 1%. A larger uncertainty would represent the dominant systematic error in some precision measurements, including the Higgs sector. To fulfil such requirement in an environment characterized by up to 140 simultaneous interactions per bunch crossing (200 in the ultimate scenario), ATLAS will feature several luminosity detectors. At least some of them must be possible to calibrate in the van der Meer scans at low luminosity and be able to measure luminosity up to its highest values. LUCID-3 is the upgraded detector of the present main ATLAS luminometer (LUCID-2) and should fulfil such a condition. Two main detector options are under study. The first one is based on photomultipliers located at a larger distance from the beamline compared with LUCID-2 and with a smaller active area. This will reduce the acceptance of the detector and avoid the saturation of the luminosity algorithms. The second option is based on optical fibers acting as both Cherenkov radiators and light-guides to route the produced light to the readout photomultipliers. Both detectors will have photomultipliers monitored continuously with Bi-207 radioactive sources deposited on the photomultiplier window. The second detector will also use LED light injected simultaneously to the PMT and at the end of the fibers in order to monitor a possible ageing of the fibers due to radiation. Several new prototype detectors that have been installed in ATLAS are discussed, together with the first results obtained in LHC Run-3.