Development and prototype of a new luminometer for the ATLAS experiment during Run 3 and Run 4 of the LHC

The Beam Monitor for ATLAS (BMA) detector is an additional luminosity monitor for the ATLAS experiment, targeting the luminosity determination for the high luminosity phase of the Large Hadron Collider (HL-LHC). The detector is composed of small square pads with about 1 mm per side of Low Gain Avalanche Detector (LGAD) silicon sensors, intended to be placed in the ATLAS detector forward shielding at about 16m from the interaction point. The detector is innovative in several aspects: it does not need an external cooling system, and its readout preamplifiers are meant to be placed at about 25 m distance from the sensors, in an area in which the radiation levels for the electronics can be neglected. Moreover, the tiny acceptance of the small pads is intended to minimize systematic effects in the luminosity determination due to pile-up of several interactions in a single or consecutive proton-bunch crossings. A two-channel prototype of the detector has been installed already for Run 3 of the LHC and recorded data in the year 2022. Modified versions were set up for data-taking in the year 2023 and the current year 2024.